and the Theory of Mechanical Types. 503 



easy to class these ideas in the clearest manner, by the three 

 following propositions : — 



1. Experience proves that a body may lose one of its ele- 

 ments and take another in its place, equivalent for equivalent; 

 this is the general fact of substitutions. 



2. When a body is modified in this way, we may admit 

 that its molecule has always remained intact, forming a group, 

 a system in which one element has taken the place of another 

 purely and simply. 



In this point of view, which is altogether mechanical, and 

 which is that of which M. Regnault pursues the study, all the 

 bodies produced by substitution present the same grouping 

 and may be referred to the same molecular type. I look upon 

 them as constituting a natural family. 



3. Amongst the bodies produced by substitution, there are 

 a great number which evidently keep the same chemical cha- 

 racter, acting the part of acid or of base in the same manner 

 and in the same degree as they did before the modification 

 they have undergone. 



These are the bodies which I have considered as belonging 

 to one chemical type, as making a part of the same genus, to 

 speak the language of naturalists. 



Thus is explained the law of substitutions, thus do we give 

 an account of the circumstances in which it is not observed. 



Every time that a body is modified without departing from 

 its molecular type, it is modified according to the law of sub- 

 stitutions. 



Every time that a body in becoming modified passes into 

 another molecular type, the law of substitutions ceases to be 

 observed in the action which ensues. Blue indigo is not a 

 body of the same type as white indigo ; the perchloride of 

 carbon does not belong to the type of olefiant gas; aldehyd 

 has departed from the type of alcohol ; hydrated acetic acid 

 does not belong to the type of aldehyd, &c. 



The Academy will observe how, in this long series of re- 

 searches, which has required six years of labour and the con- 

 currence of the most skilful French chemists, we have risen 

 from an obscure corner of the science, gradually and by the 

 force of experience only, to the most general ideas of natural 

 philosophy. 



I admit then that through all the substitutions that a com- 

 pound molecule can have undergone, when for all its elements 

 others have been substituted successively, so long as the mole- 

 cule remains intact the bodies obtained always belong to the 

 same natural family. 



When through the effect of a substitution, a body is trans- 



