520 



SCIENCE. 



[X. S. \()i.. will. No. 4(i0. 



ists necessary. Atoms, molecules, equiva- 

 lents and»atomic weights needed to be more 

 sharply defined, and in this work many 

 chemists shared. Berzelius had proposed a 

 system of atomic weights which differed, 

 except in the value taken for its base, but 

 little from the one now in use. This was 

 cibandoned for a table devised by Gmelin, 

 in which the laws of Avogadro and of Du- 

 long and Petit were almost if not entirely 

 ignored. Laui'.ent and Gerhardt attempted 

 to reform the system, but it was left for 

 Gannizzaro, in 1858, to succeed. By 

 doubling some of the currently accepted 

 atomic weights, order was introduced into 

 the prevailing chaos, and the chemical con- 

 stants were brought into harmony with the 

 physical laws. The modern atomic weights 

 and our present chemical notation may be 

 dated from this time, even though the pre- 

 liminary anticipations of them were neither 

 few nor inconspicuous. 



The second great step forward was ac- 

 complished through the labors of several 

 men. Frankland and Kekvile were fore- 

 most among them, but Couper, Odling, Wil- 

 liamson, Wurtz and Hofmann all contrib- 

 uted their share to the upbuilding of a 

 new chemistry, of which the doctrine of 

 valency was the cornerstone. A new 

 property of the chemical atom was brought 

 to light, and structural or rational formulte 

 became possible. Each atom was shown to 

 have a fixed capacity for union with other 

 atoms, a capacity which could be given nu- 

 merical expression ; and from this discovery 

 important consequences followed. An 

 atom of hydrogen unites with one other 

 atom only, the atom of oxygen may com- 

 bine with two ; that of nitrogen with three 

 or five ; while carbon has capacity for four. 

 All unions of atoms to atoms within a 

 molecule are governed by conditions of this 

 order, and the limitations thus imposed de- 

 termine the possibilities of combination in 



a given class of compounds. In organic 

 chemistry the conception of valency has 

 been most fruitful, and it has shown the 

 prophetic power which is characteristic of 

 all good theories. It explains radicles and 

 isomers; it predicts whole classes of com- 

 pounds in advance of their actual dis- 

 covery; and it has guided economic inves- 

 tigations from which great industries have 

 sprung. The former partial theories re- 

 garding chemical constitution fell into their 

 proper places under the new generalization, 

 for that was broad enough to comprehend 

 them all. All constitutional chemistrj^ de- 

 pends upon this property of the atoms, 

 and any other adequate foundation for it 

 would be difficult to find. 



I have said that the discovery of valency 

 explained the phenomena of isomerism. 

 Indeed, it enabled chemists to foresee the 

 existence of new isomers, and it estab- 

 lished the conditions under which such 

 compounds could exist. And yet, in one 

 direction at least, its power was limited, 

 and substances were found which the theory 

 could not interpret. Tartaric acid, for ex- 

 ample, exists in two modifications, differing 

 in crystalline form and in their action upon 

 polarized light. One acid was dextrorota- 

 tory, the other lajvorotatory, while a mix- 

 ture of the two in eqiial proportions was 

 neutral to the polarized beam, and gave no 

 rotation at all. Their crystals exhibited a 

 similar difference in the arrangement of 

 certain planes, one set being right-handed, 

 the other left-handed ; and each crystal re- 

 sembled its isomer like a reflection in a mir- 

 ror, alike, but reversed. For a long time 

 this physical isomerism, as it was called, 

 remained inexplicable, for the rules of va- 

 lency gave to both molecules the same struc- 

 ture, and offered no hint as to the cause of 

 the difference. Structural formulae, how- 

 ever, said nothing of the arrangement of the 

 atoms in tridimensional space, and it was 



