422 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



The way was thus opened up for a development of the 

 theory of types, which soon culminated in the modem 

 theory of valency. 



Dumas's " types," like Laurent's " nuclei," were complex 

 organic structures, in which hydrogen could be displaced 

 by chlorine, or by a small group of atoms (such as the nitro- 

 group, NO 2 ), without causing the structure to collapse. To 

 Gerhardt mainly belongs the credit of introducing a series 

 of simple INORGANIC TYPES, from which complex organic 

 compounds could be derived by substituting organic radicals 

 (ethyl, benzoyl, etc.) for the hydrogen atoms of the simple 

 type substance. 



The ammonia-type (Hofmann, 1850). Of these simple 

 types, the AMMONIA TYPE was the first to be recognised 

 clearly. Wurtz in 1849 (Ann. Chem. Pharm., 1849, 71, 

 331) discovered the simple bases, methy famine t CH 3 'NH 2 , 

 and ethylamine, C 2 H 5 'NH 2 . These two volatile organic 

 bases showed a remarkable resemblance to ammonia, from 

 which they could be derived by replacing a hydrogen atom 

 by methyl or ethyl. Hofmann, in the following year, showed 

 that all the hydrogen atoms of ammonia could be displaced 

 by alkyl radicals : 



" I have indeed found, that aniline and the bases which 

 are analogous to it, under the influence of methyl, ethyl, or 

 amyl bromide, lose one or two equivalents of hydrogen, which 

 is replaced by the corresponding alcohol-radical. Under the 

 same conditions ammonia loses one, two, or three equivalents 

 of hydrogen, which are likewise replaced by a corresponding 

 number of equivalents of the radical " (Preliminary note in 

 Ann. Chem. Pharm., 1850, 73, 91. Published in full Phil. 

 Trans., 1850, 93-131 ; 1851, 357-398). 



Using modern formulae and writing 



C 2 H 6 =Et= Ethyl 

 C 5 H n = Ay = Amyl 

 C 6 H 5 =Ph = Phenyl 



