SOLUTION OP CARBONATE OP MAGNESIA. 
327 
which it is combined. In a dry atmosphere it looses a por- 
tion of the water, which may perhaps be considered as hygro- 
metrical, and at the same time loses its transparency ; but I 
find, as has already been remarked, that a temperature of 
212° expels only one portion, and that a high temperature is 
requisite to expel the three remaining proportions, and which 
are probably the strictly chemically combined water. 
It is also said that this compound is altered by the action of 
cold water, and by boiling water ; that, in one instance a solu- 
tion of bicarbonate of magnesia is formed, and an insoluble 
carbonate containing a smaller proportion of carbonic acid ; 
and in the other, that the same insoluble subcarbonate is pro- 
duced, but without the solution of bicarbonate, the proportion 
of carbonic acid required for this being expelled in the form 
of gas. The results of the trials I have made have not con- 
firmed either of these conclusions. It has appeared to me to 
dissolve both in hot and in cold water without undergoing 
any decomposition. I have not been able to obtain an insolu- 
ble subcarbonate of magnesia by acting on the prismatic salt 
by cold water, or carbonic acid gas from it by boiling water, — 
for instance, boiling it in distilled water in a retort connected 
with a mercurial pneumatic apparatus. It is true, that when 
this carbonate is thrown into hot water, there is a disengage- 
ment of air, but the air is common air mechanically entangled, 
not carbonic acid gas which had been chemically combined. 
Both the hot solution and the cold, on evaporation, yielded 
the prismatic compound. 1000 grains of water at the tempe- 
rature of 60°, appear capable of holding in solution about 4 
grains ; thus 326.6 grains of the solution of the carbonate after 
the excess of carbonic acid had been expelled by the air-pump, 
afforded, on spontaneous evaporation, 1.5 grain of crystalline 
salt. 
Whether this slight degree of solubility can be useful, con- 
sidering the qualities of the compound as a medicine, or whe- 
ther the crystalline spicular prismatic form which it assumes 
on separation of the excess of carbonic acid by which the car- 
bonate was brought into solution, can be injurious to the coats 
