REPORTS ON THE STATE OF SCIENCE. Preliminary Report on the Recent Progress and Present State of Organic Chemistry. By GEORGE C. FOSTER, B.A., F.C.S., Late Assistant in the Laboratory of University College, London. THE late Mr. J. F. W. Johnston presented to the Second Meeting of this Association, held at Oxford in 1832, a "Report on the Recent Progress and Present State of Chemical Science." This Report included both Organic and Inorganic Chemistry, but no subsequent Report exists in which the progress of Organic Chemistry, as a whole, is discussed. It therefore seemed advisable to take the year 1832 as the starting-point of the Report, the preparation of which was entrusted to Dr. Odling and myself, at the Meeting in Leeds last year. On commencing the task, we found that a satisfactory account of the progress of organic chemistry, since that date, would be little else than a tolerably complete history of that branch of science. Believing that such a historical account would be of great value, we made some progress in its preparation. Those, however, who have the greatest acquaintance with the subject, will be the readiest to believe that it was utterly impossible for us to bring such a Report to anything like a state of completeness in time for the present Meeting. We thought, however, that such a general preliminary account as we might be able to give, of some of the most recent discoveries, illustrating some of the ideas most lately introduced into the science, might perhaps have both interest and utility. In the following pages, therefore, in which such a general account is attempted, historical completeness has not been aimed at; the object has been rather to place in a clear light the real nature and tendency of some of the most important theoretical views which are now taking a place in the science. The reconciling of the theory of types with the theory of compound radicles, which resulted from the discovery of the compound ammonias by Wurtz and Hofmann, and the discovery of the mixed ethers (or ethers containing two distinct alcohol-radicles) by Williamson, prepared the way for Gerhardt's classification of chemical substances according to types of double decomposition. The system of ideas, of which we may regard this classification as an epitome, has exerted so great an influence on the progress of theoretical chemistry during the last seven or eight years, that it becomes an essential part of a survey like the present to consider what parts of it have been modified or confirmed by recent discoveries. 1859. B Gerhardt's classification, like every classification which rests on chemical principles, was a system of rational formule. It is very important, therefore, for our present purpose, to understand clearly at the outset what his formulæ were intended to express. As he constantly repeated, they were not attempts to represent the arrangement of the atoms of chemical compounds, but to represent the groups or atoms, which, in the double decompositions by which compounds are formed or destroyed, replace, or are replaced by, other groups or atoms. His types were selected as being the simplest or best known bodies which could be the agents or products of double decompositions similar to those of the substances classified as deriving from them. Gerhardt's formulæ are, therefore, in the strictest sense chemical, and, as such, ought to be clearly distinguished from formulæ which are intended to express the molecular arrangement of compounds, formulæ which, speaking strictly, are physical, not chemical. The nature and importance of the distinction to which we refer will perhaps be made clearer if we recall to the recollection of the Section a recent instance in which it appears to have been overlooked by one of the ablest of living chemists. Gerhardt had given two different formulæ for aldehyde, namely, C2 H3 C2 H3 O. H and H O, each of which expresses accurately the chemical nature of aldehyde in relation to a particular set of reactions. Kopp, however, found that the specific gravity of aldehyde, calculated from the formula C H3 O.H, according to a rule which he had deduced from the examination of a considerable number of substances, agreed with the specific gravity found by experiment, but that the specific gravity calculated from C2 H3 the formula HO did not agree with experiment. He therefore conН cluded that the first formula was more accurate than the second. Assuming that the rule we have referred to was founded on a sufficient num ber of accurate observations, such a conclusion would doubtless be correct, were the formulæ intended as expressions of the molecular constitution of aldehyde so long as it remains such, that is to say, so long as its chemical characters do not come into account; but the facts in question have no bearing on the relative accuracy of formula which have reference solely to the reactions by which aldehyde can be formed or decomposed *. The idea of polyatomic radicles and molecules naturally arose out of the attempt to represent polybasic acids according to types of decomposition. The first chemist who used formulæ expressing the replacement of more than one atom of hydrogen by a single atom of a compound radicle was Professor Williamsont. The views which he had expressed were extended, and the expression of them in chemical formulæ greatly facilitated, by the introduction, by Dr. Odling‡, of a special mode of notation. But the most numerous and most remarkable examples of polyatomic compounds hitherto known, have been furnished by the researches of Berthelot§ and of Wurtz ||. In order to explain the nature of polyatomic compounds and the meaning of polyatomic formulæ, we cannot take a better illustration than the formula for glycerine proposed by Wurtz, (CH)"} O'. This formula represents glycerine as deriving from three atoms of water by the substitution of the indivisible triatomic radicle C3 H' for three atoms of hydrogen; that is to say, as a hydrate, but a hydrate which differs from ordinary hydrates, just as * Comp. Kekulé, Ann. Chem. Pharm. cvi. 147, note. † Chem. Soc. Quart. Journ. iv. 350. Ibid. vii. 1. a terchloride differs from a protochloride. Thus a protohydrate, alcohol, CH} O, for example, is converted into a chloride by the action of one atom of hydrochloric acid, one atom of water being at the same time eliminated, and cannot then be any further acted on in the same way. Glycerine is similarly converted into a chloride, with elimination of an atom of water, by the action of one atom of hydrochloric acid, but the product in this case can again produce the same reaction with a second, and even with a third atom of hydrochloric acid :— C3 H' O2 CI+H CI-H2O=C3 H® O Cl2 Dichlorhydrin. And in general terms, we may express the difference between a polyatomic body and a monatomic body, deriving from the same type, by saying that, with the same reagent, both produce similar reactions, but that a greater quantity of the reagent (two, three, or four times as much, according as the substance is di-, tri-, or tetratomic) is required to react to the greatest possible extent with the polyatomic body than with the monatomic body. The consideration of the following and similar series of bodies throws great light upon the mutual relations of monatomic and polyatomic substances. The second term of the series is a monatomic chloride; it reacts with one atom, but not with more, of potash, ammonia, &c. The third is a diatomic chloride, the fourth a triatomic† chloride, and the fifth a tetratomic chloride. The radicles which these four chlorides respectively contain are (CH3)', (CH2)", (CH)", and (C)iv, all formed from marsh-gas (CH), the first term of the series, by the removal of hydrogen; and the number of atoms of hydrogen which must be removed to form each radicle denotes the atomic value of that radicle. In other words, chloride of methyl, CH3 Cl, can, under a variety of conditions, part with its chlorine in exchange for other substances, whilst its carbon and hydrogen remain in unaltered combination, having the characters of a monatomic radicle. But, under certain other conditions, chloride of methyl can exchange one-third, two-thirds, or even the whole of its hydrogen against an equivalent quantity of chlorine; and the compounds which are formed, containing C H2 Cl2, C H CI3, and C CI, * No reactions corresponding to this view of chloride of methylene are yet known, but the analogy of iodide of methylene (Comp. Buttlerow, Ann. Chim. Phys. [3] liii. 313) is sufficient for our present purpose. + Comp. Kay, Chem. Soc. Quart. Journ. vii. 224; Hofmann, Proc. Roy. Soc. ix. 229. Comp. Hofmann, Proc. Roy. Soc. ix. 284. |