278 ON THE DIFFERENT KINDS OF MAGNESIA, ETC. 
base has also a greater affinity for water than for carbonic 
acid. This explains how it occurs that we never find the 
lime in mortars saturated with carbonic acid, however old 
they may be, as M. D'Arcet has proved. 
There is between lime and magnesia an analogy of che- 
mical actions, which has not sufficiently attracted the atten- 
tion of chemists, and which has long been impressed on my 
mind. The following is what I said cf it in 1828, in my 
little work on the solidification of balsam of copabia by 
magnesia: — 
" It is necessary to prolong the calcination of the magne- 
sia until this metallic oxide is almost insoluble in the acids ; 
in this state its causticity is extreme ; put on the hand, it 
burns it almost as promptly as oxide of calcium. 77 
Thus, then, it is to my mind indisputably demonstrated 
that there exists between magnesia and lime an extremely 
great analogy of chemical properties. There is a caustic 
magnesia, a quick magnesia, as there is a caustic lime and 
a quick lime ; and there exists a slaked magnesia, just as 
there exists a slaked lime. 
The chemical considerations which I have just made 
known, which are not very important at first sight, acquire 
a different interest when examined in a therapeutical point 
of view ; this is because caustic or quick magnesia, and hy- 
drated or slaked magnesia, present very different medical 
properties. 
Caustic magnesia, like caustic lime, can never be em- 
ployed in medicine, at least in a high dose; but, on the 
other hand, it is the only preparation of magnesia suitable 
for solidifying balsam of copabia. I know that several 
samples of balsam of copabia have been unjustly regarded 
as bad, and discarded as such, and this because the magne- 
sia employed for testing it was hydrated, and consequently, 
improper for the purpose. 
The following is my reason for saying that caustic mag- 
nesia should never be employed in a high dose: — First, be- 
cause it is more difficultly soluble in the acids of the stomach 
