Meanwhile an idea of inestimable value had thrust itself upon chemists. Inspecting the typ ical formulas of compounds. they could not help noticing that certain radicles (e.g. methyl. CH,„ or ethyl. were capable of replacing each a single atom of hydrogen: others were capable of replacing end' I wo atoms of hydrogen, ete. In other words. some radicles were seen to be equivalent to an atom of hydrogen: others had double its com bining capacity, etc. Bence the idea of the ralcncy if radicles and atoms. Like most other general ideas. that of valency was not new. In a vague and more or less specialized form it may be found in the researches of Berzelius, Graham. Liebig, and others: and Frankland, who first clearly enunciated it. in 1852, justly points out that it was probably a vague recognition of the valency of radicles, as exhibited by the facts of substitution, that gave birth to the theory of types. Frankland's statements, however, at tracted no attention. In 1858 and C'ouper independently developed the same idea, the latter proposing to symbolize the combining capacity of dilterent atoms by the dashes now generally employed in graphic formulas. Re called attention to the quadrivalence of the carbon atom, as shown directly by compounds like the following: MT,. CH Cl, CIICI... CC1,: or indirectly by such compounds as CO,. in the former compounds a single atom of carbon is seen to lie equivalent to four atoms of hydrogen, and zi single chlorine atom to a. single atom of hydrogen. which is also shown by the of hydrochloric acid. 11C1. In a compound like COCI,„ the oxygen atom lutist therefore lie assumed to be divalent, and so it is shown to he by the formula 11,0. Ke huh'. ,o1111 1'111110 to t hat in practi cally all compounds one carbon atom is combined with a quantity of other elements is equivalent to four atoms of hydrogen.
This gave rise to it lively controversy, the critio Kolbe especially maintaining that the valency of an clement may not be the same in all of its view. however. was finally acceptc41 by all, and in 1'460 chemists the world over were busy determining the 'structure' of organic compounds—a problem Which since the attention of a majority of thent almost exelusively. The theory of types, the mother of structural theory, exhibited the radi cles of eompounds, and thus explained those eases of isomerism in which compounds are dif ierent because they contain different radicles. The se further cases in which the radicles them ,elves are differently constituted it could not explain. The doctrine of valency, showing the different ways in which the a turns call he linked in the radicles, has furnished a sat isfaetory solu tion of the problem of molecular constitution, and has completely explained the fact that the molecules of different compounds may be made tip of the same atoms. At first Kekulc failed to appreciate the full value of his own ideas. In the very memoir in which lie states the doctrine of valency, lie advances the view that this doe trine cannot by any means solve the problem of the constitution of compounds: the old problem, he thought, might possibly be solved some day by physical chemistry. Perhaps he was not al together wrong. For now, after half a century el experience. organic chemists are beginning to complain of the inadequavy of the structural theory. even with its more recent development stereoehemistry (q.v.)—and to look forward to some broader idea, that would correlate a larger number of known phenomena and permit of fore seein• a larger number of as yet unknown facts. What that idea will be, no one can tell as yet.