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See also:CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see See also:ALCHEMY) , the natural See also:science which has for its See also:province the study of the See also:composition of substances. In See also:common with physics it includes the determination of properties or characters which serve to distinguish one substance from another, but while the physicist is concerned with properties possessed by all substances and with processes in which the molecules remain intact, the chemist is restricted to those processes in which the molecules undergo some See also:change. For example, the physicist determines the See also:density, See also:elasticity, hardness, See also:electrical and thermal conductivity, thermal expansion, &c.; the chemist, on the other See also:hand, investigates changes in composition, such as .See also:nay be effected by an electric current, by See also:heat, or when two or more substances are mixed. A further differentiation of the provinces of chemistry and physics is shown by the classifications of See also:matter. To the physicist matter is presented in three leading forms—solids, liquids and gases; and although further sub-divisions have been rendered necessary with the growth of knowledge the same principle is retained, namely, a See also:classification based on properties having no relation to composition. The fundamental chemical classification of matter, on the other hand, recognizes two See also:groups of substances, namely, elements, which are substances not admitting of See also:analysis into other substances, and compounds, which do admit of analysis into simpler substances and also of See also:synthesis from simpler substances. Chemistry and physics, however, meet on common ground in a well-defined See also:branch of science, named See also:physical chemistry, which is primarily concerned with the correlation of physical properties and chemical composition, and, more generally, with the elucidation of natural phenomena on the molecular theory. It may be convenient here to See also:state how the whole subject of chemistry is treated in this edition of the See also:Encyclopaedia Britannica. The See also:present See also:article includes the following sections: I. See also:History.—T his See also:section is confined to tracing the See also:general trend of the science from its See also:infancy to the See also:foundations of the See also:modern theory. The history of the alchemical See also:period is treated in more detail in the article ALCHEMY, and of the iatrochemical in the article See also:MEDICINE. The See also:evolution of the notion of elements is treated under See also:ELEMENT; the molecular See also:hypothesis of matter under See also:MOLECULE; and the See also:genesis of, and deductions from, the atomic theory of See also:Dalton receive detailed analysis in the article See also:ATOM. 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. VI. 2 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. See also: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 tom-pounds is also noticed. VI. Physical Chemistry.—This section is restricted to an See also:account of the relations existing between physical properties and chemical composition. Other branches of this subject are treated in the articles CHEMICAL See also:ACTION; See also:ENERGETICS; See also:SOLUTION; Annoys; See also:THERMOCHEMISTRY. I. HISTORY Although chemical actions must have been observed by See also:man in the most remote times, and also utilized in such processes as the extraction of metals from their ores and in the arts of tanning and See also:dyeing, there is no See also:evidence to show that, beyond an unordered accuml}lation of facts, the See also:early developments of these See also:industries were attended by any real knowledge of the nature of the processes involved. All observations were the result of See also:accident or See also:chance, or possibly in some cases of experimental trial, but there is no See also:record of a theory or even a general classification of the phenomena involved, although there is no doubt that the ancients .had a See also:fair knowledge of the properties and uses of the commoner substances. The origin of chemistry is intimately See also:bound up with the arts which we have indicated; in this respect it is essentially an experimental science. A unifying principle of chemical and physical changes was provided by metaphysical conceptions of the structure of matter. We find the notion of " elements," or See also:primary qualities, which confer upon all See also:species of matter their distinctive qualities by appropriate See also:combination, and also the See also:doctrine that See also:Greek matter is composed of See also:minute discrete particles, See also:plum. prevailing in the Greek See also:schools. These " elements, sophy. sops however, had not the significance of the elements of to-See also:day; they connoted physical appearances or qualities rather than chemical relations; and the atomic theory of the ancients is a See also:speculation based upon metaphysical considerations, having, in its origin, nothing in common with the modern molecular theory, which was based upon experimentally observed properties of gases (see ELEMENT; MOLECULE). 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, and others as material; and a school of philosophers termed alchemists arose who attempted the See also: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 " See also:water," " See also:air," " See also:earth " and " See also:fire " were regarded as clinging most tenaciously to the essence, while " See also:cold," " heat," " moistness " and " dryness " were more easily See also: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. See also:Berthelot, " alchemy rested partly on the See also:industrial processes of the See also:ancient Egyptians, partly on the speculative theories of the Greek philosophers, and partly on the mystical reveries of the Gnostics and Alexandrians." The See also:search for this essence subsequently resolved itself into the See also:desire to effect the trans-mutation of metals, more especially the See also:base metals, into See also:silver and See also:gold. It seems that this secondary principle became the dominant See also:idea in alchemy, and in this sense the word is used in See also:Byzantine literature of the 4th See also:century; Suidas, See also:writing in
II
the 1 rth century, defines chemistry as the " preparation of silver and gold " (see ALCHEMY).
From the Alexandrians the science passed to the See also:Arabs, who made discoveries and improved various methods of separating substances, and afterwards, from the 11th century, became seated in See also: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 See also:wealth upon the fortunate discoverer. Some alchemists honestly laboured to effect the transmutation and to discover the " philosopher's See also: It is really not extraordinary that See also: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 See also:Roger See also: 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 See also: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 sub-stances distinctly new corpuscles of a compound were formed, and that each corpuscle had a certain See also:affinity for other corpuscles.
Although Boyle practised the methods which he expounded, he was unable to gain general See also:acceptance of his doctrine of
elements; and, strangely enough, the theory which 1Ytogl next dominated chemical thought was an alchemical Ptheorystic
invention, and lacked the lucidity and perspicuity
of Boyle's views. This theory, named the phlogistic theory, was primarily based upon certain experiments on See also: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 4,lwyurros, 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 destruction or loss of the igneous principle. Georg See also:Ernst See also:Stahl, following in some measure the views held by Johann See also:Joachim See also: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 See also:resolution into two components, the inflammable principle phlogiston, and another element—" water," " See also: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 See also:coal or See also: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 See also: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 shouldhave gained the esteem of the notable chemists who flourished in the 18th century. See also: See also:Black, K. W. See also:Scheele, A. S. See also:Marggraf, J. See also:Priestley and many others who might be mentioned. Libavius (d. 1616), chiefly famous for his See also:Opera Omnia Medicochymica (1595) ; See also:Jean See also:Baptiste See also:van See also:Helmont (1577-1644), celebrated for his researches on gases ; F. de la Boe Sylvlus (1614-1672), who regarded' medicine as applied chemistry; and See also:Otto Tachenius, who elucidated the nature of salts. Descriptive chemistry was now assuming considerable See also:pro-portions; the experimental inquiries suggested by Boyle were See also:Lavoisier. being assiduously See also: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 See also:Antoine See also: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 See also:weight, and sometimes it has not; sometimes it is See also: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, See also:colours and their See also:absence; it is a veritable See also:Proteus changing in See also:form at each instant." Lavoisier may be justly regarded as the founder of modern or quantitative chemistry. First and foremost, he demanded that the See also: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 See also: The same results were obtained with lead and tin; and a more elaborate repetition indubitably established their correctness. He also showed that on See also: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 See also:charcoal, the metal was recovered and a See also: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 " (oEus, 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 " dephlogisticated air," nitrogen " phlogisticated air," &c.—and this tended to retard its promotion. Yet really the transition from the one theory to the other was See also: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 See also:mass." Matter can neither be created nor destroyed; however a chemical See also:system be changed, the weights before and after areequal.l 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. It is unnecessary in this See also: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 See also:biographical headings; in this article only the salient features in the history of our science can be treated. The beginning of the 19th century was attended by far-reaching discoveries in the nature of the composition of compounds. Investigations proceeded in two directions:—(1) the nature of chemical affinity, (2) the See also:laws of chemical combination. The first question has not yet been solved, although it has been speculated upon cnem/ca/ a//m/ty. 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 See also: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 See also:Hermann See also:Boerhaave, and made more precise by See also:Sir Isaac See also: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 See also:constant, and the same opinion was the basis of much experimental inquiry at the hands of See also:Joseph See also: a The theory of Berthollet was essentially See also:mechanical, and he attempted to prove that the course of a reaction depended not on See also: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 by the See also:Swedish chemists Guldberg and Waage, and the See also:American, See also:Willard See also:Gibbs (see CHEMICAL ACTION). In his classical thesis Berthollet vigorously attacked the results deduced by See also:Bergman, who had followed in his table of elective attractions the path traversed by Stahl and S. F. See also:Geoffroy. elements received symbols composed of circles, arcs of circles, and lines, while certain class symbols, such as 'tZ' for metals, +f or acids, for alkalies, c for salts,/ for calces, &c., were used. Compounds were represented by copulating simpler symbols, e.g. mercury calx was .3 Bergman's symbolism was obviously cumbrous, and the system used in 1782 by Lavoisier was equally abstruse, since the forms gave no See also:clue as to composition; for instance water, oxygen, and nitric acid were 7 +i, and es.. deduced the relative weight of the oxygen atom to be 6.5; from See also: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 over-estimated, notwithstanding the fact that in several cases they needed correction. The first step in this direction was effected by the co-ordination of See also:Gay Lussac's observations on the combining volumes of gases. He discovered that gases always combined in volumes having simple ratios, and that the See also: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 See also:steam, three volumes of hydrogen combined with one of nitrogen to give two volumes of See also:ammonia, one volume of hydrogen combined with one of See also: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. (i) 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 See also:Avogadro in 1811, and by See also: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 See also: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 See also:powers of 2 is accident or See also:design. Notwithstanding Avogadro's perspicuous investigation, and a similar exposition of the atom and molecule by A. M. Ampere, See also:Berzelius. the views therein expressed were ignored both by their own and the succeeding See also: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 See also: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 See also: The " compound acidifiable bases," i.e. the hypothetical radicals of acids, were denoted by squares enclosing the initial See also:letter of the base; an See also: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 See also:vertical See also: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.' 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 See also:case of only one atom) by placing Arabic numerals before the symbols; for example, copper See also: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 See also: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+SO3, See also:potassium sulphate 2S03+PoO2 (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 See also:mineralogy. He also introduced barred symbols, i.e. letters traversed by a See also:horizontal See also:bar, to denote the See also: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 See also:complete accept- See also:Extension ante. Berzelius objected to the hypothesis that if of the two elements form only one compound, then the at"' atoms combine one and one; and although he agreed theory. with the See also:adoption of simple rules as a first See also:attempt at representing a compound, he availed himself of other data in See also:order to gain further See also:information as to the structure of compounds. For example, at first he represented ferrous and ferric oxides by the formulae FeO,, FeO,, and by the analogy of See also:zinc and other basic oxides he regarded these substances as constituted similarly to FeO,, and the acidic oxides alumina and See also:chromium oxide as similar to FeO,. He found, however, that chromic acid, which he had represented as CrOs, neutralized a base containing s the 3 The following symbols were also used by Bergman: b, Yf, °--°, V, , which represented zinc, See also:manganese, See also:cobalt, See also:bismuth, See also:nickel, See also:arsenic, See also:platinum, water, See also:alcohol, phlogiston. 4 The following are the symbols employed by Dalton: O.0, 0.'®, ®, O, O, ®, ®, 0, 0, O which represent in order, hydrogen, nitrogen, carbon, oxygen, phosphorus, sulphur, See also:magnesia, See also: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. ~+ 1 . V and V.-, quantity of oxygen. He inferred that chromic acid must contain only three atoms of oxygen, as did sulphuric acid SO3 ; consequently chromic oxide, which contains half the amount of oxygen, must be Cr203, and hence ferric oxide must be Fe203. The basic oxides must have the general See also:formula MO. To these results he was aided by the law of isomorphism formulated by E. See also:Mitscherlich in 182o; and he confirmed his conclusions by showing the agreement with the law of atomic heat formulated by See also:Dulong and See also: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 See also:electrolysis (q.v.) he assumed that all substances had two components, one bearing a negative See also:charge, the other a See also:positive charge. Combination was associated with the coalescence of these charges, and the nature of the resulting compound showed the nature of the residual See also: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 2H2+02=See also:H2O+H20 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 See also: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 See also:caustic See also: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 See also:gravimetric and other methods. Chemists gradually tired of the notion of atomic weights on account of the uncertainty which surrounded them; and the See also:suggestion made by W. H. See also:Wollaston as early as 1814 to See also: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. See also:Gmelin in his famous See also:text-See also:book. Simultaneously with this discussion of the atom and molecule, great controversy was ranging over the constitution of See also:corn-Atom/c pounds, more particularly over the carbon or organic and See also:mole- compounds. This subject is discussed in section IV., cular Organic Chemistry. The See also:gradual See also:accumulation of data weights. referring to organic compounds brought in its See also:train a revival of the discussion of atoms and molecules. A. Laurent and C. F. See also: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 See also:assumption led him to double the equivalents accepted by Gmelin, making 11=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 r86o there prevailed such a confusion of hypotheses as to the atom and molecule that a See also:conference was held at See also:Karlsruhe to discuss the situation. At the conclusion of the sitting, Lothar See also:Meyer obtained a See also:paper written by Stanislas See also:Cannizzaro in 1858 wherein was found the final See also: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 See also: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 See also: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 See also:fusion of the older See also:radical and type theories. This theory brought together, as it were, the most varied compounds, and stimulated inquiry into many See also: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- Valeacy. ously studied by Sir See also:Edward See also:Frankland, who from
the investigation, not of simple inorganic compounds, but of the organo-metallic derivatives, determined the See also:kernel of the theory of See also: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 atone 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 See also: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. See also:Kekule was the forerunner of his celebrated See also: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. See also: 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 See also: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 (isomer-ism), 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. See also:Pasteur, J. A. Le See also:Bel, J. See also: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 See also:differences. Such See also:isomerism, named stereo-isomerism (q.v.) ,hasbeen assiduously developed duringrecentyears; 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 Periodic utilization in the "structure theory " permitted great law. clarification, and attempts were not wanting for the deduction of analogies or a periodicity between elements. See also:Frank-See also:land had recognized the analogies existing between the chemical properties of nitrogen, phosphorus, arsenic and antimony, noting that they See also:act as tri- or penta-valent. Carbon was joined with See also:silicon, See also:zirconium and See also:titanium, while See also:boron, being tri valent, was relegated to another See also: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 See also:establishment of such a periodicity, the greatest advances being made by John See also:Newlands in See also:England, Lothar Meyer in See also:Germany, and D. J. Mendeleeff in St See also:Petersburg. For the development of this classification see ELEMENT. In the above See also:sketch we have briefly treated the history of the See also:main tendencies of our science from the earliest times to the See also:summary. establishment of the modern laws and principles. We have seen that the science took its origin in the arts practised by the Egyptians, and, having come under the See also:influence of philosophers, it chose for its purpose the isolation of the pinta essentia, and subsequently the " See also: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 See also: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 also been shown in their See also: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 See also:body of working hypotheses of inestimable value. Chemical See also:Education.—It is remarkable that systematic instruction in the theory and practice of chemistry only received See also:earnest attention in our See also:academic institutions during the opening decades of the 19th century. Although for a See also:long time lecturers and professors had been attached to See also:universities, generally their duties had also included the study of physics, mineralogy and other subjects, with the result that chemistry received scanty encouragement. Of See also: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 academiclectures, appears to have been urged by the See also:French chemists L. N. See also:Vauquelin, Gay Lussac, See also:Thenard, and more especially by A. F. See also:Fourcroy and G. F. See also:Rouelle, while in England See also:Humphry. See also:Davy expounded the same idea in the experimental demonstrations which gave his lectures their brilliant See also:charm. But the real founder of systematic instruction in our science was Justus von See also:Liebig, who, having accepted the professorship at See also:Giessen in 1824, made his chemical laboratory and course of instruction the See also: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 See also:pattern set by Liebig at Giessen was adopted by F. See also:Wohler at See also:Gottingen in 1836, by R. W. See also:Bunsen at See also:Marburg in 184o, and by O. L. See also:Erdmann at See also:Leipzig in 1843; and during the 'fifties and 'sixties, many other laboratories were founded. A new era followed the erection of the laboratories at See also:Bonn and See also:Berlin according to the plans of A. W. von Hofmann in 1867, and of that at Leipzig, designed by See also:Kolbe in 1868. We may also mention the famous laboratory at See also:Munich designed by A. von See also:Baeyer in 1875.
In Great See also:Britain the first public laboratory appears to have been opened in 1817 by See also: In See also:America public laboratory instruction was first instituted at Yale College during the professorship of See also:Benjamin See also:Silliman. To the great progress made in See also:recent years F. W. See also: See also: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 18o8-1818, has been enormous. These two See also:works, and especially the latter, were the See also:models followed by Thenard, Liebig, Strecker, Wohler and many others, including Thomas See also:Graham, upon whose Elements of Chemistry was founded Otto's famous Lehrbuch der Chemie, to which H. See also:Kopp contributed the general theoretical part, Kolbe the organic, and See also: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 See also:English, Roscoe and Schorlemmer's See also:Treatise on Chemistry is a standard See also:work ; it records a successful attempt to state the theories and facts of chemistry, not in condensed epitomes, but in an easily read form. The Tecate de chimie minerale, edited by H. See also:Moissan, and the Handbuch der anorganischen Chemie, edited by Abegg, are of the same type. O. Dammer's Ilandbuch 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; See also:Fremy's Encyclopedia de chimie, Wurtz's Dictionnaire de chimie pure et appliquee, See also:Watts' See also:Dictionary of Chemistry, and Ladenburg's Handworterbuch der Chemie. The number of See also:periodicals devoted to chemistry has steadily increased since the early part of the 19th century. In England the most important is the See also:Journal- of the Chemical Society of See also:London, first published in 1848. Since 1871 abstracts of papers appearing in the other See also:journals have been printed. In 1904 a new departure was made in issuing See also:Annual Reports, containing resumes of the most important researches of the See also:year. The Chemical See also:News, founded by Sir W. See also:Crookes in 186o, may also be noted. In America the See also: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 See also:con-fined to abstracts of papers appearing in other journals, the Zeitschrift See also:fur Chemie, and Liebig's Annalen der Chemie are the most important of the general magazines. Others devoted to See also: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 Jahrssberichte and the Jahrbuch der Chemie. In France, the most important journals are the Annales de chimie et de physique, founded in 1789 with the See also: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:- See also:International Atomic Weights, 1910. Name. Atomic Atomic Symbol. Weights. Name. Symbol. Weights. 0=16. 0=16. See also:Aluminium Al 27.1 Mercury . . Hg 200.0 Antimony Sb I20.2 See also:Molybdenum Mo 96.o See also:Argon . A 39.9 Neodymium . Nd 144.3 Arsenic As 74'96 Neon . . . Ne 20 See also:Barium .. Ba 137'37 Nickel . Ni 58.68 See also:Beryllium or Be Nitrogen N See also:Glucinum G1 9.1- . Os 14.01 See also:Osmium . 190'9 Bismuth - Bi 208.0 Oxygen . O 16•oo Boron . B II.0 See also:Palladium Pd 106.7 See also:Bromine _ Br 79.92 Phosphorus P 31.0 See also:Cadmium Cd 112.40 Platinum Pt 195.0 See also:Caesium . Cs 132.81 Potassium . K 39.10 See also:Calcium Ca 40.09 Praseodymium Pr 140.6 Carbon .0 I2.O See also:Radium . Ra 226.4 See also:Cerium Ce 140.25 See also:Rhodium Rh 102.9 Chlorine . Cl 35.46 See also:Rubidium . Rb 85.45 Chromium Cr 52.0 See also:Ruthenium Ru I0I.7 Cobalt . . Co 58.97 Samarium . Sa 150.4 See also:Columbium . Cb See also:Scandium . Sc 44'1 or Niobium Nb 93'5 See also:Selenium . Se 79.2 Copper . . Cu 63.57 Silicon . Si 28.3 Dysprosium . Dy 162.5 Silver . . Ag Io7.88 See also:Erbium . Er 167.4 See also:Sodium . Na 23.0 See also:Europium . Eu 152.0 See also:Strontium . Sr 87.62 See also:Fluorine . F 19.0 Sulphur . S 32.07 See also:Gadolinium Gd 157.3 See also:Tantalum Ta 181.o See also:Gallium . . Ga 69'9 See also:Tellurium Te 127.5 See also:Germanium Ge 72.5 See also:Terbium . Tb 159.2 Gold . . Au 197.2 See also:Thallium _ T1 204.0 See also:Helium . He 4'0 See also:Thorium . Th 232.42 Hydrogen H 1:008. Thulium . Tm 168.5 See also:Indium In 114.8 Tin . Sn 119.0 See also:Iodine 126..92 Titanium. Ti 48.1 See also:Iridium Ir 193.1 See also:Tungsten. W 184.0 Iron . Fe 55.85 See also:Uranium . U 238.5 Krypton . Kr 83.o See also:Vanadium . V 51.2 Lanthanum La 139.0 Xenon . . Xe 130.7 Lead . . Pb 207.10 See also:Ytterbium (Neo- See also:Lithium . Li 7•.00 ytterbium) . Yb 172 Lutecium Lu 174 See also:Yttrium . . Y 89•o See also:Magnesium Mg 24.32 Zinc . . Zn 65'37 Manganese Mn 54.93 Zirconium . Zr 90.6 The elements are usually divided into two classes, the metallic and the non-metallic elements; the following are classed as non-metals, and the See also:remainder as metals: Hydrogen Oxygen Boron Neon Chlorine Sulphur Carbon Krypton Bromine Selenium Silicon Xenon Iodine Tellurium Phosphorus Helium Fluorine Nitrogen Argon 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 See also:peculiar brilliancy or metallic lustre, and they are See also:good conductors of heat and electricity; the non-metals, on the other hand, are mostly bodies of See also:low specific gravity, and See also: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. edoos, 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 See also: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 See also:regular alteration in properties from See also: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 See also:appearance at opposite poles. Those elements which are disengaged at the negative See also: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 See also:merge into each other; moreover the electric relations of elements are not See also:absolute, but vary according to the state of combination in which they exist, so that it is just as impossible to See also:divide the elements into two classes according to this See also:property as it is to See also: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 See also:power of displacing one another in See also: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. Compounds.-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 temperate .es) being mercury and the gases of the argon group. The laws of chemical combination are as follows:- 1. 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, See also:bear a simple ratio to one another. Thus, r 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 14 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 32 or 8 X 4 parts of oxygen and 14 of nitrogen; and lastly, in nitric anhydride a compound of 40 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 See also: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 See also: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, N2O, NO, N2O3, NO2, N2O3; 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 See also:production of salts but with noevolution 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, NH4, 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, HNO2, nitric acid, HNO3. If more than two be known, the one inferior in oxygen content has the prefix hypo- and the termination -ous, and the one See also:superior in oxygen content has the prefix per- and the termination -ic. This is illustrated in the four oxyacids of chlorine, HC1O, HC102, HC103, HC1O4, 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 HNO3 is monobasic, sulphuric acid H2SO4 dibasic, phosphoric acid H3PO4 tribasic. An acid terminating in -ous forms a salt ending in -See also:tile, and an oxyacid ending in -ic forms a salt ending in -See also:ate. Thus the chlorine oxyacids enumerated above form salts named respectively hypochlorites, chlorites, See also: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 See also: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 z. atom or x part by weight of hydrogen, the letter N for x atom or 14 parts of nitrogen, and the symbol Cl for r atom or 35'5 parts of chlorine.' Compounds are in like manner represented by writing the symbols of their constituent elements See also:side by side, and if more than one atom of each element be present, the number is indicated by a See also: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 r part by weight of hydrogen with 35'5 parts by weight of chlorine; again, sulphuric acid is represented by the formula H2SO4, 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,SO4 means H2SO4 taken twice. The See also:distribution of weight in chemical change is readily expressed in the form of equations by the aid of these symbols; the See also:equation 2HC1+Zn = ZnCl2 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 See also:express combination or action upon, the mearring usu dly attached to the use of the sign = being that from such and such bodies such and such other bodies are formed. Approximate values of the atomic weights are empfoyed 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 H2 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 Hz+C12=2HCI; 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 r.elative weights of the elements which enter into reaction, and is not a complete expression of what is supposed to take place: H+Cl = HCI. 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 C2H402, 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 See also: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 2H2+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 CH2O, and its molecular formula is C2H402, or twice CH2O. In addition to empirical and molecular formulae, chemists are in the See also: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 whichit 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 See also: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 orvalency (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 PC13 and PC15, nitrogen the series of oxides represented by the formulae N20, NO, (N203), N2O4, N205, molybdenum forms the chlorides MoC12, MoC13, MoC14, MoC15, MoCl6(?), and tungsten the chlorides WC12, WC14, WC15, WC16. In explanation of these facts it is supposed that each element has a certain number of " See also: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 See also:odd number, and See also: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 See also: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 1So° C. its vapour entirely consists of molecules of the composition NO2; while at temperatures between this and o° C. it consists of a mixture in different proportions of the two kinds of molecules, N2O4 and NO2. The oxide NO2 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 See also: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. Hither-to 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, 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, PbO2, Ni203, MnO2 and As205 do not exist as See also:stable compounds, but the See also:lower chlorides, PbCl2, NiC12, MnCl2 and AsC13, are very stable. The valency of an element is usually expressed by dashes or See also:Roman numerals placed on the right of its symbol, thus: H', 0", B", Mo''; 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 (Gl), 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-, di- and tri-valent; T1, 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 BiC12. Group VI.: 0, usually divalent, but tetravalent and possibly hexavalent in oxonium and other salts; S, Se, Te, di-, tetra- and hexa-valent; Cr, di-, tri- and hexa-valent; Mo, W, di-, tri-, tetra-, penta- and hexa-valent. Group VII.: H (?), monovalent; the See also:halogens F, Cl, Br, I, usually monovalent, but possibly also tri- and 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-, tetra- and 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 =S0 When this oxide is brought into contact with water it combines with it forming sulphuric acid, H2SO4. 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 H•0 S ,O H.O- O. 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•O Na-0 ,O O ~O H.O>S 0, Na•O> O zn<o>S<O. Again, the reactions of acetic acid, C2H402, show that the four atoms of hydrogen which it contains have not all the same See also: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.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 See also: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 See also: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 S02(OH)2, which indicates that it may be regarded as a compound of the group SO2 with twice the group 0H. 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 SO2 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 SO3HC1; and that by the action of water on the compound SO2C12 twice the group OH, or water minus an atom of hydrogen, is introduced in place of the two monad atoms of chlorine S02Cl2-1-2HOH= S02(OH)2 +2HC1. Water. Sulphuric acid. Constitutional formulae like these, in fact, are nothing more than symbolic expressions of the See also: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 SO2 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 C2115, thus-- OH 0c C2H5 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 See also: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 H H•C—CCO.H serves in a measure to express this, three of the atoms of hydrogen being represented as associated with one of the atoms of carbon, PRINCIPLES] are all compounds of similar atoms united together by one or more units of affinity, according to their valendes. 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 disc placed 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 H + Cl = HC1 Hydrogen. Chlorine. Hydrochloric acid. KI + Cl = KCl + 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. 21(I + C12 = 2KCI +. I2. Potassium iodide. Chlorine. Potassium chloride. Iodine. In all cases of chemical change See also: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 844ocalories aredeveloped ; and in the formation cf 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+C12HCl+HCI, 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 i$ 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 See also: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+12=HI+HI, so much energy is expended in the decomposition of the hydrogen and iodine molecules and in the See also: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 See also:expenditure, and the final result is 43 therefore negative; hence it is necessary in forming hydriodic acid from its elements to apply heat continuou$ly. 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 add 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. See also: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 pro-portions to form water 2H2+02 =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(NO3)a + 3KCNS = Fe(CNS), + 3KNO3; 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) a+3KNO3 = Fe (NO3) s+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 BiC1a + OH2 = BiOC1 + 2HC1, 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 BiOCl+2HCI = BiC13 +OH2, Chemical change which merely involves simple decomposition is thus seen to be influenced by the masses of the reacting sub-stances 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 See also:equilibrium. Such reactions are termed reversible (see CHEMICAL ACTION). M. 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 See also:alloys, by smelting the ores; mercury is mentioned by See also:Theophrastus (c. 300 B.C.). The manufacture of See also:glass, also practised in See also:Egypt, demanded a knowledge of sodium or potassium See also:carbonates; the former occurs as an efflorescence on the shores of certain lakes; the latter was obtained from See also:wood ashes. Many substances were used as See also:pigments: See also:Pliny records white lead, See also:cinnabar, See also: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 See also:alum was used as a See also:mordant in dyeing. Many substances were employed in ancient medicine: See also:galena was the basis of a valuable See also:Egyptian cosmetic and See also:drug; the arsenic sulphides, See also:realgar and See also:orpiment, litharge, alum, See also:saltpetre, iron See also: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 See also: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-See also: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, spirit/us sails, by heating See also: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 sub-stances, e.g. alum, See also:vitriol, sulphur and nitric acid, by See also: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 See also:dates from this period, especial See also: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). The metals of the alkaline-earths were somewhat neglected; we find Georg Agricola considering See also: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 See also:direct consequence of their high medicinal importance; See also:mercurial and silver compounds were investigated for the same See also:reason. The general tendency of this period appears to have taken the form of improving and developing the methods of the alchemists; • The definite distinction between potash and soda was first established by See also: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 See also:discovery of phosphorus by See also:Brand, a See also:Hamburg alchemist, in '669 excited chemists to an unwonted degree; it was also independently prepared by Robert Boyle and J. Kunckel, Brand having kept his See also:process See also:secret. Towards the See also:middle of the '8th century two new elements were isolated: cobalt by G. Brandt in 1742, and nickel by A. F. Cronstedt in 175o. These discoveries were followed by See also:Daniel See also:Rutherford's isolation of nitrogen in 1772, and by K. Scheele's isolation of chlorine and oxygen in 1774 (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 See also:Don Fausto d'Elhuyar; manganese was isolated by J. G. Gahn in 1774. 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 know-ledge of oxides and oxidation products was considerably developed. The synthesis of nitric acid by passing electric See also: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 See also:residue of air which would not combine, although ignored for about a century, was subsequently examined by See also:Lord See also:Rayleigh and Sir See also: 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. See also:Klaproth's detection of a previously unknown constituent of the mineral See also:pitchblende. He extracted a substance to which he assigned the character of an element, naming it uranium (from Ovpavos, See also: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. See also: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 '896, 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 See also:Norman See also:Lockyer during a spectroscopic examination of the sun's See also: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 See also: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 See also: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 See also:zircon, See also:hyacinth, jacynth and See also: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 See also:Antonio de Ulloa in 1748, and subsequently discussed by H. T. See also: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 900. In the following year he discovered rhodium; and at about the same time See also:Smithson See also:Tennant added two more to the list—iridium and osmium; the former was so named from the changing tints of its oxides (iprr, See also:rainbow), and the latter from the odour of its oxide (kW), See also: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 See also: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 r8o8 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 See also:ELECTROMETALLURGY). The discovery of boron by Gay Lussac and Davy in 18o9 led Berzelius to investigate See also: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 See also:furnace; its preparation in a pure condition he first accomplished in 1823, when he invented the method of heatingdouble 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 See also:gem-stone See also:beryl. In 1812 B. See also:Courtois isolated the element iodine from " See also:kelp," the burnt ashes of marine See also:plants. The chemical analogy of this substance to chlorine was quickly perceived, especially after its investigation by Davy and Gay Lussac. See also: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, See also: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 See also:deposit from sulphuric acid See also: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 See also:pupil of Berzelius, detected a new element, which he named lithium, in various minerals—notably See also: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 See also:crimson See also:flame-coloration was observed by C. G. Gmelin in 1818. The discovery of bromine in 1826 by A. J. See also:Balard completed for many years Berzelius's group of " halogen " elements; the remaining member, fluorine, notwithstanding many attempts, remained unisolated until 1886, when See also: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 " See also:bleaching-See also:powder " or " chloride of lime " was a compound or mixture in equimolecular proportions of calcium chloride and hypo-See also:chlorite, with a little calcium See also:hydrate. Gay Lussac investigated chloric acid; See also: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. See also: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 18ox) in certain lead ores by A. M. del Rio; subsequent elaborate researches by Sir Henry Roscoe showed many in-accuracies 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 See also: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. See also:Rose, who regarded niobium oxide as the element. The subject was revived in 1866 by C. W. Blomstrand and J. C. See also:Marignac, to whom is due the See also: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 See also: 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 trichloride).
In 1840 C. F. See also:Schonbein investigated See also:ozone, a gas of peculiar odour (named from the Gr. 1g- See also:ELY, to smell) observed in 1785 by See also: 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 See also:Carey See also:Lea, E. A. See also: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 See also:joint discovery in 1859 of the powerful method of spectrum analysis (see See also:SPECTROSCOPY) by G. R. See also: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 See also:lepidolite and in the See also:Durkheim mineral water. In 1861 Sir William Crookes detected thallium (named from the Gr. 06XXos, a See also: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, determined by F. Reich and H. T. See also: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 See also: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., 1895, p. 468), and various papers in the same journal by Sir William Crookes, Bohus]av 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 See also:crystallization; the striking resemblances of the compounds of these elements rarely admitting of a complete separation by simple precipitation and filtration. The extraordinary See also: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 See also: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 spark- and absorption-spectra were the See also: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 See also:phosphorescence induced spectroscopically. By such a study in the ultra-See also: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 1794 fromthe 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, See also: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 See also: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 samarskite, 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 See also: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 See also:vegetable and See also: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 sub-stances, 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 See also:Nicolas See also: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, See also: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 (5669), stated that mineral, vegetable and animal matter contained the same elements, but that more simple combinations prevailed in the mineral See also: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 See also: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 See also: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 See also:glycerin (Olsiiss) in all See also: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 See also:hydrocarbon radicals: he considered, for instance, See also: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, how-ever, he adhered to the classification of See also:emery; and it was only when' identical compounds were obtained from both vegetable and animal See also: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 See also:dogma was shaken by W&hler's synthesis of See also: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 See also: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, See also:ethylene and cyanogen were regarded as inorganic, and at a later date many other See also: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 See also:ether, which pointed to the conclusion that these substances consisted of one molecule of water and one and two of ethylene respectively; and from See also:Pierre Jean Robiquet and Jean Jacques See also:Colin, showing, in 1816, that See also: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 See also: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 See also:Friedrich Wohler on the radical of benzoic acid (" Uber das Radikal der Benzoe- saure," See also:Ann. Chem., 1832, 3, p. 249) 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. DM, matter), formed the basis of See also: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 See also:ALIZARIN; See also:INDIGO; See also:PURIN and See also:TERPENES for illustrations of the manner in which chemists have artificially prepared important animal and vegetable products. 2 These observations were generalized by J. B. Dumas and Polydore Boullay (18o6-1835) in their "ctherin theory" (vide infra). that he was compelled to reject the theory that oxygen could not See also: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. sepwt and 6pep6s, at See also: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 C14HN, which he named " picramyl " (from rrucpos, See also:bitter, and &µuyS&Xrl, See also: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, (C2115)20, which was analogous to 1(20, while alcohol was an oxide of a radical C21I6; 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 See also: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 See also: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 (µeTb.Xfl/ets, See also:exchange); and framed the following empirical laws to explain the reactions:—(r) 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 (I) 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 See also: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 See also: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 See also: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 See also:chloral, marsh gas and See also: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 pos`s"i.ble; this illustrious chemist perceived that the validity of his electrochemical theory was called in question, and therefore he waged vigorous See also: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 Wohler, 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 See also: A. Wurtz discovered the See also: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 185o A. W. See also: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, See also:aldehydes and See also:ketones; the hydrochloric acid type, the chlorides, bromides and iodides; the water type, the See also:alcohols, See also: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(OC2HL)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 See also: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 re-habilitation, 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, See also:C2H6, and not methyl, See also:CH3. 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 C2H3• C2. (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 See also: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 oxy- and amino-propionic acids respectively; glycollic acid and glycocoll, oxy- and amino-acetic acids; salicylic and benzamic acids, oxy- and 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 See also:epitome of present-day conclusions, not chronologically, but as exhibiting the principles and subject-matter of our science. Classification of Organic Compounds. An See also: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 See also:chain of carbon atoms—named the " aliphatic series " (aXec¢ap, oil or See also:fat), and (2) those consisting of a closed chain—the " carbocyclic series." The second series can be further divided ' This must not be confused with the modern acetyl, CH,.CO, which at that time was known as acetoxyl. into two groups: (I) those exhibiting properties closely analogous to the aliphatic series—the See also:polymethylenes (q.v.), 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 See also: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 See also: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 See also:CH4, named methane, in which the hydrogen atoms are of equal value, and which may be pictured as placed at the vertices of a See also:tetrahedron, the carbon atom occupying the centre, This See also: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 See also:plane square to its corners, and on the existence of two See also:optical isomers of the formula C. A. B. D. E., C being a carbon atom and A. B.D. E. being differen t 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, CH3NO2, 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 Cl HbH,Hd. 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(NOi),HbH~Hd. The same methyl iodide gave with potassium See also:cyanide, acetonitril, which was hydrolysed to acetic acid; this must be C(COOH),HbHJHd. 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, CHa(NO2)bH,Hd, 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, CHa.Hb(NO2),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 H3C•CH3i ethane. This compound may be considered as derived from methane, CH4, by replacing a hydrogen atom by the monovalent group CH3, known as methyl; hence ethane may be named " methylmethane." If the carbon atoms are connected by two valencies, we obtain a compound See also:H2C:CH2i ethylene; if by three valencies, HC:CH, See also: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 H3C• CH2• CH3j propane; ethylene gives H2C:CH•CH3, propylene; and acetylene gives HC C•CH3, allylene. By continuing the introduction of methyl groups we obtain three series of homologous hydro-carbons given by the general formulae CaHlt.+2, Cal-12n, and C,H2,,-2, each member differing from the. preceding one of the same series by See also:CH2. 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 See also:consideration of the compound propane. We derived this substance from ethane by introducing a methyl group; hence it may be termed " methylethane." Equally well we may derive it from methane by replacing a hydrogen atom by the monovalent group CH2.CH2, named ethyl; hence propane may be considered as " ethylmethane." Further, since methane may be regarded as formed by the See also: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, frequently 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 CH3•CH2•CH2•CH3, known as " normal " or n-butane, substitution occurring at a terminal atom, or CH3.CH(CH3).CH3, isobutane, substitution occurring at the medial atom. From n-butane we may derive, by a similar substitution of methyl groups, the two hydrocarbons: (1) CH3• CH2• CH2• CH2• CH3, and (2) CH3• CH( CH3)•CH2.CH3i from isobutane we may also derive two compounds, one identical with (2), and a new one (3) CH3(CH3)C(CH3)CH3. 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: (I) is built up solely of methyl and •CH2• (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 hydro-carbons 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 hydro-carbons, 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 hydro-carbon 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 -CH3, > CH2, 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 > CH2. 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—CHz•OH, >CH•OH, and a C•OH; 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 —CH2.OH and >CH•OH, with the production of radicals of the form —CH(OH)2 and >C(OH)z. 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:0 (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):O; 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 See also:mercaptans (q.v.) contain the group —CH2•SH; 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, R2S. Oxidation of thio-ethers results in the formation of sulphoxides, See also:R2: S: 0, and sulphones, R2: SO2; oxidation of mercaptans yields sulphonic acids, R•S0,H, and of sodium mercaptides sulphinic acids, R.SO(OH). We may also notice that thio-ethers combine with alkyl iodides to form sulphine or sulphonium compounds, R, : 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, R•OH, 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 See also:esters (q.v.), 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 R•Cl(Br,I); nitric acid forms nitrates, R•O•NO2; nitrous acid, nitrites, R•O•NO; sulphuric acid gives normal sulphates R2SO4, or acid sulphates, R•SO,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•NH2; secondary, R2: NH; and tertiary, R3 i N ; the oxamines, R3N : O, 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. R4N•I. 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 (q.v.) and See also:azo compounds (q.v.); closely related are the hydrazines (q.v.). Secondary amines yield nitrosamines, R2N•NO, with nitrous acid. By the action of See also: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 See also:hydrazine. Thus with hydroxylamine aldehydes yield aldoxirnes, R•CH : N.OH, and ketones, ketoximes, R2C:N•OH (see See also:OXIMES), while phenyl hydrazine gives phenylhydrazones, R2C:N•NH•C6H, (see HYnRn-ZONES). 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 starting-point 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 R.CO— (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 —NH.See also:NH2, acid-hydrazides. The structural relations of these compounds are here shown : R•CO.OH; R•CO•Cl; (R.CO)20; R•CO•SH; acid; acid-chloride; acid-anhydride; thin-acid; R•CO•NH2; R•CO•NH•NH2. acid-See also: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, R•CC12•NH2, from which are derived the imido-chlorides, R.CCI: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, R•C(OR'):NH; and by an amino group, the See also:amidines, R•C(NH2) :NH. The carbonyl oxygen may also be replaced by the oxime group, : N•OH; thus the acids yield the hydroxamic acids, R•C(OH) : NOH, and the acid-amides the amidoximes, R•C(NH2): NOH. Closely related to the amidoximes are the nitrolic acids, R•C(NO2) : NOH. Cyclic Hydrocarbons and Nuclei. Having passed in rapid See also: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: (1) those in which the carbon atoms are completely saturated—these are known by the generic term polymethylenes, their general formula being (CH2),,: 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 See also:Freund by the action of sodium on trimethylene bromide, Br(CH2)3Br, came somewhat as a surprise, especially in view of its behaviour with bromine and hydrogen bromide. In comparison with the isomeric propylene, CH3•HC:CH2, 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, CH3•CH2•CH2Br. 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 tri- and tetra-methylene rings. Perkin has himself contributed largely to our knowledge of such compounds; penta- and hexa-methylene derivatives have also received considerable attention (see P OLYMETHYLENES) . A. von Baeyer has sought to explain the See also:variations in stability See also:manifest in the various polymethylene rings by a purely mechanical hypothesis, the " See also: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 tetra. hedron towards its four corners, the See also: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 See also:harmony, he considers, with the circumstance that the greatest distortion must be involved in its formation, as if deflected into See also:parallelism each valency will be drawn out of its. position through 2.109° 28'. The values in other cases are calculable from the formula 1(1o9° 28'—a), where a is the See also:internal angle of the regular See also:polygon contained by sides equal in number to the number of the carbon atoms composing the ring. These values are: Trimethylene. Tetramethylene. ;(Io9° 28'—60°) =24° 44'. 1(109° 28'—90 ) =9° 44'. Pentamethylene. Hexamethylene. 1(109° 28'—toe) =0° 44'. i(109° 28'—I2O°)= -5° 16'. The general behaviour of the several types of hydrocarbons is certainly in accordance with this conception, and it is a remark-able 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. 1); and many other cases of the conversion of six-carbon rings into five-carbon rings have been recorded (see below, Decompositions of the Benzene Ring). Similar considerations will apply to rings containing other elements besides carbon. As an See also:illustration it may be pointed out that in the case of the two known types of lactones—the 'y-See also: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 E-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. Baeyer 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, C6H6, 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. Additional information and CommentsThere are no comments yet for this article.
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