98 ©. Marignac—Chemical Equivalents and Atomic Weights. 
It is true that there was a time when atomic weights 
had to be changed, and it is doubtless, on this account, that 
atomic weights were dropped and equivalents adopted. Never- 
theless, the history of chemistry shows that for more than 
thirty years no changes have been judged necessary for well 
known bodies, and that those which have been admitted for 
elements, whose properties or whose combinations had previ- 
ously been raises nown, were so thoroughly justified by 
their chemical properties, that even the equivalents of these 
bodies have had to be modified. Such was the case for bis- 
state; on the specific heat of their combinations, or on isomor- 
phism as was done in the first instance by M. Regnault.* We 
may see by this that, on the score of invariability, the two 
systems are on a par. 
As to the advantage which results from the fact that equiva- 
lents express ratios of real chemical equivalence, in cases 
where they are not indicated by atomic weights, it would be 
an important one if chemical equivalence were indicated in all 
cases; but we know that this is not so. It is really not more 
difficult to conceive and to remember that an atom of oxygen 
is worth two of chlorine, and an atom of lead two of silver 
than to know that an equivalent of nitrogen is worth three of 
oxygen, and that two equivalents of aluminium are worth 
three of magnesium. So there is really no advantage, on these 
two heads, which can counterbalance those which I have 
shown for atomic notations. 
It may be said that the preceding is a contradiction of what 
I said before. I said that the system of equivalents presents 
conditions of invariability that are not presented by atomic 
weights. Further on I have shown that every change of atomic 
weight had necessitated a corresponding change in equivalents. 
If we look for the cause of this apparent contradiction, it 
seems to me that we shall be led to make an observation whic 
gives the kev to the discussion actually going on. It is that, 
in reality, if we keep out of sight every question as to the 
origin of the terms equivalents and atomic weights, there is no 
difference between the two systems, and the partisans of equiva- 
and occur with great frequency. 
* Annales de Chimie et de Physique, 1841, III, vol. i, p. 191. 
