502 M. Dumas on the Law of Substitutions^ 
genus, to the same chemical type : the others could not take 
their place there. What place can be properly assigned to 
these kinds of bodies ? 
We have not to wait long for a reply, and it carries the 
law of substitutions to a degree of generality and importance 
which it does not belong to me to develop here, but which 
the order of ideas obliges me to indicate. 
The admirable work of M. Regnault on the aethers has 
indeed given an unexpected development to the theory of 
types. There is nothing more natural than to class in one 
genus bodies which approach so near as acetic acid and chlor- 
acetic acid : but there must be good reasons for admitting 
that there is a true analogy between 
Marsh gas 
Methylic aether C* O H® 
Formic acid C'^ 
Chloroform Ch" 
Bromoform 0“^ Br® 
Iodoform F 
{ Q4 0 JJ4 
Ch2 
r c** o 
Bichloridated methylic aether < 
Perchloridated methylic aether O Ch^ 
Hydro-chlorate of methylene Ch^ 
Chloridated hydrochlorate of methylene {C-C 
f Ch^ H^ 
Bichloridated hydrochlorate of methylene 
Chloride of carbon C^ Ch^ Ch® 
Amongst these bodies, to which, without forcing anything, 
we might add prussic acid and ammonia, we meet with acids, 
bases, neutral bodies, and consequently substances the most 
unlike in an ordinary chemical view. 
M. Regnault admits, and, he purposes to prove, that all the 
bodies so unlike, chemically speaking, which this series con- 
tains, that all those which can be united in analogous series, 
have this in common, that they belong to the same mechanical 
system. 
I repeat that it is not for me to explain views which 
will be explained hereafter by their inventor, but I have to 
show in what these views differ from those which preceded 
them, to cause it to be felt how they complete what may now 
be called the general theory of substitutions. Moreover, it is 
