503 
and the Theory of Mechanical Types, 
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- 
