THE ATOMIC THEOEY. 251 



to Avogadrc/s law, and made its complete recog-nition b}^ t-hemists 

 necessary. Atoms, molecules, equivalents, and atomic weights needed 

 to be more sharply detined, and in this work many chemists shared. 

 Berzelius had proposed a system of atomic weights whicli ditfered, 

 except in the yalue taken for its base, l)ut little from the one now in 

 use. This was abandoned for a table deyised by Gmelin, in which the 

 laws of Ayogadro and of Dulong and Petit were almost if not entirely 

 ignored. Laurent and Gerhardt attempted to reform the system, but 

 it was left for Cannizzaro, in 1858, to succeed. By doul)ling some of 

 the currently accepted atomic weights order was introduced into the 

 preyailing chaos, and the chemical constants were ])rought into har- 

 mony with the ph3'sical laws. The modern atomic weights and our 

 present chemical notation may be dated from this time, eyen though 

 the preliminary anticipations of them were neither few" nor incon- 

 spicuous. 



The second great step forward was accomplished through the labors 

 of several men. Frankland and Kekule were foremost among them, 

 but Couper, Odling, Williamson, Wurtz, and Hofmann all contributed 

 their share to the upbuilding of a new chemistry" of which the doctrine 

 of valency was the corner stone, A new property of the chemical 

 atom was brought to light, and structural or rational formula^ became 

 possible. Each atom was shown to have a lixed capacit}^ for union 

 with other atoms, a capacity which could be given numerical expression, 

 and from this discovery important consequences followed. An atom 

 of h3'drogen unites with one other atom onh^; the atom of oxygen 

 may combine with two; that of nitrogt^n with three or five; while 

 carl)on has capacity for four. All unions of atoms to atoms within a 

 molecule ai-e governed by conditions of this order, and the limitations 

 thus imposed determine 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 compounds in advance of their actual dis- 

 covery; and it has guided economic investigations from which great 

 industries have sprung. The former partial theories regarding chem- 

 ical constitution fell into their proper places under the ucav generali- 

 zation, for that was broad enough to comprehend them all. All con- 

 stitutional (chemistry depends upon this property of the atoms, and 

 any other adequate foundation for it would be difticult to tind. 



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 established the conditions under which such com- 

 pounds 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 example, exists in two moditications, ditiering in 



