PREPARATION OF CITRIC AND TARTARIC ACIDS>^ .I^^j. 
ly the case in the preparation of tartaric acid ; but the theory 
of its cause is so nearly identical that it is not necessary to 
describe it. 
Mr. Philips, junior, has shown (see the first number of this 
Journal,) that this tendency of carbonic acid to form soluble 
super-salts is commonly the occasion of a considerable loss in 
the preparation of the saccha rated carbonate of iron. 
I may be permitted, in conclusion, to notice, that it is this 
property of carbonic acid which most probably influenced the 
compilers of our pharmacopoeias to direct the solutions of sul- 
phate of magnesia and carbonate of potash to be mixed boiling, 
in the manufacture of carbonate of magnesia. The magnesia 
alba, or carbonate of magnesia of commerce, is a sub-carbon- 
ate, consisting of a combination of neutral carbonate of magne- 
sia and hydrate of magnesia ; the neutral carbonate of magnesia 
is a crystallisable salt, which may be frequently observed 
depositing in the bottles containing solutions of the super-car- 
bonate introduced by Sir James Murray, and so much employ- 
ed in pharmacy at present. This neutral carbonate is decom- 
posed when put into water ; if the water be cold, it is decom- 
posed into magnesia alba and bicarbonate ; if the water be 
boiling, it is resolved wholly into magnesia alba, or the sub- 
carbonate, the excess of carbonic acid being expelled. 
Now, precisely a similar series of changes occur when solu- 
tions of sulphate of magnesia and carbonate of potash are mix- 
ed together. These neutral salts yield by double decomposi- 
tion the materials for producing neutral salts. But the neutral 
carbonate of magnesia is decomposed by the water, and it de- 
pends on the temperature whether the whole of the magnesia 
be precipitated as a sub-carbonate, or a portion of it be wash- 
ed away in the form of soluble bicarbonate. — Ibid, from the 
Dublin Hospital Gazette. 
