328 M. Dumas on the Law of Suhstitutio7is^ ^c. 
These are the foresights, always justified by experience, 
which characterize the law of substitutions. If it be connected 
with the theory of equivalents, it then results that every 
chemical phaenomenon is represented by equivalents, and that 
the facts of substitution are chemical phsenomena ; that every 
possible event in chemistry is translated into the language of 
equivalents, and that after all a true fact must be a possible 
fact. In the same manner that the possible comprehends the 
true, in like manner, and not otherwise, the theory of equiva- 
lents comprehends the law of substitutions. 
So far, I have reasoned as if the law of substitutions only 
applied in reality to the substitution of hydrogen, which has 
furnished the first examples of it. But chemists know that 
in an organic substance not only can hydrogen undergo sub- 
stitution, but also oxygen and azote, of which it would be 
easy to cite numerous examples. 
Still more, we can cause carbon to undergo real substi- 
tutions, which sufficiently shows how artificial that classifi- 
cation of organic substances would be, which would rest 
solely on the permanency of the number of the equivalents of 
carbon in all the compounds of the same family. 
In an organic compound, all the elements may then be 
displaced, and others substituted for them in succession. Those 
which disappear most easily, abstraction being caused by 
certain conditions of stability which we cannot yet foresee, are 
those of which the affinities are the most energetic. This is 
why hydrogen is one of the easiest to subtract and have an- 
other element substituted for it ; this is why carbon is one of 
the most rebellious, for we know few bodies which can act 
upon carbon and not upon hydrogen. 
In fine, I add that the law of substitutions allows us not 
only to foresee the disappearance of a certain number, or of 
all the elements of the organic compound, and new elements 
being substituted for them, but also the intervention, playing 
the part of the same elements, of certain compound bodies. 
Thus, cyanogen, carbonic oxide, sulphuric acid, the 
binoxide of azote, nitrous gas, amidogen, and many other 
compound groups may intervene as the elements would do, 
take the place of hydrogen, and give rise to new bodies. 
The law of substitutions is then an almost inexhaustible 
source of discoveries. It guides the hand of the chemist 
who trusts to it, it rectifies his errors by showing him the 
cause of them ; and in a number of possible but uncertain 
actions, it points out some which are proximate, easy to pro- 
duce, and of the highest interest. 
This future, so rich in facts which may be realised, so full 
