CHEMISTRY AND ITS DEVELOPMENT 223 



the light of more knowledge. Berzelius subdivided organic 

 radicles, like the elements, into electro-positive and electro- 

 negative. On the other hand, it was generally expected 

 that radicles would eventually be isolated and thus consti- 

 tute a series of simple compounds whose molecules would 

 bear the same relation to the substances of organic chem- 

 istry as the atoms of the elements bear to the compounds of 

 inorganic chemistry. But when the electro-chemical theory 

 was overthrown, while attempts to isolate radicles remained 

 fruitless, the opinion began to spread that the theory of 

 radicles had made of organic chemistry a science of imagi- 

 nary substances, and, hence, the sooner the theory was 

 abolished the better for the young science. But how, then, 

 were organic compounds to be correlated? A solution of 

 this problem was suggested by Dumas in 1839. Continuing 

 his researches on the substitution of different elements for 

 one another in organic compounds, Dumas found that in 

 acetic acid hydrogen could be exchanged for chlorine, and 

 that the resulting compound (trichlor-acetic acid) was very 

 much like acetic acid itself. Similar facts had already been 

 observed since 1834, by himself as well as by Laurent. It 

 now occurred to Dumas that in correlating their substances 

 chemists could be guided solely by the phenomena of sub- 

 stitution. Acetic acid and its chlorine-substitution product 

 obviously belong to the same ''type," and similar relations 

 exist between other substances as well. If, therefore, the 

 phenomena of substitution were investigated in connection 

 with organic compounds in general, the result would be a 

 grouping of compounds free from all hypothesis, but based 

 on and exhibiting clearly their natural relationship. Such 

 were, in nuce, Dumas' views, on the basis of which the 

 celebrated * ' theory of types ' ' was gradually built up in 

 course of the fourth and fifth decades. The most impor- 

 tant contributions to the theory were made by Gerhardt, 

 Wurtz, Hofmann, and Williamson. Gerhardt realized that 

 Dumas' ideas were worthy of being developed, but he also 

 realized that this could not be done without the aid of the 



