Bible Encyclopedias
Chemistry

II. Principles

This section treats of such subjects as nomenclature, formulae, chemical equations, chemical change and similar subjects. It is intended to provide an introduction, necessarily brief, to the terminology and machinery of the chemist.

III. Inorganic Chemistry

Here is treated the history of descriptive inorganic chemistry; reference should be made to the articles on the separate elements for an account of their preparation, properties, &c.

IV. Organic Chemistry

This section includes a brief history of the subject, and proceeds to treat of the principles underlying the structure and interrelations of organic compounds.

V. Analytical Chemistry

This section treats of the qualitative detection and separation of the metals, and the commoner methods employed in quantitative analysis. The analysis of organic compounds is also noticed.

VI. Physical Chemistry

This section is restricted to an account of the relations existing between physical properties and chemical composition. Other branches of this subject are treated in the articles Chemical Action; Energetics; Solution; Alloys; Thermochemistry.

I History

Although such hypotheses could contribute nothing directly to the development of a science which laid especial claim to experimental investigations, yet indirectly they stimulated inquiry into the nature of the " essence " with which the four " elements " were associated. This quinta essentia had been speculated upon by the Greeks, some regarding it as immaterial or aethereal, andothers as material; and a school of philosophers termed alchemists arose who attempted the isolation of this essence. The existence of a fundamental principle, unalterable and indestructible, prevailing alike through physical and chemical changes, was generally accepted. Any change which a substance may chance to undergo was simply due to the discarding or taking up of some proportion of the primary " elements " or qualities: of these coverings " water," " air," " earth " and " fire " were regarded as clinging most tenaciously to the essence, while " cold," " heat," " moistness " and " dryness " were more easily cast aside or assumed. Several origins have been suggested for the word alchemy, and there seems to Alchemy. have been some doubt as to the exact nature and import of the alchemical doctrines. According to M. P. E. Berthelot, " alchemy rested partly on the industrial processes of the ancient Egyptians, partly on the speculative theories of the Greek philosophers, and partly on the mystical reveries of the Gnostics and Alexandrians." The search for this essence subsequently resolved itself into the desire to effect the transmutation of metals, more especially the base metals, into silver and gold. It seems that this secondary principle became the dominant idea in alchemy, and in this sense the word is used in Byzantine literature of the 4th century; Suidas, writing in the 11th century, defines chemistry as the " preparation of silver and gold " (see Alchemy) .

From the Alexandrians the science passed to the Arabs, who made discoveries and improved various methods of separating substances, and afterwards, from the 11th century, became seated in Europe, where the alchemical doctrines were assiduously studied until the 15th and 16th centuries. It is readily understood why men imbued with the authority of tradition should prosecute the search for a substance which would confer unlimited wealth upon the fortunate discoverer. Some alchemists honestly laboured to effect the transmutation and to discover the " philosopher's stone," and in many cases believed that they had achieved success, if we may rely upon writings assigned to them. The period, however, is one of literary forgeries; most of the MSS. are of uncertain date and authorship, and moreover are often so vague and mystical that they are of doubtful scientific value, beyond reflecting the tendencies of the age. The retaining of alchemists at various courts shows the high opinion which the doctrines had gained. It is really not extraordinary that Isaac Hollandus was able to indicate the method of the preparation of the " philosopher's stone " from " adamic " or " virgin " earth, and its action when medicinally employed; that in the writings assigned to Roger Bacon, Raimon Lull, Basil Valentine and others are to be found the exact quantities of it to be used in transmutation; and that George Ripley, in the 15th century, had grounds for regarding its action as similar to that of a ferment.

In the view of some alchemists, the ultimate principles of matter were Aristotle's four elements; the proximate constituents were a " sulphur " and a " mercury," the father and mother of the metals; gold was supposed to have attained to the perfection of its nature by passing in succession through the forms of lead, brass and silver; gold and silver were held to contain very pure red sulphur and white quicksilver, whereas in the other metals these materials were coarser and of a different colour. From an analogy instituted between the healthy human being and gold, the most perfect of the metals, silver, mercury, copper, iron, lead and tin, were regarded in the light of lepers that required to be healed.

Iatrochemistry

Notwithstanding the false idea which prompted the researches of the alchemists, many advances were made in descriptive chemistry, the metals and their salts receiving much attention, and several of our important acids being discovered. Towards the end of the 16th century the failure of the alchemists to achieve their cherished purpose, and the general increase in medical knowledge, caused attention to be given to the utilization of chemical preparations as medicines. As early as the 15th century the alchemist Basil Valentine had suggested this application, but the great exponent of this doctrine was Paracelsus, who set up a new definition: " The true use of chemistry is not to make gold but to prepare medicines." This relation of chemistry to medicine prevailed until the 17th century, and what in the history of chemistry is termed the iatrochemical period (see Medicine) was mainly fruitful in increasing the knowledge of compounds; the contributions to chemical theory are of little value, the most important controversies ranging over the nature of the " elements," which were generally akin to those of Aristotle, modified so as to be more in accord with current observations. At the same time, however, there were many who, opposed to the Paracelsian edefinition of chemistry, still labored at the problem of the alchemists, while others gave much attention to the chemical industries. Metallurgical operations, such as smelting, roasting, and refining, were scientifically investigated, and in some degree explained, by Georg Agricola and Carlo Biringuiccio; ceramics was studied by Bernard Palissy, who is also to be remembered as an early worker in agricultural chemistry, having made experiments on the effect of manures on soils and crops; while general technical chemistry was enriched by Johann Rudolf Glauber.1

Footnote 1: The more notable chemists of this period were Turquet de Mayerne (1573-1665), a physician of Paris, who rejected the Galenian doctrines and accepted the exaggeratons of Paracelsus; Andreas Libavius (d. 1610), chefly famous for his Opera Omnia Medicochymica (1595); Jean Baptiste van Helmont (1577-1644), celebrated for his researches on gases; F. de la Boë Sylvius (1614-1672), who regarded medicine as applied chemistry, and Otto Tachenius, who elucidated the nature of salts. (end footnote.)

Boyle

The second half of the 17th century witnessed remarkable transitions and developments in all branches of natural science,and the facts accumulated by preceding generations during their generally unordered researches were re placed by a co-ordination of experiment and deduction. From the mazy and incoherent alchemical and iatrochemical doctrines, the former based on false conceptions of matter, the latter on erroneous views of life processes and physiology, a new science arose - the study of the composition of substances. The formulation of this definition of chemistry was due to Robert Boyle. In his Sceptical Chemist (1662) he freely criticized the prevailing scientific views and methods, with the object of showing that true knowledge could only be gained by the logical application of the principles of experiment and deduction. Boyle's masterly exposition of this method is his most important contribution to scientific progress. At the same time he clarified the conception of elements and compounds, rejecting the older notions, the four elements of the " vulgar Peripateticks " and the three principles of the " vulgar Stagyrists," and defining an element as a substance incapable of decomposition, and a compound as composed of two or more elements. He explained chemical combination on the hypotheses that matter consisted of minute corpuscles, that by the coalescence of corpuscles of different substances distinctly new corpuscles of a compound were formed, and that each corpuscle had a certain affinity for other corpuscles.

Phlogistic theory.

Although Boyle practised the methods which he expounded,. he was unable to gain general acceptance of his doctrine of elements; and, strangely enough, the theory which next dominated chemical thought was an alchemical invention, and lacked the lucidity and perspicuity of Boyle's views. This theory, named the phlogistic theory,. was primarily based upon certain experiments on combustion and calcination, and in effect reduced the number of the alchemical principles, while setting up a new one, a principle of combustibility, named phlogiston (from (PXoyun-6s, burnt). Much discussion had centred about fire or the "igneous principle." On the one hand, it had been held that when a substance was burned or calcined, it combined with an " air "; on the other hand, the operation was supposed to be attended by the destruc tion or loss of the igneous principle. Georg Ernst Stahl, following in some measure the views held by Johann Joachim Becher, as, for instance, that all combustibles contain a " sulphur " (which notion is itself of older date than Becher's terra pinguis), regarded all substances as capable of resolution into two components,. the inflammable principle phlogiston, and another element- " water," " acid " or " earth." The violence or completeness of combustion was proportional to the amount of phlogiston present. Combustion meant the liberation of phlogiston.. Metals on calcination gave calces from which the metals could. be recovered by adding phlogiston, and experiment showed that this could generally be effected by the action of coal or carbon, which was therefore regarded as practically pure phlogiston; the other constituent being regarded as an acid. At the hands of Stahl and his school, the phlogistic theory, by exhibiting a fundamental similarity between all processes of combustion and by its remarkable flexibility, came to be a general theory of chemical action. The objections of the antiphlogistonists, such as the fact that calces weigh more than the original metals instead of less as the theory suggests, were answered by postulating that phlogiston was a principle of levity, or even completely ignored as an accident, the change of qualities being regarded as the only matter of importance. It is remarkable that this theory should have gained the esteem of the notable chemists who flourished in the 18th century. Henry Cavendish, a careful and accurate experimenter, was a phlogistonist, as were J. Black, K. W. Scheele, A. S. Marggraf, J. Priestley and many others who might be mentioned.

Lavoisier

Descriptive chemistry was now assuming considerable proportions; the experimental inquiries suggested by Boyle were being assiduously developed; and a wealth of observa tions was being accumulated, for the explanation of which the resources of the dominant theory were sorely taxed. To quote Antoine Laurent Lavoisier, " ... chemists have turned phlogiston into a vague principle, ... which consequently adapts itself to all the explanations for which it may be required. Sometimes this principle has weight, and sometimes it has not; sometimes it is free fire and sometimes it is fire combined with the earthy element; sometimes it passes through the pores of vessels, sometimes these are impervious to it; it explains both causticity and non-causticity, transparency and opacity, colours and their absence; it is a veritable Proteus changing in form at each instant." Lavoisier may be justly regarded as the founder of modern or quantitative chemistry. First and foremost, he demanded that the balance must be used in all investigations into chemical changes. He established as fundamental that combustion and calcination were attended by an increase of weight, and concluded, as did Jean Rey and John Mayow in the 17th century, that the increase was due to the combination of the metal with the air. The problem could obviously be completely solved only when the composition of the air, and the parts played by its components, had been determined. At all times the air had received attention, especially since van Helmont made his far-reaching investigations on gases. Mayow had suggested the existence of two components, a spiritus nitroaerus which supported combustion, and a spiritus nitri acidi which extinguished fire; J. Priestley and K. W. Scheele, although they isolated oxygen, were fogged by the phlogistic tenets; and H. Cavendish, who had isolated the nitrogen of the atmosphere, had failed to decide conclusively what had really happened to the air which disappeared during combustion.

Lavoisier adequately recognized and acknowledged how much he owed to the researches of others; to himself is due the co-ordination of these researches, and the welding of his results into a doctrine to which the phlogistic theory ultimately succumbed. He burned phosphorus in air standing over mercury, and showed that (1) there was a limit to the amount of phosphorus which could be burned in the confined air, (2) that when no more phosphorus could be burned, one-fifth of the air had disappeared, (3) that the weight of the air lost was nearly equal to the difference in the weights of the white solid produced and the phosphorus burned, (4) that the density of the residual air was less than that of ordinary air. The same results were obtained with lead and tin; and a more elaborate repetition indubitably established their correctness. He also showed that on heating mercury calx alone an " air " was liberated which differed from other " airs," and was slightly heavier than ordinary air; moreover, the weight of the " air " set free from a given weight of the calx was equal to the weight taken up in forming the calx from mercury, and if the calx be heated with charcoal, the metal was recovered and a gas named " fixed air," the modern carbon dioxide, was formed. The former experiment had been performed by Scheele and Priestley, who had named the gas " phlogisticated air "; Lavoisier subsequently named it oxygen, regarding it as the " acid producer " (OE , sour). The theory advocated by Lavoisier came to displace the phlogistic conception; but at first its acceptance was slow. Chemical literature was full of the phlogistic modes of expression - oxygen was '" phlogisticated air," nitrogen " dephlogisticated air," &c. - and this tended to retard its promotion. Yet really the transition from the one theory to the other was simple, it being only necessary to change the " addition or loss of phlogiston " into the " loss or addition of oxygen." By his insistence upon the use of the balance as a quantitative check upon the masses involved in all chemical reactions, Lavoisier was enabled to establish by his own investigations and the results achieved by others the principle now known as the " conservation of mass." Matter can neither be created nor destroyed; however a chemical system be changed, the weights before and after are equal.' To him is also due a rigorous examination of the nature of elements and compounds; he held the same views that were laid down by Boyle, and with the same prophetic foresight predicted that some of the elements which he himself accepted might be eventually found to be compounds.

Chemical affinity.

It is unnecessary in this place to recapitulate the many results which had accumulated by the end of the 18th century, or to discuss the labours and theories of individual workers since these receive attention under biographical headings; in this article only the salient features in the history of our science can be treated. The beginning of the i 9th century was attended by far-reaching discoveries in the nature of the composition of compounds. Investigations proceeded in two directions: - (I) the nature of chemical affinity, (2) the laws of chemical combination. The first question has not yet been solved, although it has been speculated upon y ? g p p from the earliest times. The alchemists explained chemical action by means of such phrases as " like attracts like," substances being said to combine when one " loved " the other, and the reverse when it " hated " it. Boyle rejected this terminology, which was only strictly applicable to intelligent beings; and he used the word " affinity " as had been previously done by Stahl and others. The modern sense of the word, viz. the force which holds chemically dissimilar substances together (and also similar substances as is seen in di-, tri-, and poly-atomic molecules), was introduced by Hermann Boerhaave, and made more precise by Sir Isaac Newton. The laws of chemical combination were solved, in a measure, by John Dalton, and the solution expressed as Dalton's " atomic theory." Lavoisier appears to have assumed that the composition of every chemical compound was constant, and the same opinion was the basis of much experimental inquiry at the hands of Joseph Louis Proust during 1801 to 1809, who vigorously combated the doctrine of Claude Louis Berthollet (Essai de statique chimique, 1803), viz. that fixed proportions of elements and compounds combine only under exceptional conditions, the general rule being that the composition of a compound may vary continuously between certain limits.2

Dalton

This controversy was unfinished when Dalton published the first part of his New System of Chemical Philosophy in 1808, although the per saltum theory was the most popular. Led thereto by speculations on gases, Dalton assumed that matter was composed of atoms, that in the elements the atoms were simple, and in compounds complex, being composed of elementary atoms. Dalton furthermore perceived that the same two elements or substances may combine in different proportions, and showed that these proportions had always a simple ratio to one another. This is the " law of multiple proportions." He laid down the following arbitrary rules for determining the number of atoms in a compound: - if only one compound of two elements exists, it is a binary compound and its atom is composed of one atom of each element; if two compounds exist one is binary (say A + B) and the other ternary (say A + 2B); if three, then one is binary and the others may be ternary (A ± 2B, and 2A + B), and so on. More important is his deduction of equivalent weights, i.e. the relative weights of atoms. He took hydrogen, the lightest substance known, to be the standard. From analyses of water, which he regarded as composed of one atom of hydrogen and one of oxygen, he This dictum was questioned by the researches of H. Landolt, A. Heydweiller and others. In a series of 75 reactions it was found that in 61 there was apparently a diminution in weight, but in 1908, after a most careful repetition and making allowance for all experimental errors, Landolt concluded that no change occurred (see Element).

2 The theory of Berthollet was essentially mechanical, and he attempted to prove that the course of a reaction depended not on affinities alone but also on the masses of the reacting components. In this respect his hypothesis has much in common with the " law of mass-action " developed at a much later date b y the Swedish chemists Guldberg and Waage, and the American, Willard Gibbs (see Chemical Action). In his classical thesis Berthollet vigorously attacked the results deduced by Bergman, who had followed in his table of elective attractions the path traversed by Stahl and S. F. Geoffroy.

deduced the relative weight of the oxygen atom to be 6.5; from marsh gas and olefiant gas he deduced carbon = 5, there being one atom of carbon and two of hydrogen in the former and one atom of hydrogen to one of carbon in the latter; nitrogen had an equivalent of 5, and so on.' The value of Dalton's generalizations can hardly be overestimated, notwithstanding the fact that in several cases they needed correction. The first step in this direction was effected by the co-ordination of Gay Lussac's observations on the combining volumes of gases. He discovered that gases always combined in volumes having simple ratios, and that the volume of the product had a simple ratio to the volumes of the reacting gases. For example, one volume of oxygen combined with two of hydrogen to form two volumes of steam, three volumes of hydrogen combined with one of nitrogen to give two volumes of ammonia, one volume of hydrogen combined with one of chlorine to give two volumes of hydrochloric acid. An immediate inference was that the Daltonian " atom " must have parts which enter into combination with parts of other atoms; in other words, there must exist two orders of particles, viz. (1) particles derived by limiting mechanical subdivision, the modern molecule, and (2) particles derived from the first class by chemical subdivision, i.e. particles which are incapable of existing alone, but may exist in combination. Additional evidence as to the structure of the molecule was discussed by Avogadro in 1811, and by Ampere in 1814. From the gas-laws of Boyle and J. A. C. Charles - viz. equal changes in temperature and pressure occasion equal changes in equal volumes of all gases and vapours - Avogadro deduced the law: Under the same conditions of temperature and pressure, equal volumes of gases contain equal numbers of molecules; and he showed that the relative weights of the molecules are determined as the ratios of the weights of equal volumes, or densities. He established the existence of molecules and atoms as we have defined above, and stated that the number of atoms in the molecule is generally 2, but may be 4, 8, &c. We cannot tell whether his choice of the powers of 2 is accident or design.

Notwithstanding Avogadro's perspicuous investigation, and a similar exposition of the atom and molecule by A. M. Ampere,. the views therein expressed were ignored both by their own and the succeeding generation. In place of the relative molecular weights, attention was concentrated on relative atomic or equivalent weights. This may be due in some measure to the small number of gaseous and easily volatile substances then known, to the attention which the study of the organic compounds received, and especially to the energetic investigations of J. J. Berzelius, who, fired with enthusiasm by the original theory of Dalton and the law of multiple proportions, determined the equivalents of combining ratios of many elements in an enormous number of compounds.2 He prosecuted his labours in this field for thirty years; as proof of his industry it may be mentioned that as early as 1818 he had determined the combining ratios of about two thousand simple and compound substances.

We may here notice the important chemical symbolism or notation introduced by Berzelius, which greatly contributed to the definite and convenient representation of chemical composition and the tracing of chemical reactions. The denotation of elements by symbols had been practised by the alchemists, and it is interesting to note that the symbols allotted to the well-known elements are identical with the astrological symbols of the sun and the other members of the solar system. Gold, the most perfect metal, had the symbol of the Sun, 0; silver, the semiperfect metal, had the symbol of the Moon, 0j; copper, iron and antimony, the imperfect metals of the gold class, had the symbols of Venus Mars and the Earth tin and lead, the imperfect metals of the silver class, had the symbols of Jupiter 94, and Saturn h; while mercury, the imperfect metal of both the gold and silver class, had the symbol of the planet,. Torbern Olof Bergman used an elaborate system in his Opuscula physica et chemica (1783); the 1 Dalton's atomic theory is treated in more detail in the article Atom.

2 Berzelius, however, appreciated the necessity of differentiating the atom and the molecule, and even urged Dalton to amend his doctrine, but without success.

elements received symbols composed of circles, arcs of circles, and lines, while certain class symbols, such as1W for metals, - - foracids, for alkalies, for salts, U for calces, &c., were used. Compounds were represented by copulating simpler symbols, e.g. mercury calx was LJ .3 Bergman's symbolism was obviously cumbrous, and the system used in 1782 by Lavoisier was equally abstruse, since the forms gave no clue as to composition; for instance water, oxygen, and nitric acid werev 4), and e-f.

A partial clarification was suggested in 1787 by J. H. Hassenfratz and Adet, who assigned to each element a symbol, and to each compound a sign which should record the elements present and their relative quantities. Straight lines and semicircles were utilized for the non-metallic elements, carbon, nitrogen, phosphorus and sulphur! (the " simple acidifiable bases " of Lavoisier), and circles enclosing the initial letters of their names for the metals. The " compound acidifiable bases," i.e. the hypothetical radicals of acids, were denoted by squares enclosing the initial letter of the base; an alkali was denoted by a triangle, and the particular alkali by inserting the initial letter. Compounds were denoted by joining the symbols of the components, and by varying the manner of joining compounds of the same elements were distinguished The symbol V was used to denote a liquid, and a vertical line to denote a gas. As an example of the complexity of this system we may note the five oxides of nitrogen, which were symbolized as the first three representing the gaseous oxides, and the last two the liquid oxides.

A great advance was made by Dalton, who, besides introducing simpler symbols, regarded the symbol as representing not only the element or compound but also one atom of that element or compound; in other words, his symbol denoted equivalent weights.4 This system, which permitted the correct representation of molecular composition, was adopted by Berzelius in 1814, who, having replaced the geometric signs of Dalton by the initial letter (or letters) of the Latin names of the elements, represented a compound by placing a plus sign between the symbols of its components, and the number of atoms of each component (except in the case of only one atom) by placing Arabic numerals before the symbols; for example, copper oxide was Cu +0, sulphur trioxide S+30. If two compounds combined, the + signs of the free compounds were discarded, and the number of atoms denoted by an Arabic index placed after the elements, and from these modified symbols the symbol of the new compound was derived in the same manner as simple compounds were built up from their elements. Thus copper sulphate was CuO+S0 3, potassium sulphate 2S0 3 +P00 2 (the symbol Po for potassium was subsequently discarded in favour of K from kalium). At a later date Berzelius denoted an oxide by dots, equal in number to the number of oxygen atoms present, placed over the element; this notation survived longest in mineralogy. He also introduced barred symbols, i.e. letters traversed by a horizontal bar, to denote the double atom (or molecule). Although the system of Berzelius has been modified and extended, its principles survive in the modern notation.

In the development of the atomic theory and the deduction of the atomic weights of elements and the formulae of compounds, Dalton's arbitrary rules failed to find complete acceptance. Berzelius objected to the hypothesis that if two elements form only one compound, then the atoms combine one and one; and although he agreed theory. with the adoption of simple rules as a first attempt at representing a compound, he availed himself of other data in order to gain further information as to the structure of compounds. For example, at first he represented ferrous and ferric oxides by the formulae Fe02, Fe03, and by the analogy of zinc and other basic oxides he regarded these substances as constituted similarly to Fe02, and the acidic oxides alumina and chromium oxide as similar to FeO 3. He found, however, that chromic acid, which he had represented as Cr06, neutralized a base containing 3 the 3 The following symbols were also used by Bergman: W, V, " + ", which represented zinc, manganese, cobalt, bismuth, nickel, arsenic, platinum, water, alcohol, phlogiston.

4 The following are the symbols employed by Dalton: which represent in order, hydrogen, nitrogen, carbon, oxygen, phosphorus, sulphur, magnesia, lime, soda, potash, strontia, baryta, mercury; iron, zinc, copper, lead, silver, platinum, and gold were represented by circles enclosing the initial letter of the element.

quantity of oxygen. He inferred that chromic acid must contain only three atoms of oxygen, as did sulphuric acid SO 3; consequently chromic oxide, which contains half the amount of oxygen, must be Cr 2 O 3, and hence ferric oxide must be Fe203. The basic oxides must have the general formula MO. To these results he was aided by the law of isomorphism formulated by E. Mitscherlich in 1820; and he confirmed his conclusions by showing the agreement with the law of atomic heat formulated by Dulong and Petit in 1819.

While successfully investigating the solid elements and their compounds gravimetrically, Berzelius was guilty of several inconsistencies in his views on gases. He denied that gaseous atoms could have parts, although compound gases could. This attitude was due to his adherence to the " dualistic theory" of the structure of substances, which he deduced from electrochemical researches. From the behaviour of substances on electrolysis he assumed that all substances had two components, one bearing a negative charge, the other a positive charge. Combination was associated with the coalescence of these charges, and the nature of the resulting compound showed the nature of the residual electricity. For example, positive iron combined with negative oxygen to form positive ferrous oxide; positive sulphur combined with negative oxygen to form negative sulphuric acid; positive ferrous oxide combined with negative sulphuric acid to form neutral ferrous sulphate. Berzelius elevated this theory to an important position in the history of our science. He recognized that if an elementary atom had parts, his theory demanded that these parts should carry different electric charges when they entered into reaction, and the products of the reaction should vary according as a positive or negative atom entered into combination. For instance if the reaction 2112+02=1120+1120 be true, the molecules of water should be different, for a negative oxygen atom would combine in one case, and a positive oxygen atom in the other. Hence the gaseous atoms of hydrogen and oxygen could not have parts. A second inconsistency was presented when he was compelled by the researches of Dumas to admit Avogadro's hypothesis; but here he would only accept it for the elementary gases, and denied it for other substances. It is to be noticed that J. B. Dumas did not adopt the best methods for emphasizing his discoveries. His terminology was vague and provoked caustic criticism from Berzelius; he assumed that all molecules contained two atoms, and consequently the atomic weights deduced from vapour density determinations of sulphur, mercury, arsenic, and phosphorus were quite different from those established by gravimetric and other methods.

Chemists gradually tired of the notion of atomic weights on account of the uncertainty which surrounded them; and the suggestion made by W. H. Wollaston as early as 1814 to deal only with " equivalents," i.e. the amount of an element which can combine with or replace unit weight of hydrogen, came into favour, being adopted by L. Gmelin in his famous text-book.

Simultaneously with this discussion of the atom and molecule, great controversy was ranging over the constitution of com pounds, more particularly over the carbon or organic compounds. This subject is discussed in section IV., Organic Chemistry. The gradual accumulation of data referring to organic compounds brought in its train a revival of the discussion of atoms and molecules. A. Laurent and C. F. Gerhardt attempted a solution by investigating chemical reactions. They assumed the atom to be the smallest part of matter which can exist in combination, and the molecule to be the smallest part which can enter into a chemical reaction. Gerhardt found that reactions could be best followed if one assumed the molecular weight of an element or compound to be that weight which occupied the same volume as two unit weights of hydrogen, and this assumption led him to double the equivalents accepted by Gmelin, making H= 1, 0 =16, and C = 12, thereby agreeing with Berzelius, and also to halve the values given by Berzelius to many metals. Laurent generally agreed, except when the theory compelled the adoption of formulae containing fractions of atoms; in such cases he regarded the molecular weight as the weight occupying a volume equal to four unit weights of hydrogen. The bases upon which Gerhardt and Laurent founded their views were not sufficiently well grounded to lead to the acceptance of their results; Gerhardt himself returned to Gmelin's equivalents in his Lehrbuch der Chemie (1853) as they were in such general use.

In 1860 there prevailed such a confusion of hypotheses as to the atom and molecule that a conference was held at Karlsruhe to discuss the situation. At the conclusion of the sitting, Lothar Meyer obtained a paper written by Stanislas Cannizzaro in 1858 wherein was found the final link required for the determination of atomic weights. This link was the full extension of Avogadro's theory to all substances, Cannizzaro showing that chemical reactions in themselves would not suffice. He chose as his unit of reference the weight of an atom of hydrogen, i.e. the weight contained in a molecule of hydrochloric acid, thus differing from Avogadro who chose the weight of a hydrogen molecule. From a study of the free elements Cannizzaro showed that an element may have more than one molecular weight; for example, the molecular weight of sulphur varied with the temperature. And from the study of compounds he showed that each element occurred in a definite weight or in some multiple of this weight. He called this proportion the " atom," since it invariably enters compounds without division, and the weight of this atom is the atomic weight. This generalization was of great value inasmuch as it permitted the deduction of the atomic weight of a non-gasifiable element from a study of the densities of its gasifiable compounds.

From the results obtained by Laurent and Gerhardt and their predecessors it immediately followed that, while an element could have but one atomic weight, it could have several equivalent weights. From a detailed study of organic compounds Gerhardt had promulgated a " theory of types " which represented a fusion of the older radical and type theories. This theory brought together, as it were, the most varied compounds, and stimulated inquiry into many fields. According to this theory, an element in a compound had a definite saturation capacity, an idea very old in itself, being framed in the law of multiple proportions. These saturation capacities were assidu- valency. ously studied by Sir Edward Frankland, who from the investigation, not of simple inorganic compounds, but of the organo-metallic derivatives, determined the kernel of the theory of valency. Frankland showed that any particular element preferentially combined with a definite number (which might vary between certain limits) of other atoms; for example, some atoms always combined with one atom of oxygen, some with two, while with others two atoms entered into combination with one of oxygen. If an element or radical combined with one atom of hydrogen, it was termed monovalent; if with two (or with one atom of oxygen, which is equivalent to two atoms of hydrogen) it was divalent, and so on. The same views were expressed by Cannizzaro, and also by A. W. von Hofmann, who materially helped the acceptance of the doctrine by the lucid exposition in his Introduction to Modern Chemistry, 1865.

The recognition of the quadrivalency of carbon by A. Kekule was the forerunner of his celebrated benzene theory in particular, and of the universal application of structural formulae to the representation of the most complex organic compounds equally lucidly as the representation of the simplest salts. Alexander Butlerow named the " structure theory," and contributed much to the development of the subject. He defined structure " as the manner of the mutual linking of the atoms in the molecule," but denied that any such structure could give information as to the orientation of the atoms in space. He regarded the chemical properties of a substance as due to (1) the chemical atoms composing it, and (2) the structure, and he asserted that while different compounds might have the same components (isomerism), yet only one compound could have a particular structure. Identity in properties necessitated identity in structure.

While the principle of varying valency laid down by Frankland is still retained, Butlerow's view that structure had no spatial significance has been modified. The researches of L. Pasteur, J. A. Le Bel, J. Wislicenus, van't Hoff and others showed that substances having the same graphic formulae vary in properties and reactions, and consequently the formulae need modification in order to exhibit these differences. Such isomerism, named stereoisomerism (q.v.),hasbeen assiduously developed during recentyears; it prevails among many different classes of organic compounds and many examples have been found in inorganic chemistry.

The theory of valency as a means of showing similarity of properties and relative composition became a dominant feature of chemical theory, the older hypotheses of types, radicals, &c.

being more or less discarded. We have seen how its utilization in the " structure theory " permitted great clarification, and attempts were not wanting for the deduction of analogies or a periodicity between elements. Frankland had recognized the analogies existing between the chemical properties of nitrogen, phosphorus, arsenic and antimony, noting that they act as trior penta-valent. Carbon was joined with silicon, zirconium and titanium, while boron, being trivalent, was relegated to another group. A general classification of elements, however, was not realized by Frankland, nor even by Odling, who had also investigated the question from the valency standpoint. The solution came abOut by arranging the elements in the order of their atomic weights, tempering the arrangement with the results deduced from the theory of valencies and experimental observations. Many chemists contributed to the establishment of such a periodicity, the greatest advances being made by John Newlands in England, Lothar Meyer in Germany, and D. J. Mendeleeff in St Petersburg. For the development of this classification see Element.

In the above sketch we have briefly treated the history of the main tendencies of our science from the earliest times to the Summary. establishment of the modern laws and principles. We Su have seen that the science took its origin in the arts practised by the Egyptians, and, having come under the influence of philosophers, it chose for its purpose the isolation of the quinta essentia, and subsequently the " art of making gold and silver." This spirit gave way to the physicians, who regarded " chemistry as the art of preparing medicines," a denotation which in turn succumbed to the arguments of Boyle, who regarded it as the " science of the composition of substances," a definition which adequately fits the science to-day. We have seen how his classification of substances into elements and compounds, and the definitions which he assigned to these species, have similarly been retained; and how Lavoisier established the law of the "conservation of mass," overthrew the prevailing phlogistic theory, and became the founder of modern chemistry by the overwhelming importance which he gave to the use of the balance. The development of the atomic theory and its concomitants - the laws of chemical combination and the notion of atoms and equivalents - at the hands of Dalton and Berzelius, the extension to the modern theory of the atom and molecule, and to atomic and molecular weights by Avogadro, Ampere, Dumas, Laurent, Gerhardt, Cannizzaro and others, have been noted. The structure of the molecule, which mainly followed investigations in organic compounds, Frankland's conception of valency, and finally the periodic law, have alsobeen shown in their chronological order. The principles outlined above constitute the foundations of our science; and although it may happen that experiments may be made with which they appear to be not in complete agreement, yet in general they constitute a body of working hypotheses of inestimable value.

Chemical Education

It is remarkable that systematic instruction in the theory and practice of chemistry only received earnest attention in our academic institutions during the opening decades of the 19th century. Although for a long time lecturers and professors had been attached to universities, generally their duties had also included the study of physics, mineralogy and other subjects, with the result that chemistry received scanty encouragement. Of practical instruction there was none other than that to be gained in a few private laboratories and in the shops of apothecaries. The necessity for experimental demonstration and practical instruction, in addition to academic lectures, appears to have been urged by the French chemists L. N. Vauquelin, Gay Lussac, Thenard, and more especially by A. F. Fourcroy and G. F. Rouelle, while in England Humphry Davy expounded the same idea in the experimental demonstrations which gave his lectures their brilliant charm. But the real founder of systematic instruction in our science was Justus von Liebig, who, having accepted the professorship at Giessen in 1824, made his chemical laboratory and course of instruction the model of all others. He emphasized that the practical training should include (1) the qualitative and quantitative analysis of mixtures, (2) the preparation of substances according to established methods, (3) original research - a course which has been generally adopted. The pattern set by Liebig at Giessen was adopted by F. Wohler at Göttingen in 1836, by R. W. Bunsen at Marburg in 1840, and by O. L. Erdmann at Leipzig in 1843; and during the 'fifties and 'sixties many other laboratories were founded. A new era followed the erection of the laboratories at Bonn and Berlin according to the plans of A. W. von Hofmann in 1867, and of that at Leipzig, designed by Kolbe in 1868. We may also mention the famous laboratory at Munich designed by A. von Baeyer in 1875.

In Great Britain the first public laboratory appears to have been opened in 1817 by Thomas Thomson at Glasgow. But the first important step in providing means whereby students could systematically study chemistry was the foundation of the College of Chemistry in 1845. This institution was taken over by the Government in 1853, becoming the Royal College of Chemistry, and incorporated with the Royal School of Mines; in 1881 the names were changed to the Normal School of Science and Royal School of Mines, and again in 1890 to the Royal College of Science. In 1907 it was incorporated in the Imperial College of Science and Technology. Under A. W. von Hofmann, who designed the laboratories and accepted the professorship in 1845 at the instigation of Prince Albert, and under his successor (in 1864) Sir Edward Frankland, this institution became one of the most important centres of chemical instruction. Oxford and Cambridge sadly neglected the erection of convenient laboratories for many years, and consequently we find technical schools and other universities having a far better equipment and offering greater facilities. In the provinces Victoria University at Manchester exercised the greater impetus, numbering among its professors Sir W. H. Perkin and Sir Henry Roscoe.

In America public laboratory instruction was first instituted at Yale College during the professorship of Benjamin Silliman. To the great progress made in recent years F. W. Clarke, W. Gibbs, E. W. Morley, Ira Remsen, and T. W. Richards have especially contributed.

In France the subject was almost entirely neglected until late in the 19th century. The few laboratories existing in the opening decades were ill-fitted, and the exorbitant fees constituted a serious bar to general instruction, for these institutions received little government support. In 1869 A. Wurtz reported the existence of only one efficient laboratory in France, namely the Ecole Normale Superieure, under the direction of H. Sainte Claire Deville. During recent years chemistry has become one of the most important subjects in the curriculum of technical schools and universities, and at the present time no general educational institution is complete until it has its full equipment of laboratories and lecture theatres.

Chemical Literature

The growth of chemical literature since the publication of Lavoisier's famous Traite de chimie in 1789, and of Berzelius' Lehrbuch der Chemie in 1808-1818, has been enormous. These two works, and especially the latter, were the models followed by Thenard, Liebig, Strecker, Wohler and many others, including Thomas Graham, upon whose Elements of Chemistry was founded Otto's famous Lehrbuch der Chemie, to which H. Kopp contributed the general theoretical part, Kolbe the organic, and Buff and Zamminer the physico-chemical. Organic chemistry was especially developed by the publication of Gerhardt's Traite de chimie organique in 1853-1856, and of Kekule's Lehrbuch der organischen Chemie in 1861-1882. General theoretical and physical chemistry was treated with conspicuous acumen by Lothar Meyer in his Moderne Theorien, by W. Ostwald in his Lehrbuch der allgem. Chemie (1884-1887), and by Nernst in his Theoretische Chemie. In English, Roscoe and Schorlemmer's Treatise on Chemistry is a standard work; it records Periodic law. a successful attempt to state the theories and facts of chemistry, not in condensed epitomes, but in an easily read form. The Traite de chimie mine'rale, edited by H. Moissan, and the Handbuch der anorganischen Chemie, edited by Abegg, are of the same type. O. Dammer's Handbuch der anorganischen Chemie and F. Beilstein's Handbuch der organischen Chemie are invaluable works of reference. Of the earlier encyclopaedias we may notice the famous HandwOrterbuch der reinen and angewandten Chemie, edited by Liebig; Fremy's Encyclopedie de chimie, Wurtz's Dictionnaire de chimie pure et applique'e, Watts' Dictionary of Chemistry, and Ladenburg's HandwOrterbuch der Chemie. The number of periodicals devoted to chemistry has steadily increased since the early part of the 19th century. In England the most important is the Journal of the Chemical Society of London, first published in 1848. Since 1871 abstracts of papers appearing in the other journals have been printed. In 1904 a new departure was made in issuing Annual Reports, containing resumes of the most important researches of the year. The Chemical News, founded by Sir W. Crookes in 1860, may also be noted. In America the chief periodical is the American Chemical Journal, founded in 1879. Germany is provided with a great number of magazines. The Berichte der deutschen chemischen Gesellschaft, published by the Berlin Chemical Society, the Chemisches Centralblatt, which is confined to abstracts of papers appearing in other journals, the Zeitschrift fur Chemie, and Liebig's Annalen der Chemie are the most important of the general magazines. Others devoted to special phases are the Journal fur praktische Chemie, founded by Erdmann in 1834, the Zeitschrift fur anorganische Chemie and the Zeitschrift fur physikalische Chemie. Mention may also be made of the invaluable Jahresberichte and the Jahrbuch der Chemie. In France, the most important journals are the Annales de chimie et de physique, founded in 1789 with the title Annales de chimie, and the Comptes rendus, published weekly by the Academie francaise since 1835.

II. General Principles The substances with which the chemist has to deal admit of classification into elements and compounds. Of the former about eighty may be regarded as well characterized, although many more have been described.

Elements.-The following table gives the names, symbols and atomic weights of the perfectly characterized elements: International Atomic Weights, 1910. The elements are usually divided into two classes, the metallic and the non-metallic elements; the following are classed as non-metals, and the remainder as metals: Of these hydrogen, chlorine, fluorine, oxygen, nitrogen, argon, neon, krypton, xenon and helium are gases, bromine is a liquid, and the remainder are solids. All the metals are solids at ordinary temperatures with the exception of mercury, which is liquid. The metals are mostly bodies of high specific gravity; they exhibit, when polished, a peculiar brilliancy or metallic lustre, and they are good conductors of heat and electricity; the nonmetals, on the other hand, are mostly bodies of low ,specific gravity, and bad conductors of heat and electricity, and do not exhibit metallic lustre. The non-metallic elements are also sometimes termed metalloids, but this appellation, which signifies metal-like substances (Gr. eilos, like), strictly belongs to certain elements which do not possess the properties of the true metals, although they more closely resemble them than the non-metals in many respects; thus, selenium and tellurium, which are closely allied to sulphur in their chemical properties, although bad conductors of heat and electricity, exhibit metallic lustre and have relatively high specific gravities. But when the properties of the elements are carefully contrasted together it is found that no strict line of demarcation can be drawn dividing them into two classes; and if they are arranged in a series, those which are most closely allied in properties being placed next to each other, it is observed that there is a more or less regular alteration in properties from term to term in the series.

When binary compounds, or compounds of two elements, are decomposed by an electric current, the two elements make their appearance at opposite poles. Those elements which are disengaged at the negative pole are termed electro-positive, or positive, or basylous elements, whilst those disengaged at the positive pole are termed electro-negative, or negative, or chlorous elements. But the difference between these two classes of elements is one of degree only, and they gradually merge into each other; moreover the electric relations of elements are not absolute, but vary according to the state of combination in which they exist, so that it is just as impossible to divide the elements into two classes according to this property as it is to separate them into two distinct classes of metals and non-metals. The following, however, are negative towards the remaining elements which are more or less positive:-Fluorine, chlorine, bromine, iodine, oxygen, sulphur, selenium, tellurium.

The metals may be arranged in a series according to their power of displacing one another in salt solutions, thus Cs, Rb, K, Na, Mg, Al, Mn, Zn, Cd, Tl, Fe, Co, Ni, Sn, Pb, (H), Sb, Bi, As, Cu, Hg, Ag, Pd, Pt, Au.

Elements which readily enter into reaction with each other, and which develop a large amount of heat on combination, are said to have a powerful affinity for each other. The tendency of positive elements to unite with positive elements, or of negative elements to unite with negative elements, is much less than that of positive elements to unite with negative elements, and the greater the difference in properties between two elements the more powerful is their affinity for each other. Thus, the affinity of hydrogen and oxygen for each other is extremely powerful, much heat being developed by the combination of these two elements; when binary compounds of oxygen are decomposed by the electric current, the oxygen invariably appears at the positive pole, being negative to all other elements, but the hydrogen of hydrogen compounds is always disengaged at the negative pole. Hydrogen and oxygen are, therefore, of very opposite natures, and this is well illustrated by the circumstance that oxygen combines, with very few exceptions, with all the remaining elements, whilst compounds of only a limited number with hydrogen have been obtained.

Com pounds .-A chemical compound contains two or more elements; consequently it should be possible to analyse it, i.e. separate it into its components, or to synthesize it, i.e. build it up from its components. In general, a compound has properties markedly different from those of the elements of which it is composed.

Laws of Chemical Combination

A molecule may be defined as the smallest part of a substance which can exist alone; an atom as the smallest part of a substance which can exist in combination. The molecule of every compound must obviously contain at least two atoms, and generally the molecules of the elements are also polyatomic, the elements with monatomic molecules (at moderate temperatures) being mercury and the gases of the argon group. The laws of chemical combination are as follows: I. Law of Definite Proportions. - The same compound always contains the same elements combined together in the same mass proportion. Silver chloride, for example, in whatever manner it may be prepared, invariably consists of chlorine and silver in the proportions by weight of 35'45 parts of the former and 107.93 of the latter.

2. Law of Multiple Proportions

When the same two elements combine together to form more than one compound, the different masses of one of the elements which unite with a constant mass of the other, bear a simple ratio to one another. Thus, i part by weight of hydrogen unites with 8 parts by weight of oxygen, forming water, and with 16 or 8 X 2 parts of oxygen, forming hydrogen peroxide. Again, in nitrous oxide we have a compound of 8 parts by weight of oxygen and 14 of nitrogen; in nitric oxide a compound of 16 or 8 X 2 parts of oxygen and 1 4 of nitrogen; in nitrous anhydride a compound of 24 or 8 X 3 parts of oxygen and 14 of nitrogen; in nitric peroxide a compound of 3 2 or 8 X 4 parts of oxygen and 14 of nitrogen; and lastly, in nitric anhydride a compound of 4 o or 8 X 5 parts of oxygen and 14 of nitrogen.

3. Law of Reciprocal Proportions

The masses of different elements which combine separately with one and the same mass of another element, are either the same as, or simple multiples of, the masses of these different elements which combine with each other. For instance, 35'45 parts of chlorine and 79.96 parts of bromine combine with 107.93 parts of silver; and when chlorine and bromine unite it is in the proportion of 35'45 parts of the former to 79.96 parts of the latter. Iodine unites with silver in the proportion of 126.97 parts to 107.93 parts of the latter, but it combines with chlorine in two proportions, viz. in the proportion of 126.97 parts either to 35'45 or to three times 35'45 parts of chlorine.

There is a fourth law of chemical combination which only applies to gases. This law states that: - gases combine with one another in simple proportions by volume, and the volume of the product (if gaseous) has a simple ratio to the volumes of the original mixtures; in other words, the densities of gases are simply related to their combining weights.

Nomenclature

If a compound contains two atoms it is termed a binary compound, if three a ternary, if four a quaternary, and so on. Its systematic name is formed by replacing the last syllable of the electro-negative element by ide and prefixing the name of the other element. For example, compounds of oxygen are oxides, of chlorine, chlorides, and so on. If more than one compound be formed from the same two elements, ,the difference is shown by prefixing such words as mono-, di-, tri-, sesqui-, per-, sub-, &c., to the last part of the name, or the suffixes -ous and -ic may be appended to the name of the first element. For example take the oxides of nitrogen, N 2 0, NO, N 2 0 3, NO 2, N 2 0 5; these are known respectively as nitrous oxide, nitric oxide, nitrogen trioxide, nitrogen peroxide and nitrogen pentoxide. The affixes -ous and sub- refer to the compounds containing more of the positive element, -ic and per- to those containing less.

An acid (q.v.) is a compound of hydrogen, which element can be replaced by metals, the hydrogen being liberated, giving substances named salts. An alkali or base is a substance which neutralizes an acid with the production of salts but with no evolution of hydrogen. A base may be regarded as water in which part of the hydrogen is replaced by a metal, or by a radical which behaves as a metal. (The term radical is given to a group of atoms which persist in chemical changes, behaving as if the group were an element; the commonest is the ammonium group, NH 4, which forms salts similar to the salts of sodium and potassium.) If the acid contains no oxygen it is a hydracid, and its systematic name is formed from the prefix hydro- and the name of the other element or radical, the last syllable of which has been replaced by the termination -ic. For example, the acid formed by hydrogen and chlorine is termed hydrochloric acid (and sometimes hydrogen chloride). If an acid contains oxygen it is termed an oxyacid. The nomenclature of acids follows the same general lines as that for binary compounds. If one acid be known its name is formed by the termination -ic, e.g. carbonic acid; if two, the one containing the less amount of oxygen takes the termination -ous and the other the termination -ic, e.g. nitrous acid, HN02, nitric acid, HN03. If more than two be known, the one inferior in oxygen content has the prefix hypo- and the termination -ous, and the one superior in oxygen content has the prefix per- and the termination -ic. This is illustrated in the four oxyacids of chlorine, HC10, HC10 2, HC10 3, HC10 4, which have the names hypochlorous, chlorous, chloric and perchloric acids. An acid is said to be monobasic, dibasic, tribasic, &c., according to the number of replaceable hydrogen atoms; thus HNO 3 is monobasic, sulphuric acid H 2 SO 4 dibasic, phosphoric acid H 3 PO 4 tribasic.

An acid terminating in -ous forms a salt ending in -ite, and an oxyacid ending in -ic forms a salt ending in -ate. Thus the chlorine oxyacids enumerated above form salts named respectively hypochlorites, chlorites, chlorates and perchlorates. Salts formed from hydracids terminate in -ide, following the rule for binary compounds. An acid salt is one in which the whole amount of hydrogen has not been replaced by metal; a normal salt is one in which all the hydrogen has been replaced; and a basic salt is one in which part of the acid of the normal salt has been replaced by oxygen.

Chemical Formulae. - Opposite the name of each element in the second column of the above table, the symbol is given which is always employed to represent it. This symbol, however, not only represents the particular element, but a certain definite quantity of it. Thus, the letter H always stands for 1 atom or part by weight of hydrogen, the letter N for 1 atom or 14 parts of nitrogen, and the symbol Cl for 1 atom or 35'5 parts of chlorine.' Compounds are in like manner represented by writing the symbols of their constituent elements side by side, and if more than one atom of each element be present, the number is indicated by a numeral placed on the right of the symbol of the element either below or above the line. Thus, hydrochloric acid is represented by the formula HC1, that is to say, it is a compound of an atom of hydrogen with an atom of chlorine, or of i part by weight of hydrogen with 35'5 parts by weight of chlorine; again, sulphuric acid is represented by the formula H 2 SO 4, which is a statement that it consists of 2 atoms of hydrogen, 1 of sulphur, and 4 of oxygen, and consequently of certain relative weights of these elements. A figure placed on the right of a symbol only affects the symbol to which it is attached, but when figures are placed in front of several symbols all are affected by it, thus 2H 2 SO 4 means H 2 SO 4 taken twice.

The distribution of weight in chemical change is readily expressed in the form of equations by the aid of these symbols; the equation 2HC1+Zn =ZnCl2+H2, for example, is to be read as meaning that from 73 parts of hydrochloric acid and 65 parts of zinc, 136 parts of zinc chloride and 2 parts of hydrogen are produced. The + sign is invariably employed in this way either to express combination or action upon, the meaning usually attached to the use of the sign = being that from such and such bodies such and such other bodies are formed.

1 Approximate values of the atomic weights are employed here.

Usually, when the symbols of the elements are written or printed with a figure to the right, it is understood that this indicates a molecule of the element, the symbol alone representing an atom. Thus, the symbols 14 2 and P4 indicate that the molecules of hydrogen and phosphorus respectively contain 2 and 4 atoms. Since, according to the molecular theory, in all cases of chemical change the action is between molecules, such symbols as these ought always to be employed. Thus, the formation of hydrochloric acid from hydrogen and chlorine is correctly represented by the equation H2+C12=2HC1; that is to say, a molecule of hydrogen and a molecule of chlorine give rise to two molecules of hydrochloric acid; whilst the following equation merely represents the relative weights of the elements which enter into reaction, and is not a complete expression of what is supposed to take place: H+CI =HC1.

In all cases it is usual to represent substances by formulae which to the best of our knowledge express their molecular composition in the state of gas, and not merely the relative number of atoms which they contain; thus, acetic acid consists of carbon, hydrogen and oxygen in the proportion of one atom of carbon, two of hydrogen, and one of oxygen, but its molecular weight corresponds to the formula C211402, which therefore is always employed to represent acetic acid. When chemical change is expressed with the aid of molecular formulae not only is the distribution of weight represented, but by the mere inspection of the symbols it is possible to deduce from the law of gaseous combination mentioned above, the relative volumes which the agents and resultants occupy in the state of gas if measured at the same temperature and under the same pressure. Thus, the equation 2112+02 =2H20 not only represents that certain definite weights of hydrogen and oxygen furnish a certain definite weight of the compound which we term water, but that if the water in the state of gas, the hydrogen and the oxygen are all measured at the same temperature and pressure, the volume occupied by the oxygen is only half that occupied by the hydrogen, whilst the resulting water-gas will only occupy the same volume as the hydrogen. In other words, 2 volumes of oxygen and 4 volumes of hydrogen furnish 4 volumes of water-gas. A simple equation like this, therefore, when properly interpreted, affords a large amount of information. One other instance may be given; the equation 2NH3=N2+3H2 represents the decomposition of ammonia gas into nitrogen and hydrogen gases by the electric spark, and it not only conveys the information that a certain relative weight of ammonia, consisting of certain relative weights of hydrogen and nitrogen, is broken up into certain relative weights of hydrogen and nitrogen, but also that the nitrogen will be contained in half the space which contained the ammonia, and that the volume of the hydrogen will be one and a half times as great as that of the original ammonia, so that in the decomposition of ammonia the volume becomes doubled.

Formulae which merely express the relative number of atoms of the different elements present in a compound are termed empirical formulae, and the formulae of all compounds whose molecular weights are undetermined are necessarily empirical. The molecular formula of a compound, however, is always a simple multiple of the empirical formula, if not identical with it; thus, the empirical formula of acetic acid is CH 2 O, and its molecular formula is C2H402, or twiceTCH 2 O. In addition to empirical and molecular formulae, chemists are in the habit of employing various kinds of rational formulae, called structural, constitutional or graphic formulae, &c., which not only express the molecular composition of the compounds to which they apply, but also embody certain assumptions as to the manner in which the constituent atoms are arranged, and convey more or less information with regard to the nature of the compound itself, viz. the class to which it belongs, the manner in which it is formed, and the behaviour it will exhibit under various circumstances. Before explaining these formulae it will be necessary, however, to consider the differences in combining power exhibited by the various elements.

Valency

It is found that the number of atoms of a given element, of chlorine, for example, which unite with an atom of each of the other elements is very variable. Thus, hydrogen unites with but a single atom of chlorine, zinc with two, boron with three, silicon with four, phosphorus with five and tungsten with six. Those elements which are equivalent in combining or displacing power to a single atom of hydrogen are said to be univalent or monad elements; whilst those which are equivalent to two atoms of hydrogen are termed bivalent or dyad elements; and those equivalent to three, four, five or six atoms of hydrogen triad, tetrad, pentad or hexad elements. But not only is the combining power or valency (atomicity) of the elements different, it is also observed that one element may combine with another in several proportions, or that its valency may vary; for example, phosphorus forms two chlorides represented by the formulae PC1 3 and PC1 51 nitrogen the series of oxides represented by the formulae N 2 0, NO, (N203), N 2 O 4, N205, molybdenum forms the chlorides MoC1 2, MoC1 3, MoC1 4, MoC1 5, MoCls(?), and tungsten the chlorides WC1 2, WCl 4, WC1 5, WC16.

. In explanation of these facts it is supposed that each element has a certain number of " units of affinity," which may be entirely, or only in part, engaged when it enters into combination with other elements; and in those cases in which the entire number of units of affinity are not engaged by other elements, it is supposed that those which are thus disengaged neutralize each other, as it were. For example, in phosphorus pentachloride the five units of affinity possessed by the phosphorus atom are satisfied by the five monad atoms of chlorine, but in the trichloride two are disengaged, and, it may be supposed, satisfy each other. Compounds in which all the units of affinity of the contained elements are engaged are said to be saturated, whilst those in which the affinities of the contained elements are not all engaged by other elements are said to be unsaturated. According to this view, it is necessary to assume that, in all unsaturated compounds, two, or some even number of affinities are disengaged; and also that all elements which combine with an even number of monad atoms cannot combine with an odd number, and vice versa, - in other words, that the number of units of affinity active in the case of any given element must be always either an even or an odd number, and that it cannot be at one time an even and at another an odd number. There are, however, a few remarkable exceptions to this " law." Thus, it must be supposed that in nitric oxide, NO, an odd number of affinities are disengaged, since a single atom of dyad oxygen is united with a single atom of nitrogen, which in all its compounds with other elements acts either as a triad or pentad. When nitric peroxide, N204, is converted into gas, it decomposes, and at about 180° C. its vapour entirely consists of molecules of the composition N02; while at temperatures between this and o C. it consists of a mixture in different proportions of the two kinds of molecules, N 2 O 4 and N02. The oxide NO 2 must be regarded as another instance of a compound in which an odd number of affinities of one of the contained elements are disengaged, since it contains two atoms of dyad oxygen united with a single atom of triad or pentad nitrogen. Again, when tungsten hexachloride is converted into vapour it is decomposed into chlorine and a pentachloride, having a normal vapour density, but as in the majority of its compounds tungsten acts as a hexad, we apparently must regard its pentachloride as a compound in which an odd number of free affinities are disengaged. Hitherto no explanation has been given of these exceptions to what appears to be a law of almost universal application, viz. that the sum of the units of affinity of all the atoms in a compound is an even number.

The number of units of affinity active in the case of any particular element is largely dependent, however, upon the nature of the element or elements with which it is associated. Thus, an atom of iodine only combines with one of hydrogen, VI. 2 a but may unite with three of chlorine, which never combines with more than a single atom of hydrogen; an atom of phosphorus unites with only three atoms of hydrogen, but with five of chlorine, or with four of hydrogen and one of iodine; and the chlorides corresponding to the higher oxides of lead, nickel, manganese and arsenic, Pb0 2, Ni 2 0 3, Mn0 2 and As 2 0 5 do not exist as stable compounds, but the lower chlorides, PbCl 2j NiC12, MnC1 2 and AsC1 3j are very stable.

The valency of an element is usually expressed by dashes or Roman numerals placed on the right of its symbol, thus: H', O", B"', C I ", P", mow; but in constructing graphic formulae the symbols of the elements are written with as many lines attached to each symbol as the element which it represents has units of affinity.

The periodic law (see Element) permits a grouping of the elements according to their valency as follows: - Group 0: helium, neon, argon, krypton and xenon appear to be devoid of valency. Group I.: the alkali metals Li, Na, K, Rb, Cs, and also Ag, monovalent; Cu, monovalent and divalent; Au, monovalent and trivalent. Group II.: the alkaline earth metals Ca, Sr, Ba, and also Be (GI), Mg, Zn, Cd, divalent; Hg, monovalent and divalent. Group III.: B, trivalent; Al, trivalent, but possibly also tetra-or penta-valent; Ga, divalent and trivalent; In, mono-, diand tri-valent; Tl, monovalent and trivalent.. Group IV.: C, Si, Ge, Zr, Th, tetravalent; Ti, tetravalent and hexavalent; Sn, Pb, divalent and tetravalent; Ce, trivalent and tetravalent. Group V.: N, trivalent and pentavalent, but divalent in nitric oxide; P, As, Sb, Bi, trivalent and pentavalent, the last being possibly divalent in BiO and BiC1 2. Group VI.: 0, usually divalent, but tetravalent and possibly hexavalent in oxonium and other salts; S, Se, Te, di-, tetraand hexa-valent; Cr, di-, triand hexa-valent; Mo, W, di-, tri-, tetra-, pentaand hexa-valent. Group VII.: H (?), monovalent; the halogens F, Cl, Br, I, usually monovalent, but possibly also triand pentavalent; Mn, divalent and trivalent, and possibly heptavalent in permanganates. Group VIII.: Fe, Co, divalent and trivalent; Ni, divalent; Os, Ru, hexavalent and octavalent; Pd, Pt, divalent and tetravalent; Ir, tri-, tetraand hexa-valent. (See also Valency.) Constitutional Formulae. - Graphic or constitutional formulae are employed to express the manner in which the constituent atoms of compounds are associated together; for example, the trioxide of sulphur is usually regarded as a compound of an atom of hexad sulphur with three atoms of dyad oxygen, and this hypothesis is illustrated by the graphic formula 0=S`O.

When this oxide is brought into contact with water it combines with it forming sulphuric acid, H2S04.

In this compound only two of the oxygen atoms are wholly associated with the sulphur atom, each of the remaining oxygen atoms being united by one of its affinities to the sulphur atoms, and by the remaining affinity to an atom of hydrogen; thus HO S ?O HO> The graphic formula of a sulphate is readily deduced by remembering that the hydrogen atoms are partially or entirely replaced. Thus acid sodium sulphate, normal sodium sulphate, and zinc sulphate have the formulae Na-0 O NaO ,O O ¦ O H.o> S <<O NaO >S ?O, zn Again, the reactions of acetic acid, C 2 H 4 0 2, show that the four atoms of hydrogen which it contains have not all the same function, and also that the two atoms of oxygen have different functions; the graphic formula which we are led to assign to acetic acid, viz.

HOH serves in a measure to express this, three of the atoms of hydrogen being represented as associated with one of the atoms of carbon, whilst the fourth atom is associated with an atom of oxygen which is united by a single affinity to the second atom of carbon, to which, however, the second atom of oxygen is united by both of its affinities. It is not to be supposed that there are any actual bonds of union between the atoms; graphic formulae such as these merely express the hypothesis that certain of the atoms in a compound come directly within the sphere of attraction of certain other atoms, and only indirectly within the sphere of attraction of others, - an hypothesis to which chemists are led by observing that it is often possible to separate a group of elements from a compound, and to displace it by other elements or groups of elements.

Rational formulae of a much simpler description than these graphic formulae are generally employed. For instance, sulphuric acid is usually represented by the formula S0 2 (OH) 2, which indicates that it may be regarded as a compound of the group SO 2 with twice the group OH. Each of these OH groups is equivalent in combining or displacing power to a monad element, since it consists of an atom of dyad oxygen associated with a single atom of monad hydrogen, so that in this case the S02 group is equivalent to an atom of a dyad element. This formula for sulphuric acid, however, merely represents such facts as that it is possible to displace an atom of hydrogen and an atom of oxygen in sulphuric acid by a single atom of chlorine, thus forming the compound SO 3 HC1; and that by the action of water on the compound SO 2 C1 2 twice the group OH, or water minus an atom of hydrogen, is introduced in place of the two monad atoms of chlorine S0 2 C1 2 +2HOH = S0 2 (OH) 2 +2HC1.

Water. Sulphuric acid.

Constitutional formulae like these, in fact, are nothing more than symbolic expressions of the character of the compounds which they represent, the arrangement of symbols in a certain definite manner being understood to convey certain information with regard to the compounds represented.

Groups of two or more atoms like SO 2 and OH, which are capable of playing the part of elementary atoms (that is to say, which can be transferred from compound to compound), are termed compound radicals, the elementary atoms being simple radicals. Thus, the atom of hydrogen is a monad simple radical, the atom of oxygen a dyad simple radical, whilst the group OH is a monad compound radical.

It is often convenient to regard compounds as formed upon certain types; alcohol, for example, may be said to be a compound formed upon the water type, that is to say, a compound formed from water by displacing one of the atoms of hydrogen by the group of elements C 2 H 5, thus - H C2H5 O H O H Water Alcohol.

Constitutional formulae become of preponderating importance when we consider the more complicated inorganic and especially organic compounds. Their full significance is treated in the section of this article dealing with organic chemistry, and in the articles Isomerism and STEREO-Isomerism.

Chemical Action. - Chemical change or chemical action may be said to take place whenever changes occur which involve an alteration in the composition of molecules, and may be the result of the action of agents such as heat, electricity or light, or of two or more elements or compounds upon each other.

Three kinds of changes are to be distinguished, viz. changes which involve combination, changes which involve decomposition or separation, and changes which involve at the same time both decomposition and combination. Changes of the first and second kind, according to our views of the constitution of molecules, are probably of very rare occurrence; in fact, chemical action appears almost always to involve the occurrence of both these kinds of change, for, as already pointed out, we must assume that the molecules of hydrogen, oxygen and several other elements are diatomic, or that they consist of two atoms. Indeed, it appears probable that with few exceptions the elements are all compounds of similar atoms united together by one or more units of affinity, according to their valencies. If this be the case, however, it is evident that there is no real distinction between the reactions which take place when two elements combine together and when an element in a compound is displaced by another. The combination, as it is ordinarily termed, of chlorine with hydrogen, and the displacement of iodine in potassium iodide by the action of chlorine, may be cited as examples; if these reactions are represented, as such reactions very commonly are, by equations which merely express the relative weights of the bodies which enter into reaction, and of the products, thus Cl = HC1 Hydrogen. Chlorine. Hydrochloric acid.

KI -ICl = KC1 + I Potassium iodide. Chlorine. Potassium chloride. Iodine. they appear to differ in character; but if they are correctly represented by molecular equations, or equations which express the relative number of molecules which enter into reaction and which result from the reaction, it will be obvious that the character of the reaction is substantially the same in both cases, and that both are instances of the occurrence of what is ordinarily termed double decomposition H2 + C12 = 2HC1 Hydrogen. Chlorine. Hydrochloric acid.

2KI + C12 = 2KC1 ?- 12.

Potassium iodide. Chlorine. Potassium chloride. Iodine.

In all cases of chemical change energy in the form of heat is either developed or absorbed, and the amount of heat developed or absorbed in a given reaction is as definite as are the weights of the substance engaged in the reaction. Thus, in the production of hydrochloric acid from hydrogen and chlorine 22,000 calories are developed; in the production of hydrobromic acid from hydrogen and bromine, however, only 8440 caloriesare developed; and in the formation of hydriodic acid from hydrogen and iodine 6040 calories are absorbed.

This difference in behaviour of the three elements, chlorine, bromine and iodine, which in many respects exhibit considerable resemblance, may be explained in the following manner. We may suppose that in the formation of gaseous hydrochloric acid from gaseous chlorine and hydrogen, according to the equation H2 +C1 2 = HCI+HC1, a certain amount of energy is expended in separating the atoms of hydrogen in the hydrogen molecule, and the atoms of chlorine in the chlorine molecule, from each other; but that heat is developed by the combination of the hydrogen atoms with the chlorine atoms, and that, as more energy is developed by the union of the atoms of hydrogen and chlorine than is expended in separating the hydrogen atoms from each other and the chlorine atoms from one another, the result of the action of the two elements upon each other is the development of heat, - the amount finally developed in the reaction being the difference between that absorbed in decomposing the elementary molecules and that developed by the combination of the atoms of chlorine and hydrogen. In the formation of gaseous hydrobromic acid from liquid bromine and gaseous hydrogen H2+Br2=HBr+HBr, in addition to the energy expended in decomposing the hydrogen and bromine molecules, energy is also expended in converting the liquid bromine into the gaseous condition, and probably less heat is developed by the combination of bromine and hydrogen than by the combination of chlorine and hydrogen, so that the amount of heat finally developed is much less than is developed in the formation of hydrochloric acid. Lastly, in the production of gaseous hydriodic acid from hydrogen and solid iodine H2 - 1 - 12=HI+HI, so much energy is expended in the decomposition of the hydrogen and iodine molecules and in the conversion of the iodine into the gaseous condition, that the heat which it may be supposed is developed by the combination of the hydrogen and iodine atoms is insufficient to balance the expenditure, and the final result is therefore negative; hence it is necessary in forming hydriodic acid from its elements to apply heat continuously.

These compounds also afford examples of the fact that, generally speaking, those compounds are most readily formed, and are most stable, in the formation of which the most heat is developed. Thus, chlorine enters into reaction with hydrogen, and removes hydrogen from hydrogenized bodies, far more readily than bromine; and hydrochloric acid is a far more stable substance than hydrobromic acid, hydriodic acid being greatly inferior even to hydrobromic acid in stability. Compounds formed with the evolution of heat are termed exothermic, while those formed with an absorption are termed endothermic. Explosives are the commonest examples of endothermic compounds.

When two substances which by their action upon each other develop much heat enter into reaction, the reaction is usually complete without the employment of an excess of either; for example, when a mixture of hydrogen and oxygen, in the proportions to form water 2E12+0, =20H2, is exploded, it is entirely converted into water. This is also the case if two substances are brought together in solution, by the action of which upon each other a third body is formed which is insoluble in the solvent employed, and which also does not tend to react upon any of the substances present; for instance, when a solution of a chloride is added to a solution of a silver salt, insoluble silver chloride is precipitated, and almost the whole of the silver is removed from solution, even if the amount of the chloride employed be not in excess of that theoretically required.

But if there be no tendency to form an insoluble compound, or one which is not liable to react upon any of the other substances present, this is no longer the case. For example, when a solution of a ferric salt is added to a solution of potassium thiocyanate, a deep red coloration is produced, owing to the formation of ferric thiocyanate. Theoretically the reaction takes place in the case of ferric nitrate in the manner represented by the equation Fe(NOs) 3 + 3KCNS = Fe(CNS) 3 + 3KNOs; Ferric nitrate. Potassium thiocyanate. Ferric thiocyanate. Potassium nitrate.

but it is found that even when more than sixty times the amount of potassium thiocyanate required by this equation is added, a portion of the ferric nitrate still remains unconverted, doubtless owing to the occurrence of the reverse change Fe (CNS) 3 +3KNOs =Fe(N03) 3+3KCNS.

In this, as in most other cases in which substances act upon one another under such circumstances that the resulting compounds are free to react, the extent to which the different kinds of action which may occur take place is dependent upon the mass of the substances present in the mixture. As another instance of this kind, the decomposition of bismuth chloride by water may be cited. If a very large quantity of water be added, the chloride is entirely decomposed in the manner represented by the equation BiC1 3 -fOH, = BiOCI -F2HC1, Bismuth chloride. Bismuth oxychloride.

the oxychloride being precipitated; but if smaller quantities of water be added the decomposition is incomplete, and it is found that the extent to which decomposition takes place is proportional to the quantity of water employed, the decomposition being incomplete, except in presence of large quantities of water, because of the occurrence of the reverse action BiOC1-1-2HC1= BiC13-{-02H.

Chemical change which merely involves simple decomposition is thus seen to be influenced by the masses of the reacting substances and the presence of the products of decomposition; in other words the system of reacting substances and resultants form a mixture in which chemical action has apparently ceased, or the system is in equilibrium. Such reactions are termed reversible (see Chemical Action).

III. Inorganic Chemistry Inorganic chemistry is concerned with the descriptive study o f the elements and their compounds, except those of carbon. Reference should be made to the separate articles on the different elements and the more important compounds for their preparation, properties and uses. In this article the development of this branch of the science is treated historically.

The earliest discoveries in inorganic chemistry are to be found in the metallurgy, medicine and chemical arts of the ancients. The Egyptians obtained silver, iron, copper, lead, zinc and tin, either pure or as alloys, by smelting the ores; mercury is mentioned by Theophrastus (c. 300 B.C.). The manufacture of glass, also practised in Egypt, demanded a knowledge of sodium or potassium carbonates; the former occurs as an efflorescence on the shores of certain lakes; the latter was obtained from wood ashes. Many substances were used as pigments: Pliny records white lead, cinnabar, verdigris and red oxide of iron; and the preparation of coloured glasses and enamels testifies to the uses to which these and other substances were put. Salts of ammonium were also known; while alum was used as a mordant in dyeing. Many substances were employed in ancient medicine: galena was the basis of a valuable Egyptian cosmetic and drug; the arsenic sulphides, realgar and orpiment, litharge, alum, saltpetre, iron rust were also used. Among the Arabian and later alchemists we find attempts made to collate compounds by specific properties, and it is to these writers that we are mainly indebted for such terms as "alkali," " sal," &c. The mineral acids, hydrochloric, nitric and sulphuric acids, and also aqua regia (a mixture of hydrochloric and nitric acids) were discovered, and the vitriols, alum, saltpetre, sal-ammoniac, ammonium carbonate, silver nitrate (lunar caustic) became better known. The compounds of mercury attracted considerable attention, mainly on account of their medicinal properties; mercuric oxide and corrosive sublimate were known to pseudo-Geber, and the nitrate and basic sulphate to " Basil Valentine." Antimony and its compounds formed the subject of an elaborate treatise ascribed to this last writer, who also contributed to our knowledge of the compounds of zinc, bismuth and arsenic. All the commonly occurring elements and compounds appear to have received notice by the alchemists; but the writings assigned to the alchemical period are generally so vague and indefinite that it is difficult to determine the true value of the results obtained.

In the succeeding iatrochemical period, the methods of the alchemists were improved and new ones devised. Glauber showed how to prepare hydrochloric acid, spiritus salis, by heating rock-salt with sulphuric acid, the method in common use to-day; and also nitric acid from saltpetre and arsenic trioxide. Libavius obtained sulphuric acid from many substances, e.g. alum, vitriol, sulphur and nitric acid, by distillation. The action of these acids on many metals was also studied; Glauber obtained zinc, stannic, arsenious and cuprous chlorides by dissolving the metals in hydrochloric acid, compounds hitherto obtained by heating the metals with corrosive sublimate, and consequently supposed to contain mercury. The scientific study of salts dates from this period, especial interest being taken in those compounds which possessed a medicinal or technical value. In particular, the salts of potassium, sodium and ammonium were carefully investigated, but sodium and potassium salts were rarely differentiated. 1 The metals of the alkaline-earths were somewhat neglected; we find Georg Agricola considering gypsum (calcium sulphate) as a compound of lime, while calcium nitrate and chloride became known at about the beginning of the 17th century. Antimonial, bismuth and arsenical compounds were assiduously studied, a direct consequence of their high medicinal importance; mercurial and silver compounds were investigated for the same reason. The general tendency of this period appears to have taken the form of improving and developing the methods of the alchemists; 1 The definite distinction between potash and soda was first established by Duhamel de Monceau (1700-1781).

few new fields were opened, and apart from a more complete knowledge of the nature of salts, no valuable generalizations were attained.

The discovery of phosphorus by Brand, a Hamburg alchemist, in 1669 excited chemists to an unwonted degree; it was also independently prepared by Robert Boyle and J. Kunckel, Brand having kept his process secret. Towards the middle of the 18th century two new elements were isolated: cobalt by G. Brandt in 1742, and nickel by A. F. Cronstedt in 1750. These discoveries were followed by Daniel Rutherford's isolation of nitrogen in 1772, and by K. Scheele's isolation of chlorine and oxygen in 1 774 (J. Priestley discovered oxygen independently at about the same time), and his investigation of molybdic and tungstic acids in the following year; metallic molybdenum was obtained by P. J. Hjelm in 1783, and tungsten by Don Fausto d'Elhuyar; manganese was isolated by J. G. Gahn in 1 774. In 1784 Henry Cavendish thoroughly examined hydrogen, establishing its elementary nature; and he made the far-reaching discovery that water was composed of two volumes of hydrogen to one of oxygen.

The phlogistic theory, which pervaded the chemical doctrine of this period, gave rise to continued study of the products of calcination and combustion; it thus happened that the knowledge of oxides and oxidation products was considerably developed. The synthesis of nitric acid by passing electric sparks through moist air by Cavendish is a famous piece of experimental work, for in the first place it determined the composition of this important substance, and in the second place the minute residue of air which would not combine, although ignored for about a century, was subsequently examined by Lord Rayleigh and Sir William Ramsay, who showed that it consists of a mixture of elementary substances - argon, krypton, neon and xenon (see Argon).

The 18th century witnessed striking developments in pneumatic chemistry, or the chemistry of gases, which had been begun by van Helmont, Mayow, Hales and Boyle. Gases formerly considered to be identical came to be clearly distinguished, and many new ones were discovered. Atmospheric air was carefully investigated by Cavendish, who showed that it consisted of two elementary constituents: nitrogen, which was isolated by Rutherford in 1772, and oxygen, isolated in 1774; and Black established the presence, in minute quantity, of carbon dioxide (van Helmont's gas sylvestre). Of the many workers in this field, Priestley occupies an important position. A masterly device, initiated by him, was to collect gases over mercury instead of water; this enabled him to obtain gases previously only known in solution, such as ammonia, hydrochloric acid, silicon fluoride and sulphur dioxide. Sulphuretted hydrogen and nitric oxide were discovered at about the same time.

Returning to the history of the discovery of the elements and their more important inorganic compounds, we come in 1789 to M. H. Klaproth's detection of a previously unknown constituent of the mineral pitchblende. He extracted a substance to which he assigned the character of an element, naming it uranium (from O'pavos, heaven); but it was afterwards shown by E. M. Peligot, who prepared the pure metal, that Klaproth's product was really an oxide. This element was investigated at a later date by Sir Henry Roscoe, and more thoroughly and successfully by C. Zimmermann and Alibegoff. Pitchblende attained considerable notoriety towards the end of the 19th century on account of two important discoveries. The first, made by Sir William Ramsay in 1896, was that the mineral evolved a peculiar gas when treated with sulphuric acid; this gas, helium (q.v.), proved to be identical with a constituent of the sun's atmosphere, detected as early as 1868 by Sir Norman Lockyer during a spectroscopic examination of the sun's chromosphere. The second discovery, associated with the Curies, is that of the peculiar properties exhibited by the impure substance, and due to a constituent named radium. The investigation of this substance and its properties (see Radioactivity) has proceeded so far as to render it probable that the theory of the unalterability of elements, and also the hitherto accepted explanations of various celestial phenomena - the source of solar energy and the appearances of the tails of comets - may require recasting.

In the same year as Klaproth detected uranium, he also isolated zirconia or zirconium oxide from the mineral variously known as zircon, hyacinth, jacynth and jargoon; but he failed to obtain the metal, this being first accomplished some years later by Berzelius, who decomposed the double potassium zirconium fluoride with potassium. In the following year, 1795, Klaproth announced the discovery of a third new element, titanium; its isolation' (in a very impure form), as in the case of zirconium, was reserved for Berzelius.

Passing over the discovery of carbon disulphide by W. A. Lampadius in 1796, of chromium by L. N. Vauquelin in 1797, and Klaproth's investigation of tellurium in 1798, the next important series of observations was concerned with platinum and the allied metals. Platinum had been described by Antonio de Ulloa in 1748, and subsequently discussed by H. T. Scheffer in 1752. In 1803 W. H. Wollaston discovered palladium, especially interesting for its striking property of absorbing (" occluding ") as much as 376 volumes of hydrogen at ordinary temperatures, and 643 volumes at 90 0. In the following year he discovered rhodium; and at about the same time Smithson Tennant added two more to the list - iridium and osmium; the former was so named from the changing tints of its oxides (ipcs, rainbow), and the latter from the odour of its oxide (ovµA, smell). The most recently discovered " platinum metal," ruthenium, was recognized by C. E. Claus in 1845. The great number and striking character of the compounds of this group of metals have formed the subject of many investigations, and already there is a most voluminous literature. Berzelius was an early worker in this field; he was succeeded by Bunsen, and Deville and Debray, who worked out the separation of rhodium; and at a later date by P. T. Cleve, the first to make a really thorough study of these elements and their compounds. Of especial note are the curious compounds formed by the union of carbon monoxide with platinous chloride, discovered by Paul Schiitzenberger and subsequently investigated by F. B. Mylius and F. Foerster and by Pullinger; the phosphoplatinic compounds formed primarily from platinum and phosphorus pentachloride; and also the " ammino " compounds, formed by the union of ammonia with the chloride, &c., of these metals, which have been studied by many chemists, especially S. M. Jorgensen. Considerable uncertainty existed as to the atomic weights of these metals, the values obtained by Berzelius being doubtful. K. F. O. Seubert redetermined this constant for platinum, osmium and iridium; E. H. Keiser for palladium, and A. A. Joly for ruthenium.

The beginning of the 19th century witnessed the discovery of certain powerful methods for the analysis of compounds and the isolation of elements. Berzelius's investigation of the action of the electric current on salts clearly demonstrated the invaluable assistance that electrolysis could render to the isolator of elements; and the adoption of this method by Sir Humphry Davy for the analysis of the hydrates of the metals of the alkalis and alkaline earths, and the results which he thus achieved, established its potency. In 1808 Davy isolated sodium and potassium; he then turned his attention to the preparation of metallic calcium, barium, strontium and magnesium. Here he met with greater difficulty, and it is to be questioned whether he obtained any of these metals even in an approximately pure form (see Electrometallurgy). The discovery of boron by Gay Lussac and Davy in 1809 led Berzelius to investigate silica (silex). In the following year he announced that silica was the oxide of a hitherto unrecognized element, which he named silicium, considering it to be a metal. This has proved to be erroneous; it is non-metallic in character, and its name was altered to silicon, from analogy with carbon and boron. At the same time Berzelius obtained the element, in an impure condition, by fusing silica with charcoal and iron in a blast furnace; its preparation in a pure condition he first accomplished in 1823, when he invented the method of heating double potassium fluorides with metallic potassium. The success which attended his experiments in the case of silicon led him to apply it to the isolation of other elements. In 1824 he obtained zirconium from potassium zirconium fluoride; the preparation of (impure) titanium quickly followed, and in 1828 he obtained thorium. A similar process, and equally efficacious, was introduced by F. WOhler in 1827. It consisted in heating metallic chlorides with potassium, and was first applied to aluminium, which was isolated in 1827; in the following year, beryllium chloride was analysed by the same method, beryllium oxide (berylla or glucina) having been known since 1798, when it was detected by L. N. Vauquelin in the gem-stone beryl.

In 1812 B. Courtois isolated the element iodine from " kelp," the burnt ashes of marine plants. The chemical analogy of this substance to chlorine was quickly perceived, especially after its investigation by Davy and Gay Lussac. Cyanogen, a compound which in combination behaved very similarly to chlorine and iodine, was isolated in 1815 by Gay Lussac. This discovery of the first of the then-styled " compound radicals " exerted great influence on the prevailing views of chemical composition. Hydrochloric acid was carefully investigated at about this time by Davy, Faraday and Gay Lussac, its composition and the elementary nature of chlorine being thereby established.

In 1817 F. Stromeyer detected a new metallic element, cadmium, in certain zinc ores; it was rediscovered at subsequent dates by other observers and its chemical resemblance to zinc noticed. In the same year Berzelius discovered selenium in a deposit from sulphuric acid chambers, his masterly investigation including a study of the hydride, oxides and other compounds. Selenic acid was discovered by E. Mitscherlich, who also observed the similarity of the crystallographic characters of selenates and sulphates, which afforded valuable corroboration of his doctrine of isomorphism. More recent and elaborate investigations in this direction by A. E. H. Tutton have confirmed this view.

In 1818 L. J. Thenard discovered hydrogen dioxide, one of the most interesting inorganic compounds known, which has since been carefully investigated by H. E. SchOne, M. Traube, Wolfenstein and others. About the same time, J. A. Arfvedson, a pupil of Berzelius, detected a new element, which he named lithium, in various minerals - notably petalite. Although unable to isolate the metal, he recognized its analogy to sodium and potassium; this was confirmed by R. Bunsen and A. Matthiessen in 1855, who obtained the metal by electrolysis and thoroughly examined it and its compounds. Its crimson flame-coloration was observed by C. G. Gmelin in 1818.

The discovery of bromine in 1826 by A. J. Balard completed for many years Berzelius's group of " halogen " elements; the remaining member, fluorine, notwithstanding many attempts, remained unisolated until 1886, when Henri Moissan obtained it by the electrolysis of potassium fluoride dissolved in hydrofluoric acid. Hydrobromic and hydriodic acids were investigated by Gay Lussac and Balard, while hydrofluoric acid received considerable attention at the hands of Gay Lussac, Thenard and Berzelius. We may, in fact, consider that the descriptive study of the various halogen compounds dates from about this time. Balard discovered chlorine monoxide in 1834, investigating its properties and reactions; and his observations on hypochlorous acid and hypochlorites led him to conclude that " bleaching-powder " or " chloride of lime " was a compound or mixture in equimolecular proportions of calcium chloride and hypochlorite, with a little calcium hydrate. Gay Lussac investigated chloric acid; Stadion discovered perchloric acid, since more fully studied by G. S. Serullas and Roscoe; Davy and Stadion investigated chlorine peroxide, formed by treating potassium chlorate with sulphuric acid. Davy also described and partially investigated the gas, named by him " euchlorine," obtained by heating potassium chlorate with hydrochloric acid; this gas has been more recently examined by Pebal. The oxy-acids of iodine were investigated by Davy and H. G. Magnus; periodic acid, discovered by the latter, is characterized by the striking complexity of its salts as pointed out by Kimmins.

In 1830 N. G. Sefstrom definitely proved the existence of a metallic element vanadium, which had been previously detected (in 1801) in certain lead ores by A. M. del Rio; subsequent elaborate researches by Sir Henry Roscoe showed many inaccuracies in the conclusions of earlier workers (for instance, the substance considered to be the pure element was in reality an oxide) and provided science with an admirable account of this element and its compounds. B. W. Gerland contributed to our knowledge of vanadyl salts and the vanadic acids. Chemically related to vanadium are the two elements tantalum and columbium or niobium. These elements occur in the minerals columbite and tantalite, and their compounds became known in the early part of the 19th century by the labours of C. Hatchett, A. G. Ekeberg, W. H. Wollaston and Berzelius. But the knowledge was very imperfect; neither was it much clarified by H. Rose, who regarded niobium oxide as the element. The subject was revived in 1866 by C. W. Blomstrand and J. C. Marignac, to whom is due the credit of first showing the true chemical relations of these elements. Subsequent researches by Sainte Claire Deville and L. J. Troost, and by A. G. Kriiss and L. E. Nilson, and subsequently (1904) by Hall, rendered notable additions to our knowledge of these elements and their compounds. Tantalum has in recent years been turned to economic service, being employed, in the same manner as tungsten, for the production of the filaments employed in incandescent electric lighting.

In 1833 Thomas Graham, following the paths already traced out by E. D. Clarke, Gay Lussac and Stromeyer, published his masterly investigation of the various phosphoric acids and their salts, obtaining results subsequently employed by J. von Liebig in establishing the doctrine of the characterization and basicity of acids. Both phosphoric and phosphorous acids became known, although imperfectly, towards the end of the 18th century; phosphorous acid was first obtained pure by Davy in 1812, while pure phosphorous oxide, the anhydride of phosphorous acid, remained unknown until T. E. Thorpe's investigation of the products of the slow combustion of phosphorus. Of other phosphorus compounds we may here notice Gengembre's discovery of phosphuretted hydrogen (phosphine) in 1783, the analogy of which to ammonia was first pointed out by Davy and supported at a later date by H. Rose; liquid phosphuretted hydrogen was first obtained by Thenard in 1838; and hypophosphorous acid was discovered by Dulong in 1816. Of the halogen compounds of phosphorus, the trichloride was discovered by Gay Lussac and Thenard, while the pentachloride was obtained by Davy. The oxychloride, bromides, and other compounds were subsequently discovered; here we need only notice Moissan's preparation of the trifluoride and Thorpe's discovery of the pentafluoride, a compound of especial note, for it volatilizes unchanged, giving a vapour of normal density and so demonstrating the stability of a pentavalent phosphorus compound (the pentachloride and pentabromide dissociate into a molecule of the halogen element and phosphorus trichoride).

In 1840 C. F. Schonbein investigated ozone, a gas of peculiar odour (named from the Gr. N"Ecv, to smell) observed in 1785 by Martin van Marum to be formed by the action of a silent electric discharge on the oxygen of the air; he showed it to be an allotropic modification of oxygen, a view subsequently confirmed by Marignac, Andrews and Soret. In 1845 a further contribution to the study of allotropy was made by Anton Schrotter, who investigated the transformations of yellow and red phosphorus, phenomena previously noticed by Berzelius, the inventor of the term " allotropy." The preparation of crystalline boron in 1856 by Wohler and Sainte Claire Deville showed that this element also existed in allotropic forms, amorphous boron having been obtained simultaneously and independently in 1809 by Gay Lussac and Davy. Before leaving this phase of inorganic chemistry, we may mention other historical examples of allotropy. Of great importance is the chemical identity of the diamond, graphite and charcoal, a fact demonstrated in part by Lavoisier in 1773, Smithson Tennant in 1796, and by Sir George Steuart-Mackenzie (1780-1848), who showed that equal weights. of these three substances yielded the same weight of carbon dioxide on combustion. The allotropy of selenium was first investigated by Berzelius; and more fully in 1851 by J. W. Hittorf, who carefully investigated the effects produced by heat; crystalline selenium possesses a very striking property, viz. when exposed to the action of light its electric conductivity increases. Another element occurring in allotropic forms is sulphur, of which many forms have been described. E. Mitscherlich was an early worker in this field. A modification known as " black sulphur," soluble in water, was announced by F. L. Knapp in 1848, and a colloidal modification was described by H. Debus. The dynamical equilibrium between rhombic, liquid and monosymmetric sulphur has been worked out by H. W. Bakhuis Roozeboom. The phenomenon of allotropy is not confined to the non-metals, for evidence has been advanced to show that allotropy is far commoner than hitherto supposed. Thus the researches of Carey Lea, E. A. Schneider and others, have proved the existence of " colloidal silver "; similar forms of the metals gold, copper, and of the platinum metals have been described. The allotropy of arsenic and antimony is also worthy of notice, but in the case of the first element the variation is essentially non-metallic, closely resembling that of phosphorus. The term allotropy has also been applied to inorganic compounds, identical in composition, but assuming different crystallographic forms. Mercuric oxide, sulphide and iodide; arsenic trioxide; titanium dioxide and silicon dioxide may be cited as examples.

The joint discovery in 1859 of the powerful method of spectrum analysis (see Spectroscopy) by G. R. Kirchhoff and R. W. Bunsen, and its application to the detection and the characterization of elements when in a state of incandescence, rapidly led to the discovery of many hitherto unknown elements. Within two years of the invention the authors announced the discovery of two metals, rubidium and caesium, closely allied to sodium, potassium and lithium in properties, in the mineral lepidolite and in the Diirkheim mineral water. In 1861 Sir William Crookes detected thallium (named from the Gr. BdXXo , a green bud, on account of a brilliant green line in its spectrum) in the selenious mud of the sulphuric acid manufacture; the chemical affinities of this element, on the one hand approximating to the metals of the alkalis, and on the other hand to lead, were mainly established by C. A. Lamy. Of other metals first detected by the spectroscope mention is to be made of indium, determiped by F. Reich and H. T. Richter in 1863, and of gallium, detected in certain zinc blendes by Lecoq de Boisbaudran in 1875. The spectroscope has played an all-important part in the characterization of the elements, which, in combination with oxygen, constitute the group of substances collectively named the " rare earths." The substances occur, in very minute quantity, in a large number of sparingly-distributed and comparatively rare minerals - euxenite, samarksite, cerite, yttrotantalite, &c. Scandinavian specimens of these minerals were examined by J. Gadolin, M. H. Klaproth, and especially by Berzelius; these chemists are to be regarded as the pioneers in this branch of descriptive chemistry. Since their day many chemists have entered the lists, new and powerful methods of research have been devised, and several new elements definitely characterized. Our knowledge on many points, however, is very chaotic; great uncertainty and conflict of evidence circulate around many of the " new elements " which have been announced, so much so that P. T. Cleve proposed to divide the " rare earth " metals into two groups, (1) " perfectly characterized "; (2) " not yet thoroughly characterized." The literature of this subject is very large. The memorial address on J. C. G. de Marignac, a noted worker in this field, delivered by Cleve, a high authority on this subject, before the London Chemical Society (J. C. S. Trans., 18 95, p. 468), and various papers in the same journal by Sir William Crookes, Bohuslav Brauner and others should be consulted for details.

In the separation of the constituents of the complex mixture of oxides obtained from the " rare earth " minerals, the methods generally forced upon chemists are those of fractional precipitation or crystallization; the striking resemblances of the compounds of these elements rarely admitting of a complete separation by simple precipitation and filtration. The extraordinary patience requisite to a successful termination of such an analysis can only be adequately realized by actual research; an idea may be obtained from Crookes's Select Methods in Analysis. Of recent years the introduction of various organic compounds as precipitants or reagents has reduced the labour of the process; and advantage has also been taken of the fairly complex double salts which these metals form with compounds. The purity of the compounds thus obtained is checked by spectroscopic observations. Formerly the sparkand absorption-spectra were the sole methods available; a third method was introduced by Crookes, who submitted the oxides, or preferably the basic sulphates, to the action of a negative electric discharge in vacuo, and investigated the phosphorescence induced spectroscopically. By such a study in the ultra-violet region of a fraction prepared from crude yttria he detected a new element victorium, and subsequently by elaborate fractionation obtained the element itself.

The first earth of this group to be isolated (although in an impure form) was yttria, obtained by Gadolin in 1194 from the mineral gadolinite, which was named after its discoverer and investigator. Klaproth and Vauquelin also investigated this earth, but without detecting that it was a complex mixture - a discovery reserved for C. G. Mosander. The next discovery, made independently and simultaneously in 1803 by Klaproth and by W. Hisinger and Berzelius, was of ceria, the oxide of cerium, in the mineral cerite found at Ridderhytta, Westmannland, Sweden. These crude earths, yttria and ceria, have supplied most if not all of the " rare earth " metals. In 1841 Mosander, having in 1839 discovered a new element lanthanum in the mineral cerite, isolated this element and also a hitherto unrecognized substance, didymia, from crude yttria, and two years later he announced the determination of two fresh constituents of the same earth, naming them erbia and terbia. Lanthanum has retained its elementary character, but recent attempts at separating it from didymia have led to the view that didymium is a mixture of two elements, praseodymium and neodymium (see Didymium). Mosander's erbia has been shown to contain various other oxides - thulia, holmia, &c. - but this has not yet been perfectly worked out. In 1878 Marignac, having subjected Mosander's erbia, obtained from gadolinite, to a careful examination, announced the presence of a new element, ytterbium; this discovery was confirmed by Nilson, who in the following year discovered another element, scandium, in Marignac's ytterbia. Scandium possesses great historical interest, for Cleve showed that it was one of the elements predicted by Mendeleeff about ten years previously from considerations based on his periodic classification of the elements (see Element). Other elements predicted and characterized by Mendeleeff which have been since realized are gallium, discovered in 1875, and germanium, discovered in 1885 by Clemens Winkler.

In 1880 Marignac examined certain earths obtained from the mineral samarksite, which had already in 1878 received attention from Delafontaine and later from Lecoq de Boisbaudran. He established the existence of two new elements, samarium and gadolinium, since investigated more especially by Cleve, to whom most of our knowledge on this subject is due. In addition to the rare elements mentioned above, there are a score or so more whose existence is doubtful. Every year is attended by fresh " discoveries " in this prolific source of elementary substances, but the paucity of materials and the predilections of the investigators militate in some measure against a just valuation being accorded to such researches. After having been somewhat neglected for the greater attractions and wider field presented by organic chemistry, the study of the elements and their inorganic compounds is now' rapidly coming into favour; new investigators are continually entering the lists; the beaten paths are being retraversed and new ramifications pursued.

IV. Organic Chemistry While inorganic chemistry was primarily developed through the study of minerals - a connexion still shown by the French appellation chimie minerale - organic chemistry owes its origin to the investigation of substances occurring in the vegetable and animal organisms. The quest of the alchemists for the philosopher's stone, and the almost general adherence of the iatrochemists to the study of the medicinal characters and preparation of metallic compounds, stultified in some measure the investigation of vegetable and animal products. It is true that by the distillation of many herbs, resins and similar substances, several organic compounds had been prepared, and in a few cases employed as medicines; but the prevailing classification of substances by physical and; superficial properties led to the correlation of organic and inorganic compounds, without any attention being paid to their chemical composition. The clarification and spirit of research so clearly emphasized by Robert Boyle in the middle of the 17th century is reflected in the classification of substances expounded by Nicolas Lemery, in 1675, in his Cours de chymie. Taking as a basis the nature of the source of compounds, he framed three classes: " mineral," comprising the metals, minerals, earths and stones; " vegetable," comprising plants, resins, gums, juices, &c.; and " animal," comprising animals, their different parts and excreta. Notwithstanding the inconsistency of his allocation of substances to the different groups (for instance, acetic acid was placed in the vegetable class, while the acetates and the products of their dry distillation, acetone, &c., were placed in the mineral class), this classification came into favour. The phlogistonists endeavoured to introduce chemical notions to support it: Becher, in his Physica subterranea (1669), stated that mineral, vegetable and animal matter contained the same elements, but that more simple combinations prevailed in the mineral kingdom; while Stahl, in his Specimen Becherianum (1702), held the " earthy " principle to predominate in the mineral class, and the " aqueous " and " combustible " in the vegetable and animal classes. It thus happened that in the earlier treatises on phlogistic chemistry organic substances were grouped with all combustibles.

The development of organic chemistry from this time until almost the end of the 18th century was almost entirely confined to such compounds as had practical applications, especially in pharmacy and dyeing. A new and energetic spirit was introduced by Scheele; among other discoveries this gifted experimenter isolated and characterized many organic acids, and proved the general occurrence of glycerin (Olsiiss) in all oils and fats. Bergman worked in the same direction; while Rouelle was attracted to the study of animal chemistry. Theoretical speculations were revived by Lavoisier, who, having explained the nature of combustion and determined methods for analysing compounds, concluded that vegetable substances ordinarily contained carbon, hydrogen and oxygen, while animal substances generally contained, in addition to these elements, nitrogen, and sometimes phosphorus and sulphur. Lavoisier, to whom chemistry was primarily the chemistry of oxygen compounds, having developed the radical theory initiated by Guyton de Morveau, formulated the hypothesis that vegetable and animal substances were oxides of radicals composed of carbon and hydrogen; moreover, since simple radicals (the elements) can form more than one oxide, he attributed the same character to his hydrocarbon radicals: he considered, for instance, sugar to be a neutral oxide and oxalic acid a higher oxide of a certain radical, for, when oxidized by nitric acid, sugar yields oxalic acid. At the same time, however, he adhered to the classification of Lemery; and it was only when identical compounds were obtained from both vegetable and animal sources that this subdivision was discarded, and the classes were assimilated in the division organic chemistry.

At this time there existed a belief, held at a later date by Berzelius, Gmelin and many others, that the formation of organic compounds was conditioned by a so-called vital force; and the difficulty of artificially realizing this action explained the supposed impossibility of synthesizing organic compounds.

This dogma was shaken by Wohler's synthesis of urea in 1828. But the belief died hard; the synthesis of urea remained isolated for many years; and many explanations were attempted by the vitalists (as, for instance, that urea was halfway between the inorganic and organic kingdoms, or that the carbon, from which it was obtained, retained the essentials of this hypothetical vital force), but only to succumb at a later date to the indubitable fact that the same laws of chemical combination prevail in both the animate and inanimate kingdoms, and that the artificial or laboratory synthesis of any substance, either inorganic or organic, is but a question of time, once its constitution is determined.'. The exact delimitation of inorganic and organic chemistry engrossed many minds for many years; and on this point there existed considerable divergence of opinion for several decades. In addition to the vitalistic doctrine of the origin of organic compounds, views based on purely chemical considerations were advanced. The atomic theory, and its correlatives - the laws of constant and multiple proportions - had been shown to possess absolute validity so far as well-characterized inorganic compounds were concerned; but it was open to question whether organic compounds obeyed the same laws. Berzelius, in 1813 and 1814, by improved methods of analysis, established that the Daltonian laws of combination held in both the inorganic and organic kingdoms; and he adopted the view of Lavoisier that organic compounds were oxides of compound radicals, and therefore necessarily contained at least three elements - carbon, hydrogen and oxygen. This view was accepted in 1817 by Leopold Gmelin, who, in his Handbuch der Chemie, regarded inorganic compounds as being of binary composition (the simplest being oxides, both acid and basic, which by combination form salts also of binary form), and organic compounds as ternary, i.e. composed of three elements; furthermore, he concluded that inorganic compounds could be synthesized, whereas organic compounds could not. A consequence of this empirical division was that marsh gas, ethylene and cyanogen were regarded as inorganic, and at a later date many other hydrocarbons of undoubtedly organic nature had to be included in the same division.

The binary conception of compounds held by Berzelius received apparent support from the observations of Gay Lussac, in 1815, on the vapour densities of alcohol and ether, which pointed to the conclusion that these substances consisted of one molecule of water and one and two of ethylene respectively; and from Pierre Jean Robiquet and Jean Jacques Colin, showing, in 1816, that ethyl chloride (hydrochloric ether) could be regarded as a compound of ethylene and hydrochloric acid. 2 Compound radicals came to be regarded as the immediate constituents of organic compounds; and, at first, a determination of their empirical composition was supposed to be sufficient to characterize them. To this problem there was added another in about the third decade of the 19th century - namely, to determine the manner in which the atoms composing the radical were combined; this supplementary 'requisite was due to the discovery of the isomerism of silver fulminate and silver cyanate by Justus von Liebig in 1823, and to M. Faraday's discovery of butylene, isomeric with ethylene, in 1825.

The classical investigation of Liebig and Friedrich Wihler on the radical of benzoic acid (" Uber das Radikal der Benzoesaure," Ann. Chem., 1832, 3, p. 2 49) is to be regarded as a most important contribution to the radical theory, for it was shown that a radical containing the elements carbon, hydrogen and oxygen, which they named benzoyl (the termination yl coming from the Gr. An, matter), formed the basis of benzaldehyde, benzoic acid, benzoyl chloride, benzoyl bromide and benzoyl sulphide, benzamide and benzoic ether. Berzelius immediately appreciated the importance of this discovery, notwithstanding 1 The reader is specially referred to the articles Alizarin; Indigo; Purin and Terpenes for illustrations of the manner in which chemists have artificially prepared important animal and vegetable products.

These observations were generalized by J. B. Dumas and Polydore Boullay (2806-2835) in their " etherin theory " (vide infra). that he was compelled to reject the theory that oxygen could not play any part in a compound radical - a view which he previously considered as axiomatic; and he suggested the names " proin " or " orthrin " (from the Gr. irpcoi. and 6pOp6s, at dawn). However, in 1833, Berzelius reverted to his earlier opinion that oxygenated radicals were incompatible with his electrochemical theory; he regarded benzoyl as an oxide of the radical C 14 H 1Q, which he named " picramyl " (from 7rucp6s, bitter, and &uvyalk, almond), the peroxide being anhydrous benzoic acid; and he dismissed the views of Gay Lussac and Dumas that ethylene was the radical of ether, alcohol and ethyl chloride, setting up in their place the idea that ether was a suboxide of ethyl, (C2H5)20, which was analogous to K 2 0, while alcohol was an oxide of a radical C 2 H 6; thus annihilating any relation between these two compounds. This view was modified by Liebig, who regarded ether as ethyl oxide, and alcohol as the hydrate of ethyl oxide; here, however, he was in error, for he attributed to alcohol a molecular weight double its true value. Notwithstanding these errors, the value of the " ethyl theory " was perceived; other radicals - formyl, methyl, amyl, acetyl, &c. - were characterized; Dumas, in 1837, admitted the failure of the etherin theory; and, in company with Liebig, he defined organic chemistry as the " chemistry of compound radicals." The knowledge of compound radicals received further increment at the hands of Robert W. Bunsen, the discoverer of the cacodyl compounds.

The radical theory, essentially dualistic in nature in view of its similarity to the electrochemical theory of Berzelius, was destined to succumb to a unitary theory. Instances had already been recorded of cases where a halogen element replaced hydrogen with the production of a closely allied substance: Gay Lussac had prepared cyanogen chloride from hydrocyanic acid; Faraday, hexachlorethane from ethylene dichloride, &c. Here the electronegative halogens exercised a function similar to electro-positive hydrogen. Dumas gave especial attention to such substitutions, named metalepsy (µeraXntks, exchange); and framed the following empirical laws to explain the reactions: - (1) a body containing hydrogen when substituted by a halogen loses one atom of hydrogen for every atom of halogen introduced; (2) the same holds if oxygen be present, except that when the oxygen is present as water the latter first loses its hydrogen without replacement, and then substitution according to (1) ensues. Dumas went no further that thus epitomizing his observations; and the next development was made in 1836 by Auguste Laurent, who, having amplified and discussed the applicability of Dumas' views, promulgated his Nucleus Theory, which assumed the existence of " original nuclei or radicals " (radicaux or noyaux fondamentaux) composed of carbon and hydrogen, and " derived nuclei " (radicaux or noyaux derives) formed from the original nuclei by the substitution of hydrogen or the addition of other elements, and having properties closely related to the primary nuclei.

Vigorous opposition was made by Liebig and Berzelius, the latter directing his attack against Dumas, whom he erroneously believed to be the author of what was, in his opinion, a pernicious theory. Dumas repudiated the accusation, affirming that he held exactly contrary views to Laurent; but only to admit their correctness in 1839, when, from his own researches and those of Laurent, Malaguti and Regnault, he formulated his type theory. According to this theory a " chemical type " embraced compounds containing the same number of equivalents combined in a like manner and exhibiting similar properties; thus acetic and trichloracetic acids, aldehyde and chloral, marsh gas and chloroform are pairs of compounds referable to the same type. He also postulated, with Regnault, the existence of " molecular or mechanical types " containing substances which, although having the same number of equivalents, are essentially different in characters. His unitary conceptions may be summarized: every chemical compound forms a complete whole, and cannot therefore consist of two parts; and its chemical character depends primarily upon the arrangement and number of the atoms, and, in a lesser degree, upon their chemical nature.

More emphatic opposition to the dualistic theory of Berzelius was hardly possible; this illustrious chemist perceived that the validity of his electrochemical theory was called in question, and therefore he waged vigorous war upon Dumas and his followers. But he fought in a futile cause; to explain the facts put forward by Dumas he had to invent intricate and involved hypotheses, which, it must be said, did not meet with general acceptance; Liebig seceded from him, and invited Wohler to endeavour to correct him. Still, till the last Berzelius remained faithful to his original theory; experiment, which he had hitherto held to be the only sure method of research, he discarded, and in its place he substituted pure speculation, which greatly injured the radical theory. At the same time, however, the conception of radicals could not be entirely displaced, for the researches of Liebig and Welder, and those made subsequently by Bunsen, demonstrated beyond all doubt the advantages which would accrue from their correct recognition.

A step forward - the fusion of Dumas' type theory and the radical theory - was made by Laurent and Charles Gerhardt. As early as 1842, Gerhardt in his Précis de chimie organique exhibited a marked leaning towards Dumas' theory, and it is without doubt that both Dumas and Laurent exercised considerable influence on his views. Unwilling to discard the strictly unitary views of these chemists, or to adopt the copulae theory of Berzelius, he revived the notion of radicals in a new form. According to Gerhardt, the process of substitution consisted of the union of two residues to fo- m a unitary whole; these residues, previously termed " compound radicals," are atomic complexes which remain over from the interaction of two compounds. Thus, he interpreted the interaction of benzene and nitric acid as C6H61-HN03 = C 6 H 5 NO 2 +H 2 0, the "residues" of benzene being C 6 H 5 and H, and of nitric acid HO and N02. Similarly he represented the reactions investigated by Liebig and Wehler on benzoyl compounds as double decompositions.

This rejuvenation of the notion of radicals rapidly gained favour; and the complete fusion of the radical theory with the theory of types was not long delayed. In 1849 C. A. Wurtz discovered the amines or substituted ammonias, previously predicted by Liebig; A. W. von Hofmann continued the investigation, and established their recognition as ammonia in which one or more hydrogen atoms had been replaced by hydrocarbon radicals, thus formulating the " ammonia type." In 1850 A. W. Williamson showed how alcohol and ether were to be regarded as derived from water by substituting one or both hydrogen atoms by the ethyl group; he derived acids and the acid anhydrides from the same type; and from a comparison of many inorganic and the simple organic compounds he concluded that this notion of a " water-type " clarified, in no small measure, the conception of the structure of compounds.

These conclusions were co-ordinated in Gerhardt's " new theory of types." Taking as types hydrogen, hydrochloric acid, water and ammonia, he postulated that all organic compounds were referable to these four forms: the hydrogen type included hydrocarbons, aldehydes and ketones; the hydrochloric acid type, the chlorides, bromides and iodides; the water type, the alcohols, ethers, monobasic acids, acid anhydrides, and the analogous sulphur compounds; and the ammonia type, the amines, acid-amides, and the analogous phosphorus and arsenic compounds. The recognition of the polybasicity of acids, which followed from the researches of Thomas Graham and Liebig, had caused Williamson to suggest that dibasic acids could be referred to a double water type, the acid radical replacing an atom of hydrogen in each water molecule; while his discovery of tribasic formic ether, CH(OC 2 H 5) 3, in 1854 suggested a triple water type. These views were extended by William Odling, and adopted by Gerhardt, but with modifications of Williamson's aspects. A further generalization was effected by August Kekule, who rejected the hydrochloric acid type as unnecessary, and introduced the methane type and condensed mixed types. Pointing out that condensed types can only be fused with a radical replacing more than one atom of hydrogen, he laid the foundation of the doctrine of valency, a doctrine of incalcul able service to the knowledge of the structure of chemical compounds.

At about the same time Hermann Kolbe attempted a rehabilitation, with certain modifications, of the dualistic conception of Berzelius. He rejected the Berzelian tenet as to the unalterability of radicals, and admitted that they exercised a considerable influence upon the compounds with which they were copulated. By his own investigations and those of Sir Edward Frankland it was proved that the radical methyl existed in acetic acid; and by the electrolysis of sodium acetate, Kolbe concluded that he had isolated this radical; in this, however, he was wrong, for he really obtained ethane, C 2 H 6, and not methyl, CH 3. From similar investigations of valerianic acid he was led to conclude that fatty acids were oxygen compounds of the radicals hydrogen, methyl, ethyl, &c., combined with the double carbon equivalent C2. Thus the radical of acetic acid, acetyl,' was C 2 H 3 C 2. (It will be noticed that Kolbe used the atomic weights H= 1, C=6, 0= 8, S = 16, &c.; his formulae, however, were molecular formulae, i.e. the molecular weights were the same as in use to-day.) This connecting link, C2, was regarded as essential, while the methyl, ethyl, &c. was but a sort of appendage; but Kolbe could not clearly conceive the manner of copulation.

The brilliant researches of Frankland on the organo-metallic compounds, and his consequent doctrine of saturation capacity or valency of elements and radicals, relieved Kolbe's views of all obscurity. The doctrine of copulae was discarded, and in 1859 emphasis was given to the view that all organic compounds were derivatives of inorganic by simple substitution processes. He was thus enabled to predict compounds then unknown, e.g. the secondary and tertiary alcohols; and with inestimable perspicacity he proved intimate relations between compounds previously held to be quite distinct. Lactic acid and alanine were shown to be oxyand amino-propionic acids respectively; glycollic acid and glycocoll, oxyand amino-acetic acids; salicylic and benzamic acids, oxyand amino-benzoic acids.

Another consequence of the doctrine of valency was that it permitted the graphic representation of the molecule. The " structure theory " (or the mode of linking of the atoms) of carbon compounds, founded by Butlerow, Kekule and Couper and, at a later date, marvellously enhanced by the doctrine of stereo-isomerism, due to J. H. van't Hoff and Le Bel, occupies such a position in organic chemistry that its value can never be transcended. By its aid the molecule is represented as a collection of atoms connected together by valencies in such a manner that the part played by each atom is represented;. isomerism, or the existence of two or more chemically different substances having identical molecular weights, is adequately shown; and, most important of all, once the structure is determined, the synthesis of the compound is but a matter of time.

In this summary the leading factors which have contributed to a correct appreciation of organic compounds have so far been considered historically, but instead of continuing this method it has been thought advisable to present an epitome of present-day conclusions, not chronologically, but as exhibiting the principles and subject-matter of our science.

Classification of Organic Compounds. An apt definition of organic chemistry is that it is "the study of the hydrocarbons and their derivatives." This description, although not absolutely comprehensive, serves as a convenient starting-point for a preliminary classification, since a great number of substances, including the most important, are directly referable to hydrocarbons, being formed by replacing one or more hydrogen atoms by other atoms or groups. Two distinct types of hydrocarbons exist: (1) those consisting of an open chain of carbon atoms - named the " aliphatic series " (i.XEicAap, oil or fat), and (2) those consisting of a closed chain - the " carbocyclic series." The second series can be further divided 1 This must not be confused with the modern acetyl, CH3CO, which at that time was known as acetoxyl. into two groups: (r) those exhibiting properties closely analogous to the aliphatic series - the polymethylenes, and (2) a series exhibiting properties differing in many respects from the aliphatic and polymethylene compounds, and characterized by a peculiar stability which is to be associated with the disposition of certain carbon valencies not saturated by hydrogen - the " aromatic series." There also exists an extensive class of compounds termed the " heterocyclic series " - these compounds are derived from ring systems containing atoms other than carbon; this class is more generally allied to the aromatic series than to the aliphatic.

We now proceed to discuss the types of aliphatic compounds; then, the characteristic groupings having been established, an epitome of their derivatives will be given. Carbocyclic rings will next be treated, benzene and its allies in some detail; and finally the heterocyclic nuclei.

Accepting the doctrine of the tetravalency of carbon (its divalency in such compounds as carbon monoxide, various isocyanides, fulminic acid, &c., and its possible trivalency in M. Gomberg's triphenyl-methyl play no part in what follows), it is readily seen that the simplest hydrocarbon has the formula CH 4, named methane, in which the hydrogen atoms are of equal value, and which may be pictured as placed at the vertices of a tetrahedron, the carbon atom occupying the centre. This tetrahedral configuration is based on the existence of only one methylene dichloride, two being necessary if the carbon valencies were directed from the centre of a plane square to its corners, and on the existence of two optical isomers of the formula C. A. B. D. E., C being a carbon atom and A.B.D. E. being different monovalent atoms or radicals (see Stereo-Isomerism). The equivalence of the four hydrogen atoms of methane rested on indirect evidence, e.g. the existence of only one acetic acid, methyl chloride, and other monosubstitution derivatives - until the experimental proof by L. Henry (Zeit. f. Phys. Chem., 1888, 2, p. 553), who prepared the four nitromethanes, CH 3 N 2 0, each atom in methane being successively replaced by the nitro-group.

Henry started with methyl iodide, the formula of which we write in the form CI a H b H c H d. This readily gave with silver nitrite a nitromethane in which we may suppose the nitro-group to replace the a hydrogen atom, i.e. C(NO 2)aH b H c H d. The same methyl iodide gave with potassium cyanide, acetonitril, which was hydrolysed to acetic acid; this must be C(Coch) a H b H c H d. Chlorination of this substance gave a monochloracetic acid; we will assume the chlorine atom to replace the b hydrogen atom. This acid with silver nitrite gave nitroacetic acid, which readily gave the second nitromethane, CH a (NO 2) b H c H d, identical with the first nitromethane. From the nitroacetic acid obtained above, malonic acid was prepared, and from this a monochlormalonic acid was obtained; we assume the chlorine atom to replace the c hydrogen atom. This acid gives with silver nitrite the corresponding nitromalonic acid, which readily yielded the third nitromethane, CHaHb(N02),Hd, also identical with the first. The fourth nitromethane was obtained from the nitromalonic acid previously mentioned by a repetition of the method by which the third was prepared; this was identical with the other three.

Let us now consider hydrocarbons containing 2 atoms of carbon. Three such compounds are possible according to the number of valencies acting directly between the carbon atoms. Thus, if they are connected by one valency, and the remaining valencies saturated by hydrogen, we obtain the compound H 3 CCH 3, ethane. This compound may be considered as derived from methane, CH 4, by replacing a hydrogen atom by the monovalent group CH 3, known as methyl; hence ethane may be named " methylmethane." If the carbon atoms are connected by two valencies, we obtain a compound H2C:CH2, ethylene; if by three valencies, HC: CH, acetylene. These last two compounds are termed unsaturated, whereas ethane is saturated. It is obvious that we have derived three combinations of carbon with hydrogen, characterized by containing a single, double, and triple linkage; and from each of these, by the substitution of a methyl group for a hydrogen atom, compounds of the same nature result. Thus ethane gives H3CCH2CH3, propane; ethylene gives H 2 C:CHCH 3, propylene; and acetylene gives HC: CCH 3, allylene. By continuing the introduction of methyl groups we obtain three series of homologous hydro carbons given by the general formulae CnH2,i 4.2, CnH2n, and CnH2n 2, each member differing from the preceding one of the same series by CH 2. It will be noticed that compounds containing two double linkages will have the same general formula as the acetylene series; such compounds are known as the " diolefines." Hydrocarbons containing any number of double or triple linkages, as well as both double and triple linkages, are possible, and a considerable number of such compounds have been prepared.

A more complete idea of the notion of a compound radical follows from a consideration of the compound propane. We derived this substance from ethane by introducing a meth y l group; hence it may be termed " methylethane." Equally well we may derive it from methane by replacing a hydrogen atom by the monovalent group CH 2 CH 31 named ethyl; hence propane may be considered as " ethylmethane." Further, since methane may be regarded as formed b y the conjunction of a methyl group with a hydrogen atom, it may be named "methyl hydride "; similarly ethane is " ethyl hydride," propane, " propyl hydride," and so on. The importance of such groups as methyl, ethyl, &c. in attempting a nomenclature of organic compounds cannot be overestimated; these compound radicals, fre q uently termed alkyl radicals, serve a similar purpose to the organic chemist as the elements to the inorganic chemist.

In methane and ethane the hydrogen atoms are of equal value, and no matter which one may be substituted by another element or group the same compound will result. In propane, on the other hand, the hydrogen atoms attached to the terminal carbon atoms differ from those joined to the medial atom; we may therefore expect to obtain different compounds according to the position of the hydrogen atom substituted. By introducing a methyl group we may obtain CH 3. CH 2. CH 2. CH3, known as " normal " or n-butane, substitution occurring at a terminal atom, or CH 3. CH(CH 3) CH 3, isobutane, substitution occurring at the medial atom. From n-butane we may derive, by a similar substitution of methyl groups, the two hydrocarbons: (I) CH 3 CH 2 CH 2 CH 2 CH 3, and (2) CH 3. CH(CH3)CH2CH31 from isobutane we may also derive two compounds, one identical with (2), and a new one (3) CH 3 (CH 3)C(CH 3)CH 3. These three hydrocarbons are isomeric, i.e. they possess the same formula, but differ in constitution. We notice that they may be differentiated as follows: (r) is built up solely of methyl and CH 2. (methylene) groups and the molecule consists of a single chain; such hydrocarbons are referred to as being normal; (2) has a branch and contains the group; CH (methine) in which the free valencies are attached to carbon atoms; such hydrocarbons are termed secondary or iso-; (3) is characterized by a carbon atom linked directly to four other carbon atoms; such hydrocarbons are known as tertiary. Deferring the detailed discussion of cyclic or ringed hydrocarbons, a correlation of the various types or classes of compounds which may be derived from hydrocarbon nuclei will now be given. It will be seen that each type depends upon a specific radical or atom, and the copulation of this character with any hydrocarbon radical (open or cyclic) gives origin to a compound of the same class.

It is convenient first to consider the effect of introducing one, two, or three hydroxyl (OH) groups into the - CH 3, > CH 2, and >CH groups, which we have seen to characterize the different types of hydrocarbons. It may be noticed here that cyclic nuclei can only contain the groups > CH 2. and >- CH, the first characterizing the polymethylene and reduced heterocyclic compounds, the second true aromatic compounds.

Substituting one hydroxyl group into each of these residues, we obtain radicals of the type - CH 2. OH, >CHOH, and ?COH; these compounds are known as alcohols (q.v.), and are termed primary, secondary, and tertiary respectively. Polymethylenes can give only secondary and tertiary alcohols, benzene only tertiary; these latter compounds are known as phenols. A second hydroxyl group may be introduced into the residues - CH 2. OH and >CHOH, with the production of radicals of the form - CH(OH) 2 and >C(OH)2. Compounds containing these groupings are, however, rarely observed (see Chloral), and it is generally found that when compounds of these types are expected, the elements of water are split off, and the typical groupings are reduced to - CH :0 and >C: 0. Compounds containing the group - CH: O are known as aldehydes (q.v.), while the group >C: O (sometimes termed the carbonyl or keto group) characterizes the ketones (q.v.). A third hydroxyl group may be introduced into the - CH: 0 residue with the formation of the radical - C(OH) :0; this is known as the carboxyl group, and characterizes the organic acids.

Sulphur analogues of these oxygen compounds are known. Thus the thio-alcohols or mercaptans (q.v.) contain the group - CH2SH; and the elimination of the elements of sulphuretted hydrogen between two molecules of a thio-alcohol results in the formation of a thio-ether or sulphide, R 2 S. Oxidation of thio-ethers results in the formation of sulphoxides, R2: S: 0, and sulphones, R2: S02; oxidation of mercaptans yields sulphonic acids, RS0 3 H, and of sodium mercaptides sulphinic acids, RS0(OH). We may also notice that thio-ethers combine with alkyl iodides to form sulphide or sulphonium compounds, R3; SI. Thio-aldehydes, thio-ketones and thio-acids also exist.

We proceed to consider various simple derivatives of the alcohols, which we may here regard as hydroxy hydrocarbons, ROH, where R is an alkyl radical, either aliphatic or cyclic in nature.

Of these, undoubtedly the simplest are the ethers (q.v.), formed by the elimination of the elements of water between two molecules of the same alcohol, " simple ethers," or of different alcohols, " mixed ethers." These compounds may be regarded as oxides in just the same way as the alcohols are regarded as hydroxides. In fact, the analogy between the alkyl groups and metallic elements forms a convenient basis from which to consider many derivatives. Thus from ethyl alcohol there can be prepared compounds, termed esters, or ethereal salts, exactly comparable in structure with corresponding salts of, say, potassium; by the action of the phosphorus haloids, the hydroxyl group is replaced by a halogen atom with the formation of derivatives of the type RCl(Br,I); nitric acid forms nitrates, RONO 2; nitrous acid, nitrites, RONO; sulphuric acid gives normal sulphates R 2 SO 4, or acid sulphates, RSO 4 H. Organic acids also condense with alcohols to form similar compounds: the fats, waxes, and essential oils are naturally occurring substances of this class.

An important class of compounds, termed amines (q.v.), results from the condensation of alcohols with ammonia, water being eliminated between the alcoholic hydroxyl group and a hydrogen atom of the ammonia. Three types of amines are possible and have been prepared: primary, R. NH 2; secondary, R2: NH; and tertiary, R3: N; the oxamines, R 3 N :0, are closely related to the tertiary ammonias, which also unite with a molecule of alkyl iodide to form salts of quaternary ammonium bases, e.g. R 4 NI. It is worthy of note that phosphorus and arsenic bases analogous to the amines are known (see Phosphorus and Arsenic). From the primary amines are derived the diazo compounds and azo compounds; closely related are the hydrazines (q.v.). Secondary amines yield nitrosamines, R 2 NNO, with nitrous acid. By the action of hydroxylamine or phenylhydrazine on aldehydes or ketones, condensation occurs between the carbonyl oxygen of the aldehyde or ketone and the amino group of the hydroxylamine or hydrazine. Thus with hydroxylamine aldehydes yield aldoximes, RCH: NOH, and ketones, ketoximes, R 2 C: NOH (see Oximes), while phenyl hydrazine gives phenylhydrazones, R 2 C:NNHC 6 H 5 (see Hydrazones). Oxyaldehydes and oxyketones (viz. compounds containing an oxy in addition to an aldehydic or ketonic group) undergo both condensation and oxidation when treated with phenylhydrazine, forming compounds known as osozones; these are of great importance in characterizing the sugars (q.v.).

The carboxyl group constitutes another convenient startingpoint for the orientation of many types of organic compounds. This group may be considered as resulting from the fusion of a carbonyl (:CO) and a hydroxyl (HO-) group; and we may expect to meet with compounds bearing structural resemblances to the derivatives of alcohols and aldehydes (or ketones).

Considering derivatives primarily concerned with transformations of the hydroxyl group, we may regard our typical acid as a fusion of a radical RCO - (named acetyl, propionyl, butyl, &c., generally according to the name of the hydrocarbon containing the same number of carbon atoms) and a hydroxyl group. By replacing the hydroxyl group by a halogen, acid-haloids result; by the elimination of the elements of water between two molecules, acid-anhydrides, which may be oxidized to acid-peroxides; by replacing the hydroxyl group by the group. SH, thio-acids; by replacing it by the amino group, acid-amides (q.v.); by replacing it by the group - NHNH2, acid-hydrazides. The structural relations of these compounds are here shown: RC0. OH; R.COCl; (R.CO)20; R.COSH; acid; acid-chloride; acid-anhydride; thio-acid; RCONH 2 i RCONHNH2.

acid-amide; acid-hydrazide.

It is necessary clearly to distinguish such compounds as the amino- (or amido-) acids and acid-amides; in the first case the amino group is substituted in the hydrocarbon residue, in the second it is substituted in the carboxyl group.

By transformations of the carbonyl group, and at the same time of the hydroxyl group, many interesting types of nitrogen compounds may be correlated.

Thus from the acid-amides, which we have seen to be closely related to the acids themselves, we obtain, by replacing the carbonyl oxygen by chlorine, the acidamido-chlorides, RCC1 2 NH 2, from which are derived the imido-chlorides, RCC1:NH, by loss of one molecule of hydrochloric acid. By replacing the chlorine in the imido-chloride by an oxyalkyl group we obtain the imido-ethers, RC(OR') :NH; and by an amino group, the amidines, RC(NH 2): NH. The carbonyl oxygen may also be replaced by the oxime group,: NOH; thus the acids yield the hydroxamic acids, RC(OH): NOH, and the acid-amides the amidoximes, RC(NH 2): NOH. Closely related to the amidoximes are the nitrolic acids, RC(NO 2): NOH.

Cyclic Hydrocarbons and Nuclei. Having passed in rapid review the various types of compounds derived by substituting for hydrogen various atoms or groups of atoms in hydrocarbons (the separate articles on specific compounds should be consulted for more detailed accounts), we now proceed to consider the closed chain compounds. Here we meet with a great diversity of types: oxygen, nitrogen, sulphur and other elements may, in addition to carbon, combine together in a great number of arrangements to form cyclic nuclei, which exhibit characters closely resembling open-chain compounds in so far as they yield substitution derivatives, and behave as compound radicals. In classifying closed chain compounds, the first step consists in dividing them into: (1) carbocyclic, in which the ring is composed solely of carbon atoms - these are also known as homocyclic or isocyclic on account of the identity of the members of the ring - and (2) heterocyclic, in which different elements go to make up the ring. Two primary divisions of carbocyclic compounds may be conveniently made: (I) those in which the carbon atoms are completely saturated - these are known by the generic term polymethylenes, their general formula being (CH 2), t: it will be noticed that they are isomeric with ethylene and its homologues; they differ, however, from this series in not containing a double linkage, but have a ringed structure; and (2) those containing fewer hydrogen atoms than suffice to saturate the carbon valencies - these are known as the aromatic compounds proper, or as benzene compounds, from the predominant part which benzene plays in their constitution.

It was long supposed that the simplest ring obtainable contained six atoms of carbon, and the discovery of trimethylene in 1882 by August Freund by the action of sodium on trimethylene bromide, Br(CH 2) 3 Br, came somewhat as a surprise, especially in view of its behaviour with bromine and hydrogen bromide. In comparison with the isomeric propylene, CH 3 HC:CH 2, it is remarkably inert, being only very slowly attacked by bromine, which readily combines with propylene. But on the other hand, it is readily converted by hydrobromic acid into normal propyl bromide, CH 3 CH 2 CH 2 Br. The separation of carbon atoms united by single affinities in this manner at the time the observation was made was altogether without precedent. A similar behaviour has since been noticed in other trimethylene derivatives, but the fact that bromine, which usually acts so much more readily than hydrobromic acid on unsaturated compounds,, should be so inert when hydrobromic acid acts readily is one still. needing a satisfactory explanation. A great impetus was given to the study of polymethylene derivatives by the important and unexpected observation made by W. H. Perkin, junr., in 1883, that ethylene and trimethylene bromides are capable of acting in such a way on sodium acetoacetic ester as to form triand tetramethylene rings. Perkin has himself contributed largely to our knowledge of such compounds; pentaand hexa-methylene derivatives have also received considerable attention (see P Olymethylenes) .

A. von Baeyer has sought to explain the variations in stability manifest in the various polymethylene rings by a purely mechanical hypothesis, the " strain " or Spannungs theory (Ber., 1885, p. 2277). Assuming the four valencies of the carbon atom to be directed from the centre of a regular tetrahedron towards its four corners, the angle at which they meet. is 109° 28'. Baeyer supposes that in the formation of carbon, rings " the valencies become deflected from their positions, and that the tension thus introduced may be deduced from a comparison of this angle with the angles at which the strained valencies would meet. He regards the amount of deflection as a measure of the stability of the " ring." The readiness with which ethylene is acted on in comparison with other types of hydrocarbon, for example, is in harmony, he considers, with the circumstance that the greatest distortion must be involved in its formation, as if deflected into parallelism each valency will be drawn out of its position through 2.109° 28'. The values in other cases are calculable from the formula RI 09° 28' - a), where a is the internal angle of the regular polygon contained by sides equal in number to the number of the carbon atoms composing the ring. These values are: Trimethylene. Tetramethylene.

2(109° 28' -60°) =2 4° 44'. 2(109° 28 '-90°) = 9° 44'.

Pentamethylene. Hexamethylene.

(109 ° 28' - 108°)=0° 44'. 1(109° 28'-120°)= -5° 16'.

The general behaviour of the several types of hydrocarbons is certainly in accordance with this conception, and it is a remarkable fact that when benzene is reduced with hydriodic acid, it is converted into a mixture of hexamethylene and methylpentamethylene (cf. W. Markownikov, Ann., 1898, 302, p. I); and many other cases of the conversion of six-carbon rings into fivecarbon rings have been recorded (see below, Decompositions of the Benzene Ring) . Similar considerations will apply to rings containing other elements besides carbon. As an illustration it may be pointed out that in the case of the two known types of lactones - the y-lactones, which contain four carbon atoms and one oxygen atom in the ring, are more readily formed and more stable (less readily hydrolysed) than the S-lactones, which contain one oxygen and five carbon atoms in the ring. That the number of atoms which can be associated in a ring by single affinities is limited there can be no doubt, but there is not yet sufficient evidence to show where the limit must be placed. B aeyer has suggested that his hypothesis may also be applied to explain the instability of acetylene and its derivatives, and the still greater instability of the polyacetylene compounds.

Benzene. The ringed structure of benzene, C 6 H 61 was first suggested in 1865 by August Kekule, who represented the molecule by six CH groups placed at the six angles of a regular hexagon, the sides of which denoted the valencies saturated by adjacent carbon atoms, the fourth valencies of each carbon atom being represented as saturated along alternate sides. This formula, notwithstanding many attempts at both disproving and modifying it, has well stood the test of time; the subject has been the basis of constant discussion, many variations have been proposed, but the original conception of Kekule remains quite as convenient as any of the newer forms, especially when considering the syntheses and decompositions of the benzene complex. It will be seen, however, that the absolute disposition of the fourth valency may be ignored in a great many cases, and consequently the complex may be adequately represented as a hexagon. This symbol is in general use; it is assumed that at each corner there is a CH group which, however, is not always written in; if a hydrogen atom be substituted by another group, then this group is attached to the corner previously occupied by the displaced hydrogen. The following diagrams illustrate these statements: - C ` H COH HC /CH HC CH HC,/CH 'N/ HC CH CH CH From the benzene nucleus we can derive other aromatic nuclei, graphically represented by fusing two or more hexagons along common sides. By fusing two nuclei we obtain the formula of naphthalene, C 1 oH 8; by fusing three, the hydrocarbons anthracene and phenanthrene, C14H10; by fusing four, chrysene, C18H12, and possibly pyrene, C16H1n; by fusing five, picene, C22 H 14. But it must be here understood that each member of these condensed nuclei need not necessarily be identical in structure; thus the central nuclei in anthracene and phenanthrene differ very considerably from the terminal nuclei (see below, Condensed Nuclei). Other hydrocarbon nuclei generally classed as aromatic in character result from the union of two or more benzene nuclei joined by one or two valencies with polymethylene or oxidized polymethylene rings; instances of such nuclei are indene, hydrindene, fluorene, and fluoranthene. From these nuclei an immense number of derivatives may be obtained, for the hydrogen atoms may be substituted by any of the radicals discussed in the preceding section on the classification of organic compounds.

We now proceed to consider the properties, syntheses, decompositions and constitution of the benzene complex. It has already been stated that benzene derivatives may be regarded as formed by the replacement of hydrogen atoms by other elements or radicals in exactly the same manner as in the aliphatic series. Important differences, however, are immediately met with when we consider the methods by which derivatives are obtained. For example: nitric acid and sulphuric acid readily react with benzene and its homologues with the production of nitro derivatives and sulphonic acids, while in the aliphatic series these acids exert no substituting action (in the case of the olefines, the latter acid forms an addition product); another distinction is that the benzene complex is more stable towards oxidizing agents. This and other facts connected with the stability of benzenoid compounds are clearly shown when we consider mixed aliphatic-aromatic hydrocarbons, i.e. compounds derived by substituting aliphatic radicals in the benzene nucleus; such a compound is methylbenzene or toluene, C 6 H 5 CH 3. This compound is readily oxidized to benzoic acid, C 6 H 5 000H, the aromatic residue being unattacked; nitric and sulphuric acids produce nitro-toluenes, C6H4CH3N02j and toluene sulphonic acids, C 6 H 4 CH 3 SO 3 H; chlorination may result in the formation of derivatives substituted either in the aromatic nucleus or in the side chain; the former substitution occurs most readily, chlor-toluenes, C 6 H 4 CH 3 Cl, being formed, while the latter, which needs an elevation in temperature or other auxiliary, yields benzyl chloride, C 6 H 5 CH 2 C1, and benzal chloride, C 6 11 5 CHC1 2. In general, the aliphatic residues in such mixed compounds retain the characters of their class, while the aromatic residues retain the properties of benzene.

Further differences become apparent when various typical compounds are compared. The introduction of hydroxyl groups into the benzene nucleus gives rise to compounds generically named phenols, which, although resembling the aliphatic alcohols in their origin, differ from these substances in their increased chemical activity and acid nature. The phenols more closely resemble the tertiary alcohols, since the hydroxyl group is linked to. a carbon atom which is united to other carbon atoms by its remaining three valencies; hence on oxidation they cannot yield the corresponding aldehydes, ketones or acids (see below, Decompositions of the Benzene Ring). The amines also exhibit striking differences: in the aliphatic series these compounds may be directly formed from the alkyl haloids and ammonia, but in the benzene series this reaction is quite impossible unless the haloid atom be weakened by the presence of other substituents, e.g. nitro groups. Moreover, while methylamine, dimethylamine, and trimethylamine increase in basicity corresponding to the introduction of successive methyl groups, phenylamine or aniline, diphenylamine, and triphenylamine are in decreasing order of basicity, the salts of diphenylamine being decomposed by water. Mixed aromatic-aliphatic amines, both secondary and tertiary, are also more strongly basic than the pure aromatic amines, and less basic than the true aliphatic compounds; e.g. aniline, C 6 H 5 NH 2, monomethyl aniline, C 6 H 5 NHCH 3, and dimethyl aniline, C 6 H 5 N(CH 3) 2, are in increasing order of basicity. These observations may be summarized by saying that the benzene nucleus is more negative in character than the aliphatic residues.

Isomerism of Benzene Derivatives

Although Kekule founded his famous benzene formula in 1865 on the assumptions that the six hydrogen atoms in benzene are equivalent and that the molecule is symmetrical, i.e. that two pairs of hydrogen atoms are symmetrically situated with reference to any specified hydrogen atom, the absolute demonstration of the validity of OH these assumptions was first given by A. Ladenburg in 1874 (see Ber., 1874, 7, p. 1684; 1875, 8, p. 1666; Theorie der aromatischen Verbindungen, 1876). These results may be graphically represented as follows: numbering the hydrogen atoms in cyclical order from i to 6, then the first thesis demands that whichever atom is substituted the same compound results, while the second thesis points out that the pairs 2 and 6, and 3 and 5 are symmetrical with respect to 1, or in other words, the di-substitution derivatives 1.2 and 1.6, and also 1.3 and 1.5 are identical. Therefore three di-derivatives are possible, viz. 1.2 or 1.6, named ortho- (o), 1.3 or 1.5, named meta- (m), and 1.4, named para- compounds (p). In the same way it may be shown that three tri-substitution, three tetra-substitution, one penta-substitution, and one hexa-substitution derivative are possible. Of the tri-substitution derivatives, 1.2.3.-compounds are known as " adjacent " or " vicinal " (v), the 1.2.4 as " asymmetrical " (as), the 1.3.5 as " symmetrical " (s), of the tetrasubstitution derivatives, 1.2.3.4-compounds are known as " adjacent," 1.2.3.5 as " asymmetrical," and 1.2.4.5 as " symmetrical." Di-derivatives x x x p v as $ v as s Here we have assumed the substituent groups to be alike; when they are unlike, a greater number of isomers is possible. Thus in the tri-substitution derivatives six isomers, and no more, are possible when two of the substituents are alike; for instance, six diaminobenzoic acids, C 6 H 3 (NH 2) 2 000H, are known; when all are unlike ten isomers are possible; thus, ten oxytoluic acids, C 6 H 3 -CH 3. OH-000H, are known. In the case of tetra-substituted compounds, thirty isomers are possible when all the groups are different.

The preceding considerations render it comparatively easy to follow the reasoning on which the experimental verification of the EgaJva- above statements is based. The proof is divided into two parts: (1) that four hydrogen atoms are equal, and (2) that two pairs of hydrogen atoms are symmetrical with reference to a specified hydrogen atom. In the first thesis, phenol or oxybenzene,C 6 H 5. OH, in which we will assume the hydroxyl group to occupy position I, is converted into brombenzene, which is then converted into benzoic acid, C 6 1-1 5 -COOH. From this substance, an oxybenzoic acid (meta-), C 6 H 4. OHCOOH, may be prepared; and the two other known oxybenzoic acids (ortho- and para-) may be converted into benzoic acid. These three acids yield on heating phenol, identical with the substance started with, and since in the three oxybenzoic acids the hydroxyl groups must occupy positions other than I, it follows that four hydrogen atoms are equal in value.

R. Hubner and A. Petermann (Ann., 1869, 149, p. 129) provided the proof of the equivalence of the atoms 2 and 6 with respect to 1. From meta-brombenzoicacid two nitrobrombenzoic ac i ds are obtained on direct nitration; elimination of the bromine atom and the reduction of the nitro to an amino group in these two acids results in the formation of the same ortho-aminobenzoic acid. Hence the positions occupied by the nitro groups in the two different nitrobrombenzoic acids must be symmetrical with respect to the carboxyl group. In 1879, Hubner (Ann., 1 95, p. 4) proved the equivalence of the second pair, viz. 3 and 5, by starting out with ortho-aminobenzoic acid, previously obtained by two different methods. This substance readily yields ortho-oxybenzoic acid or salicylic acid, which on nitration yields two mononitro-oxybenzoic acids. By eliminating the hydroxy groups in these acids the same nitrobenzoic acid is obtained, which yields on reduction an aminobenzoic acid different from the starting-out acid. Therefore there must be another pair of hydrogen atoms, other than 2 and 6, which are symmetrical with respect to 1. The symmetry of the second pair was also established in 1878 by E. Wroblewsky (Ann., 192, p. 196).

Orientation of Substituent Groups.-The determination of the relative positions of the substituents in a benzene derivative constitutes an important factor in the general investigation of such compounds. Confining our attention, for the present, to di-substitution products we see that there are three distinct series of compounds to be considered. Generally if any group be replaced by another group, then the second group enters the nucleus in the position occupied by the displaced group; this means that if we can definitely orientate three di-derivatives of benzene, then any other compound, which can be obtained from or converted into one of our typical derivatives, may be definitely orientated. Intermolecular transformations-migrations of substituent groups from one carbon atom to anotherare of fairly common occurrence among oxy compounds at elevated temperatures. Thus potassium ortho-oxybenzoate is converted into the salt of para-oxybenzoic acid at 220 0; the three bromphenols, and also the brombenzenesulphonic acids, yield m-dioxybenzene or resorcin when fused with potash. It is necessary, therefore, to avoid reactions involving such intermolecular migrations when determining the orientation of aromatic compounds.

Such a series of typical compounds are the benzene dicarboxylic acids (phthalic acids), C 6 H 4 (000H) 2. C. Graebe (Ann., 1869, 149, p. 22) orientated the ortho-compound or phthalic acid from its formation from naphthalene on oxidation; the meta-compound or isophthalic acid is orientated by its production from mesitylene, shown by A. Ladenburg (Ann., 1875, 179, p. 163) to be symmetrical trimethyl benzene; terephthalic acid, the remaining isomer, must therefore be the para-compound.

P. Griess (Ber., 1872, 5, p. 192; 18 74, 7, p. 1223) orientated the three diaminobenzenes or phenylene diamines by considering their preparation by the elimination of the carboxyl group in the six diaminobenzoic acids. The diaminobenzene resulting from two of these acids is the ortho-compound; from three, the meta-; and from one the para-; this is explained by the following scheme: CH NH, NH, NH., IAO NH, N / NH HooCCNH, Cooh Nh, W. Korner (Ganz. Chem. Ital., 4, p. 305) in 1874 orientated the three dibrombenzenes in a somewhat similar manner. Starting with the three isomeric compounds, he found that one gave two tribrombenzenes, another gave three, while the third gave only one. A scheme such as the preceding one shows that the first dibrombenzene must be the ortho-compound, the second the meta-, and the third the para-derivative. Further research iri this direction was made by D. E. Noetling (Ber., 1885, 18, p. 2657), who investigated the nitro-, amino-, and oxy-xylenes in their relations to the three xylenes or dimethyl benzenes.

The orientation of higher substitution derivatives is determined by considering the diand tri-substitution compounds into which they can be transformed.

Substitution of the Benzene Ring.-As a general rule, homologues and mono-derivatives of benzene react more readily with substituting agents than the parent hydrocarbon; for example, phenol is converted into tribromphenol by the action of bromine water, and into the nitrophenols by dilute nitric acid; similar activity characterizes aniline. Not only does the substituent group modify the readiness with which the derivative is attacked, but also the nature of the product. Starting with a monoderivative, we have seen that a substituent group may enter in either of three positions to form an ortho-, meta-, or paracompound. Experience has shown that such mono-derivatives as nitro compounds, sulphonic acids, carboxylic acids, aldehydes, and ketones yield as a general rule chiefly the meta-compounds, and this is independent of the nature of the second group introduced; on the other hand, benzene haloids, amino-, homologous-, and hydroxy-benzenes yield principally a mixture of the orthoand para-compounds. These facts are embodied in the " Rule of Crum Brown and J. Gibson " (Jour. Chem. Soc. 61, p. 367): If the hydrogen compound of the substituent already in the benzene nucleus can be directly oxidized to the' corresponding hydroxyl compound, then meta-derivatives predominate on further substitution, if not, then orthoand paraderivatives. By further substitution of orthoand para-diderivatives, in general the same tri-derivative [1.2.4] is formed (Ann., 1878, 192, p. 219); meta - compounds yield [1.3.41 and [1.2. 3 1 tri-derivatives, except in such cases as when both substituent groups are strongly acid, e.g. m-dinitrobenzene, then [1.3.5]-derivatives are obtained.

Syntheses of the Benzene Ring.-The characteristic distinctions NH NH, r.-NH, [[Cooh Cooh ,_ + Nh2 Nh, H., Cooh]] x x x Tri. derivatives Tetra-derivatives x x x x x x ? x u x?X x x HZ which exist between aliphatic and benzenoid compounds make the transformations of one class into the other especially interesting.

In the first place we may notice a tendency of several aliphatic compounds, e.g. methane, tetrachlormethane, &c., to yield aromatic compounds when subjected to a high temperature, the so-called pyrogenetic reactions (from Greek 7rup, fire, and - yon, fco, I produce); the predominance of benzenoid, and related compounds-naphthalene, anthracene, phenanthrene, &c.-in coal-tar is probably to be associated with similar pyrocondensations. Long-continued treatment with halogens may, in some cases, result in the formation of aromatic compounds; thus perchlorbenzene, C 6 C1 6, frequently appears as a product of exhaustive chlorination, while hexyl iodide, C 6 H 13 I, yields perchlorand perbrom-benzene quite readily.

The trimolecular polymerization of numerous acetylene compounds-substances containing two trebly linked carbon atoms, -C: C -, to form derivatives of benzene is of considerable interest. M. P. E. Berthelot first accomplished the synthesis of benzene in 1870 by leading acetylene, HC: CH, through tubes heated to dull redness; at higher temperatures the action becomes reversible, the benzene yielding diphenyl, diphenylbenzene, and acetylene. The condensation of acetylene to benzene is also possible at ordinary temperatures by leading the gas over pyrophoric iron, nickel, cobalt, or spongy platinum (P. Sabatier and J. B. Senderens). The homologues of acetylene condense more readily; thus allylene, CH: CCH 3, and crotonylene, CH 3.0: CCH 3, yield trimethyland hexamethyl-benzene under the influence of sulphuric acid. Toluene or mono-methylbenzene results from the pyrocondensation of a mixture of acetylene and allylene. Substituted acetylenes also exhibit this form of condensation; for instance, bromacetylene, BrC: CH, is readily converted into tribrombenzene, while propiolic acid, HC: C. COOH, under the influence of sunlight, gives benzene tricarboxylic acid.

A larger and more important series of condensations may be grouped together as resulting from the elimination of the elements of water between carbonyl (CO) and methylene (CH 2) groups. A historic example is that of the condensation of three molecules of acetone, CH 3 COCH 3, in the presence of sulphuric acid, to s-trimethylbenzene or mesitylene, C 6 H 3 (CH 3) 3, first observed in 1837 by R. Kane; methylethyl ketone and methyl-n-propyl ketone suffer similar condensations to s-triethylbenzene and s-tri-n-propylbenzene respectively. Somewhat similar condensations are: of geranial or citral, (CH 3) 2 CHCH 2 CH :CHC(CH 3) :CHCHO, to p-isopropylmethylbenzene or cymene; of the condensation product of methylethylacrolein and acetone, CH3CH2CH:C(CH3)CH:CHCOCH3, to [1.3. 4]-trimethylbenzene or pseudocumene; and of the condensation product of two molecules of isovaleryl aldehyde with one of acetone, C 3 H 7 CH 2 CH:C(C 3 H 7)CH:CHCO. CH 31 to (1)-methyl2-4-di-isopropyl benzene. An analogous synthesis is that of dihydro-m-xylene from methyl heptenone,(CH 3) 2 C:CH(CH2)2.COCH3. Certain a-diketones condense to form benzenoid quinones, two molecules of the diketone taking part in the reaction; thus diacetyl, CH 3 COCOCH 3, yields p-xyloquinone, C 6 H 2 (CH 3) 2 0 2 (Ber., 1888, 21, p. 1411), and acetylpropionyl, CH 3 COCOC 2 H 5, yields duroquinone, or tetramethylquinone, C 6 (CH 3) 4 0 2, Oxymethylene compounds, characterized by the grouping > C:CH(OH), also give benzene derivatives by hydrolytic condensation between three molecules; thus oxymethylene acetone, or formyl acetone, CH 3 CO. CH :CH (OH), formed by acting on formic ester with acetone in the presence of sodium ethylate, readily yields [1.3.51-triacetylbenzene, C 6 H 3 (COCH 3) 3; oxymethylene acetic ester or formyl acetic ester or ß-oxyacrylic ester, (HO)CH :CHCO 2 C 2 H 51 formed by condensing acetic ester with formic ester, and also its dimolecular condensation product, coumalic acid, readily yields esters of [ 1.3. 5]- benzene tricarboxylic acid or trimesic acid (see Ber., 1887, 20, p. 2930).

In 1890, 0. Doebner (Ber. 2 3, p. 2 377) investigated the condensation of pyroracemic acid, CH 3 CO000H, with various aliphatic aldehydes, and obtained from two molecules of the acid and one of the aldehyde in the presence of baryta water alkylic isophthalic acids: with acetaldehyde [1.3.51-methylisophthalic acid or uvitic acid, C 6 H 3 CH 3 (000H) 2, was obtained, with propionic aldehyde [ 1.3. 5]- ethylisophthalic acid, and with butyric aldehyde the corresponding propylisophthalic acid. We may here mention the synthesis of oxyuvitic ester (5-methyl-4-oxy-I-3-benzene dicarboxylic ester) by the condensation of two molecules of sodium acetoacetic ester with one of chloroform (Ann., 1883, 222, p. 249). Of other syntheses of true benzene derivatives, mention may be made of the formation of orcinol or [3s]-dioxytoluene from dehydracetic acid; and the formation of esters of oxytoluic acid (5-methyl3-oxy-benzoic acid), C6 H3 CH3. OH COOH,when acetoneoxalic ester, CH 3 COCH 2 COCOCO 2 C 2 H 5, is boiled with baryta (Ber., 1889, 22, p. 3271). Of interest also are H. B. Hill and J. Torray's observations on nitromalonic aldehyde, N02CH(CHO)2,formed by acting on mucobromic acid, probably CHOCBr:CBr:000H, with alkaline nitrites; this substance condenses with acetone to give p-nitrophenol, and forms [I.3.5]-trinitrobenzene when its sodium salt is decomposed with an acid.

By passing carbon monoxide over heated potassium J. von Liebig discovered, in 1834, an interesting aromatic compound, potassium carbon monoxide or potassium hexaoxybenzene, the nature of which was satisfactorily cleared up by R. Nietzki and T. Benckiser (Ber. 18, p. 499) in 1885, who showed that it yielded hexaoxybenzene, C 6 (OH) 6, when acted upon with dilute hydrochloric acid further investigation of this compound brought to light a considerable number of highly interesting derivatives (see Quinones). Another hexa-substituted benzene compound capable of direct synthesis is mellitic acid or benzene carboxylic acid, C6(000H)6. This substance, first obtained from the mineral honeystone, aluminium mellitate, by M. H. Klaproth in 1799, is obtained when pure carbon (graphite or charcoal) is oxidized by alkaline permanganate, or when carbon forms the positive pole in an electrolytic cell (Ber., 1883, 16, p. 1209). The composition of this substance was determined by A. von Baeyer in 1870, who obtained benzene on distilling the calcium salt with lime.

Hitherto we have generally restricted ourselves to syntheses which result in the production of a true benzene ring; but there are many reactions by which reduced benzene rings are synthesized, and from the compounds so obtained true benzenoid compounds may be prepared. Of such syntheses we may notice: the condensation of sodium malonic ester to phloroglucin tricarboxylic ester, a substance which gives phloroglucin or trioxybenzene when fused with alkalis, and behaves both as a triketohexamethylene tricarboxylic ester and as a trioxybenzene tricarboxylic ester; the condensation of succinic ester, (CH2 C02C2H5)2, under the influence of sodium to succinosuccinic ester, a diketohexamethylene dicarboxylic ester, which readily yields dioxyterephthalic acid and hydroquinone (F. Herrmann, Ann., 1882, 211, p. 306; also see below, Configuration of the Benzene Complex); the condensation of acetone dicarboxylic ester with malonic ester to form triketohexamethylene dicarboxylic ester (E. Rimini, Gazz. Chem., 1896, 26, (2), p. 374) the condensation of acetone-di-propionic acid under the influence of boiling water to a diketohexamethylene propionic acid (von Pechmann and Sidgwick, Ber., 1904, 37, p. 3816). Many diketo compounds suffer condensation between two molecules to form hydrobenzene derivatives; thus a, 7 -di-acetoglutaric ester, C 2 H S O 2 C(CH 3 CO) CHCH 2 CH(COCH 3)CO 2 C 2 H 5, yields a methylketohexamethylene,whiles-acetobutyric ester,CH 3 CO (CH2)2C02C2N5, is converted into dihydroresorcinol or m-diketohexamethylene by sodium ethylate; this last reaction is reversed by baryta (see Decompositions of Benzene Ring). For other syntheses of hexamethylene derivatives, see Polymethylenes.

Decompositions of the Benzene Ring. - We have previously alluded to the relative stability of the benzene complex; consequently reactions which lead to its disruption are all the more interesting, and have engaged the attention of many chemists. If we accept Kekule's formula for the benzene nucleus, then we may expect the double linkages to be opened up partially, either by oxidation or reduction, with the formation of di-, tetra-, or hexa-hydro derivatives, or entirely, with the production of open chain compounds. Generally rupture occurs at more than one point; and rarely are the six carbon atoms of the complex regained as an open chain. Certain compounds withstand ring decomposition much more strongly than others; for instance,. benzene and its homologues, carboxylic acids, and nitro compounds are much more stable towards oxidizing agents than. aminoand oxy-benzenes, aminophenols, quinones, and oxycarboxylic acids.

Strong oxidation breaks the benzene complex into such compounds, as carbon dioxide, oxalic acid, formic acid, &c.; such decompositions are of little interest. More important are Kekule's observations that nitrous acid oxidizes pyrocatechol or [I.2]-dioxybenzene, and protocatechuic acid or [3.4]- dioxybenzoic acid to dioxytartaric acid, (C(OH) 2 COOH) 2 (Ann., 1883, 221, p. 230); and 0. Doebner's preparation of mesotartaric acid, the internally compensated tartaric acid, (CH(OH)COOH)21 by oxidizing phenol with dilute potassium permanganate (Ber., 1891,. 2 4, P. 1753).

For many years it had been known that a mixture of potassium chlorate and hydrochloric or sulphuric acids possessed strong. oxidizing powers. L. Carius showed that potassium chlorate and sulphuric acid oxidized benzene to trichlorphenomalic acid, a substance afterwards investigated by Kekule and 0. Strecker (Ann., 1884, 223, p. 170), and shown to be /3-trichloracetoacrylic acid, CC1 3 COCH :CHCOOH. which with baryta gave chloroform and maleic acid. Potassium chlorate and hydrochloric acid oxidize phenol, salicylic acid (o-oxybenzoic acid), and gallic acid ([2.3.4] trioxybenzoic acid) to tri chlorpyroracemic acid (isotrichlorglyceric acid), CC13C(OH)2C02H, a substance also obtained from trichloracetonitrile, CC1 3 COCN, by hydrolysis. We may also notice the conversion of picric acid. [2.4.6]-trinitrophenol) into chloropicrin, CCI 3 NO 2, by bleaching lime (calcium hypochlorite), and into bromopicrin, CBr 3 NO 2, by bromine' water.

The action of chlorine upon diand tri-oxybenzenes has been carefully investigated by Th. Zincke; and his researches have led to the discovery of many chlorinated oxidation products which admit of decomposition into cyclic compounds containing fewer carbon atoms than characterize the benzene ring, and in turn yielding openchain or aliphatic compounds. In general, the rupture occurs between a keto group (CO) and a keto-chloride group (CC1 2), into which two adjacent carbon atoms of the ring are converted by the oxidizing and substituting action of chlorine. Decompositions of this nature were first discovered in the naphthalene series, where it was found that derivatives of indene (and of hydrindene and indone) and also of benzene resulted; Zincke then extended his methods to the disintegration of the oxybenzenes and obtained analogous results, R-pentene and aliphatic derivatives being formed (Rsymbolizing a ringed nucleus).

When treated with chlorine, pyrocatechol (1.2 or ortho-dioxybenzene) (1) yields a tetrachlor ortho-quinone, which suffers further chlorination to hexachlor-o-diketo-R-hexene (2). This substance is transformed into hexachlor-R-pentene oxycarboxylic acid (3) when digested with water; and chromic acid oxidizes this substance to hexachlor-R-pentene (4). The ring of this compound is ruptured by caustic soda with the formation of perchlorvinyl acrylic acid (5), which gives on reduction ethidine propionic acid (6), a compound containing five of the carbon atoms originally in the benzene ring (see Zincke, Ber., 18 94, 27, p. 33 6 4) (the carbon atoms are omitted in some of the formulae).

Resorcin (1.3 or meta dioxybenzene) (1) is decomposed in a somewhat similar manner. Chlorination in glacial acetic acid solution yields pentachlor-m-diketo-R-hexene (2) and, at a later stage, heptachlor-m-diketo-R-hexene (3). These compounds are both decomposed by water, the former giving dichloraceto-trichlorcrotonic acid (4), which on boiling with water gives dichlormethylvinyl-a-diketone (5). The heptachlor compound when treated with chlorine water gives trichloraceto-pentachlorbutyric acid (6), which is hydrolysed by alkalis to chloroform and pentachlorglutaric acid (7), and is converted by boiling water into tetrachlor-diketo-Rpentene (8). This latter compound may be chlorinated to perchloracetoacrylic chloride (9), from which the corresponding acid (to) is obtained by treatment with water; alkalis hydrolyse the acid to chloroform and dichlormaleic acid (I I).

, CO-CCI [[Cioc Cci: Cci. Co]]CCl 3 - I 2CO (8) 1 (9) ccl=CC1?

HO 2 C CCI:CCI CO CC1 3 ' - > H02CCC1:cC1c02H+CHe13' (to) (xi) Hydroquinone (1.4 or para-dioxybenzene) (1) gives with chlorine, first, a tetrachlorquinone (2), and then hexachlor-p-diketo-R-hexene (3), which alcoholic potash converts into perchloracroylacrylic acid (4). This substance, and also the preceding compound, is converted by aqueous caustic soda into dichlormaleic acid, trichlorethylene, and hydrochloric acid (5) (Th. Zincke and 0. Fuchs, Ann., 1892, 267, p. l:).

OH 0 CI N, C1 CI ??C1 01-1 0 (t) (2) (3) (4) (5) Phloroglucin (I.3.5-trioxybenzene) (i) behaves similarly to resorcin, hexachlor [1.3.5] triketo-R-hexylene (2) being first formed. This compound is converted by chlorine water into octachloracetylacetone (3) by methyl alcohol into the ester of dichlormalonic acid and tetrachioracetone (4); whilst ammonia gives dichloracetamide (5) (Th. Zincke and 0. Kegel, Ber., 1890, 23, p. 1706).

OH C1t '-C12 (3) C1 3 Cco. CC1 2 co. CC13+C02 O?OIi O / O / (4) Cl2HCCOCHCl2+CH302CCCl2C02CH3 (5) Cl2HCCONH2 Cl (z) (2) When phenol is oxidized in acid solution by chlorine, tetrachlorquinone is obtained, a compound also obtainable from hydroquinone. By conducting the chlorination in alkaline solution, A. Hantzsch (Ber., 1889, 22, p. 1238) succeeded in ob R taining derivatives of o-diketo-R-hexene, which yield R-pentene and aliphatic compounds on decomposition.

When thus chlorinated phenol (I) yields trichlor-o-diketo-R-hexene (2), which may be hydrolysed to an acid (3), which, in turn, suffers rearrangement to trichlor-R-pentene-oxycarboxylic acid (4). Bromine water oxidizes this substance to oxalic acid and tetrabromdichloracetone (5).

'Cooh Ci, BrCCOCBr3+ H H2 HC ("CH, HC C /CH, H02CC02HCI CI (t) (2) C(3) (4) (5) The reduction of o-oxybenzoic acids by sodium in amyl alcohol solution has been studied by A. Einhorn and J. S. Lumsden (Ann., 1895, 286, p. 257). It is probable that tetrahydro acids are first formed, which suffer rearrangement to orthoketone carboxylic acids. These substances absorb water and become pimelic acids. Thus salicylic acid yields n-pimelic acid, [[Hooc(Ch 2) 5 Cooh]], while o-, m-, and p-cresotinic acids, C 6 H 3 (CH 3)(OH)(000H), yield isomeric methylpimelic acids.

Resorcin on reduction gives dihydroresorcin, which G. Merling (Ann., 1894, 278, p. 20) showed to be converted into n-glutaric acid, [[Hooc(Ch2)3000h]], when oxidized with potassium permanganate; according to D. Vorlander (Ber., 1895, 28, p. 2348) it is converted into y-acetobutyric acid, CH 3 C0(CH 2) 3 000H, when heated with baryta to 150-160°.

Configuration of the Benzene Complex

The development of the " structure theory " in about 1860 brought in its train an appreciation of the chemical structure of the derivatives of benzene. The pioneer in this field was August Kekule, who, in 1865 (Ann., 137, p. 129; see also his Lehrbuch der organischen Chemie), submitted his well-known formula for benzene, so founding the " benzene theory " and opening up a problem which, notwithstanding the immense amount of labour since bestowed upon it, still remains imperfectly solved. Arguing from the existence of only one mono-substitution derivative, and of three di-derivatives (statements of which the rigorous proof was then wanting), he was led to arrange the six carbon atoms in a ring, attaching a hydrogen atom to each carbon atom; being left with the fourth carbon valencies, he mutually saturated these in pairs, thus obtaining the symbol I (see below). The value of this ringed structure was readily perceived, but objections were raised with respect to Kekule's disposal of the fourth valencies. In 1866 Sir James Dewar proposed an unsymmetrical form (II); while in 1867, A. Claus (Theoretische Betrachtungen and deren Anwendung zur Systematik der organischen Chemie) proposed his diagonal formula (III), and two years later, A. Ladenburg (Ber., 2, p. 140) devised his prism formula (IV), the six carbon atoms being placed at the six corners of a right equilateral triangular prism, with its plane projections (V, VI).

CH CH HC ,N CH HC CH HC HC HC Hcch Hc Ch Ch Kekule t i Dewar Ladenburg One of the earliest and strongest objections urged against Kekule's formula was that it demanded two isomeric ortho-di-substitution derivatives; for if we number the carbon atoms in cyclical order from i to 6, then the derivatives 1.2 and 1.6 should be different.' Ladenburg submitted that if the 1 . 2 and 1.6 compounds were identical, then we should expect the two well-known crotonic acids, CH 3 CH: CHCOOH and CH 2: CHCH 2 000H, to be identical. This view was opposed by Victor Meyer and Kekule. The former pointed out that the supposed isomerism was not due to an arrangement of atoms, but to the disposition of a valency, and therefore it was doubtful whether such a subtle condition could exert any influence on the properties of the substance. Kekule answered Ladenburg by formulating a dynamic interpretation of valency. He assumed that if we have one atom 1 It is now established that ortho compounds do exist in isomeric forms, instances being provided by chlor-, brom-, and amino-toluene, chlorphenol, and chloraniline; but arguments, e.g. E. Knoevenagel's theory of " motoisomerism," have been brought forward to cause these facts to support Kekule.