Page:The New International Encyclopædia 1st ed. v. 04.djvu/655

* CHEMISTRY. 573 CHEMISTRY. an element may not be the same in all of its coinpouiuls. Kckulo's view, however, was finally aiieptea by all, and in IStiO chemists the world over were busy determining the "structure' of orjianic compounds — a problem which has since occupied the attention of a majority of them almost exclusively. The theory of types, the mother of structural theory, exhibited the radi- cles of comi)ounds, and thus exiflained those cases of isomerism in which compi>unds are dif- ferent because they contain dillVrcnt radicles. Those further cases in which tlic radicles them- ►elves are ditTerently constituted it could not explain. The doctrine of valency, showing the ditiercnt ways in which the atoms can be linked in the radicles, has furnished a satisfactoiy solu- tion of the problem of molecular constitution, and has completely ex|)laiiicd the fact that the molecules of dill'ereiit compounds may be made up of the same atoms. At first Kekule failed to appreciate the full value of his own ideas. In the very memoir in which he states the doctrine of valency, he advances the view that this doc- 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. Perhaiis be was not al- together wrong. For now, after half a century of experience, organic chemists are beginning to complain of the inadequacy of the structural theory, even with its more recent development — ■ stereochemistry (q.v. ) — and to look forward to some broader idea, that would correlate a larger number of knowTi phenomena and permit of fore- seeing a larger number of as yet unknown facts. What that idea will be, no one can tell as yet. Cexeral Chemistry. The doctrine of valency could not have come into existence if not for the fact that toward the end of the fifties chem- ists bad learned the true atomic weights of the elements. Without a knowledge of the true relative weights of atoms, it would liave been im- possible to know their true number in molecules, and, hence, impossible to know their true valen- cies. Atomic weights were determined, calculat- ed, and re-ealeulated ever since Dalton first es- tablished the atomic theory. Dalton himself, as stated in a previous section of this article, determined atomic weights on the basis of cer- tain simple assumptions. Soon afterwards Ber- zelins devoted himself to the problem with great assiduity. From the law of combining volumes, discovered by Oay-Lussac in 1808, Berzclius inferred that equal volumes of gaseous elements must contain equal numbers of particles. In 1819 Alitscherlich discovered the principle of isomorjihism. (See Atomic Weights.) Berze- lius bad carried out about two thousand analyses, and bad thus determincil the relative quantities of the elements contained in a great variety of compotmds. By combining the principle of iso- morphism with that of equal gasecms volumes, he was now able to calculate the atomic weights of the elements. Now, his principle ' of equal volumes was not quite correct. To him the par- ticles of a gaseous element in the uncombined state were isolated atoms. While he distin- guished between the particles of compounds and the atoms of elements, he failed to distinguish between the free particles of elements and their atoms. That the particle of an element might be made up of two or more single atoms, it would have lieen impossible for him to admit; for, ac- cording to his electro-chemical theory, only un- like atoms could exist in combination with one another. Avogadro's memoir of 181 1, in which more correct views on the subject had been ad- vanced, therefore remained unnoticed, and Ber- zelius's atomic weights were for years employed by all. Xor were most of those figures wrong; for in many cases Berzelius's error eliminated itself, owing to the fact that the molecules of the ordinaiy gaseous elements are made uj) of equal numbers of atoms. Knowing the true atomic weights of the ordinary gaseous elements, Berzclius ^as able to obtain correct ligurcs for many other elements, with the aid of the principle of isomoriibism and certain other principles that need not be explained here. Thus, his figure for mercury was 200, that for phosphorus 31, that for sulphur .32 — figures practically identical with those acceptt^d at present. In 1827, how- ever, Dumas invented his celebrated metliod of detenuining vapor densities, and undertook to apply Berzelius's principle of equal volumes to elements which are not ordinarily gaseous. Finding that the vapor of mercury is 101 times as heavy as an equal volume of hydrogen, the vapor of phosphorus 62.8 times, and the vapor of sulphur 90 times, as hea-y as h.vdrogen, Dumas concluded that the relative weights of their atoms must be, respectivelv. 101, 62.S, and 90, and not 200, 31, and 32," as Berzelius thought. The error of Berzelius's principle thus emerged in the results of Dumas. But instead of rectifying the error of his prin- ciple by introducing the concept of the mole- cules of elements, Berzelius only concluded that the principle was unreliable. The result was that cliemists began to disagree as to the true values of the atomic weights, and many even abandoned the hope of ever knowing atomic weights altogether, and decided to use nothing but ciiiiivalcnts. These represented the weights of elements that were capable of combining with, or of being replaced by, unit weight of hydrogen. For example, Berzelius's view that an atom of oxygen was 16 times as hcaATT as an atom of hydrogen was abandoned, and as hydrogen com- bined with S times its Aeight of oxygen, the lat- ter was represented by its equivalent 8. But the use of equivalents was not universal, many chemists using systems in which the figures were partly equivalents, partly atomic weights, and thus for years great confusion reigned in chem- ical notation, the tiiie purpose of which is to avoid confusion by exhibiting the composition of substances in the simplest and clearest pos- sible manner. In the forties, Laurent and Ger hardt became convinced that the progress of knowledge in organic chemistry was seriously impeded by the lack of a consistent system of atomic weights. Their researches soon led them to distinguish clearly between the atoms and molecules of elements, and to grasp the full vahie of Avogadro's principle for determining the relative weights of molecules. With the aid of this principle, Gerhardt found the true atomic weights of the elements: and, in the latter part of the fifties, his pupil Cannizzaro demonstrated clearly the consistency of the principle with all known facts. Thus was paved the way for the doctrine of valency. A few years later ( in 1869) ilendeleefl' and Lotbar Meyer established a remarkable connection between the l>roperties of the elements and their atomic weights (see