ON DIFFERENT KINDS OF MAGNESIA, ETC. 
277 
respect similar to that which comes from London, and ca- 
pable of replacing the latter in the preparation of magne- 
sian draughts, which is not the case, as I have already 
said, when hydroxide of magnesia has been instantaneously 
prepared. 
Nothing is now more easy than to give an explanation 
of the different properties which the two varieties of 
hydroxide of magnesium just mentioned, present; there is 
between these two compounds the same difference as be- 
ween lime spontaneously slacked in the air, and lime sud- 
denly slaked by a sufficient quantity of water. Every one 
knows that the latter alone can be employed in masonry, 
because it alone is susceptible of producing solidifiable mor- 
tars. Is it necessary to observe that lime spontaneously 
slaked in the air is analogous to English hydroxide of mag- 
nesium and that ordinary slaked lime is analogous to hy- 
droxide of magnesium, prepared immediately ? No ; doubt- 
less. But, perhaps, it will be asked of me, how do you con- 
ceive that magnesia can be hydratedin the air, without evi- 
dently absorbing carbonic acid ? This is, because it is no 
true that oxide of magnesium has as much affinity for this 
acid as chemical books would lead us to believe ; it is be- 
cause this metallic oxide has a much greater affinity for 
water than for carbonic acid. Its affinity is even so small 
that calcined magnesia, kept in a dry place, may be pre- 
served in tact for several years, as I had an opportunity of 
proving, in 1829, at the Pharmacie Central, in concert with 
M. 0. Henry. I will say more: having passed a current of 
carbonic acid over dry oxide of magnesium, the proportion 
of carbonic acid absorbed was entirely inapplicable. Thus, 
it is certain, that calcined magnesia has a much greater 
affinity for water than for carbonic acid ; it is quite as cer- 
tain that this base absorbs carbonic acid only after having 
passed to the state of hydrate, and that, even then, this ab- 
sorption is infinitely less than had previously been sup- 
posed. The same phenomenon is presented with lime; this 
vol. x. — no. iv. 25 
