12 Clarke, TJie Atomic TJieory. 



Cannizzaro was a time of fermentation, discussion, and 

 discovery. In chemistry, contrary to the saying of the 

 preacher, there were many new things under the sun, and 

 some of the discoveries were most suggestive. First it 

 was found that certain groups of atoms could be trans- 

 ferred from compound to compound, almost as if they 

 were veritable elements ; and radicles such as ammonium, 

 cyanogen and benzoyl were generally recognised. I say 

 "groups of atoms" advisedly, for as such they were 

 regarded, and they could hardly have been interpreted 

 otherwise. Then came the discovery of isomerism ; of the 

 fact that two substances could be strikingly different, and 

 yet composed of the same elements in exactly the same 

 proportions. This was only explicable upon the supposi- 

 tion that the atoms were differently arranged within the 

 isomeric molecules, and it led investigators more and 

 more to the study of chemical or molecular structure. 

 Without the atomic theory the phenomena would have 

 been hopelessly bewildering ; with its aid they were easy 

 to understand, and fertile in suggestions for research. 

 Still another link in the chain of chemical reasoning was 

 forged by Dumas, when he proved that the hydrogen of 

 organic compounds was often replaceable, atom for atom, 

 by chlorine. Sometimes the replacement was complete, 

 sometimes it was only partial, and the latter cases were 

 the most significant. In acetic acid, for example, one, 

 two, or three fourths of the hydrogen could be successively 

 replaced, but the last fourth was permanently retained. 

 Hydrogen, then, was combined in acetic acid in two 

 different ways, one part yielding its place to chlorine, 

 the other being unaffected. This behaviour was soon 

 found to be by no means exceptional ; indeed, it was very 

 common, and it opened a new line of attack upon the 

 problems of chemical constitution. The existence of 



