LECTURE IX.] HISTORY OF CHEMISTRY. 165 



upon which Dumas erects his theory of types. 33 Thus, accord- 

 ing to him, all substances which contain the same number of 

 equivalents combined in the same way, and of which the chief 

 characters are similar, belong to the same chemical type. 

 These are, for the most part, compounds which can be obtained 

 from one another by very simple reactions, such as acetic acid 

 and chloracetic acid ; chloroform, bromoform, and iodoform ; 

 ethylene and the products arising from it by substitution by 

 means of chlorine. 



Dumas thinks he has found, in the conception of the 

 chemical type, the basis of a new classification, which includes 

 the recently observed facts ; but he employs, at the same time, 

 the molecular type introduced by Regnault, 34 which he calls 

 also the mechanical type. To this type the following com- 

 pounds belong : 



Marsh gas C 2 H 2 H 6 



Methyl ether - C 2 O H 6 



Formic acid - - C 2 H 2 O 3 



Chloroform - C 2 H 2 C1 6 

 Chloride of methyl C 2 C1 2 H 6 



Chloride of carbon C 2 C1 2 C1 6 



These substances, which may be regarded as arising by 

 substitution from one another, and which may possess very 

 different properties, are classed in one natural family. The 

 point of view which led to the establishment of Regnault's 

 types, is a much more comprehensive one than that from 

 which Dumas was induced to advance his chemical types ; the 

 substances embraced under the latter heading constituting 

 merely a subdivision of those which must be classed under the 

 same mechanical type. Dumas also sees this clearly, for he 

 says : 35 " On every occasion when a substance undergoes 

 change without quitting its molecular type, it is changed in 

 accordance with the law of substitution. On every occasion 

 when a substance passes, on undergoing modification, into 



33 Annalen. 33, 259; 35, 129 and 281 ; 44, 66. 34 Ibid. 34, 45. 

 35 Ibid. 33, 279. 



