ORGANIC SUBSTANCES. 669 
tion,” says Dumas, “ presupposes that all the elements can sup- 
plant and replace each other, that simpla can be substituted by 
compound bodies, so that not only oxygen, but also cyanogen, 
carbonic oxide, sulphurous acid, deutoxide of nitrogen, hypo- 
nitric acid, amidogen, or other compound bodies may occur as 
fundamental principles, take the place of hydrogen, and give rise 
to new substances.” 
Dumas has not succeeded in establishing clearly the funda- 
mental idea of these types. But he assumes two kinds of them, 
chemical types and mechanical or molecular types. 
I shall quote his own words :— 
“]. Experience shows us that a body may lose one of its 
elements and take up in its place another, equivalent for equiva- 
lent. This is the general fact of the theory of substitutions. 
“2. When this occurs, we may admit that its molecule 
always remains unchanged, forming a group, in which one of 
the elements simply occupies the space previously filled by an- 
other. This constitutes in my opinion a natural family. 
«3. Amongst the bodies which are formed by substitution, 
the greater portion evidently retains the same chemical character 
as an acid or a base, and in the same degree as it did before the 
change by substitution. Such bodies form, in my opinion, a 
chemical type, or speaking in the terms of natural history, 
belong to the same genus.” 
The chemical types therefore represent, according to Dumas, 
a kind of natural historical genus of compounds, which all agree 
in possessing an equal number of equivalents united in the same 
manner, and have the same properties. 
Here then are three generic characters. The first, or the 
number of equivalents, is quite easily and accurately determined 
by the atomic weight. The second, on the contrary, or the 
similar mode of union of the elements, is without any founda- 
tion, unless it can be discovered by the isomorphism of the com- 
pounds. Here then individual opinion has free scope to con- 
sider the simple atoms united together in the same manner or 
not. With regard to the ¢hird, Dumas appears originally to 
have considered the chemical characters of those bodies pro- 
duced by substitution as so little changed (in proof of which he 
mentions acetic and chloracetic acid), that he may have meant 
merely a general accordance in the chemical characters ; when 
however objections were raised to this, he replied, that bodies 
