4M M. Dumas on the of Substitutions^ 
the remainder of the elements of the acetic acid constitute 
marsh gas rather than anything else ? There are four equa- 
tions, which, at the low temperature at which the action takes 
place, are alike admissible. The acetic acid always furnish- 
ing carbonic gas, it may give besides, 
1. Carbon and hydrogen C'^ -f H^, 
2. Methylene and hydrogen 
3. Olefiant gas and hydrogen 
4. Marsh gas H®. 
Thus, when we only consult the general forces of chemistry, 
at least four suppositions are presented between which there 
is nothing to authorize a choice ; these are the possible ac- 
tions. When the consideration of the types is allowed to in- 
tervene, it chooses between these four possible actions the 
necessary action, that which will give rise to four volumes of 
a hydrocarburetted gas corresponding to chloroform, wTich 
it represents in this decomposition. 
It is not sufficient then to explain the decomposition of the 
acetic acid by the alkalies, by saying that those determine the 
formation of all the carbonic acid which can be produced ; 
we must besides give an account of the production of marsh 
gas. Now if four equations alike possible are presented, how 
shall we choose ? 
We see well that the notion of organic types meets the 
same difficulty as the law of substitutions. The types are 
sent back to the general forces of chemistry, as the substitu- 
tions w'ere sent back to the equivalents. 
The reply is then the same: when we put into play the ge- 
neral forces of chemistry only, the decomposition of acetic 
acid into carbonic acid and marsh gas is a possible fact. 
When we set out from the analogv which exists between 
acetic acid and chloracetic acid, it is a necessary fact. In the 
first case we should have perfectly understood that some 
carbon was deposited, that some methylene or olefiant gas 
was disengaged. In the second, some marsh gas must abso- 
lutely be disengaged. 
But it is very evident, that the production of carbonic gas, 
and of marsh gas by the decomposition of acetic acid by 
means of alkalies is a fact which does not shock the general 
ideas of chemistry, who explains herself by the play of the 
general affinities at her disposal. That need not have been 
demonstrated : a true fact is always possible. 
Thus in the view taken by general chemistry, marsh gas 
might be formed ; but viewed by the theory of organic types, 
it was necessary that that compound should be formed. 
