118 HEAVY CARBONATE AND CALCINED MAGNESIA. 
formed, down to dryness, and the density of the carbonate 
of magnesia was increased ; but when the sulphate of am- 
monia and carbonic acid were expelled by calcination, the 
magnesia in this case also lost its density. It then struck 
me, that as the density of the carbonate of magnesia had 
been increased by evaporating it with the salt formed to 
dryness, that if the residue could be heated without expel- 
ling it, the density would be retained. In order to do this 
by the Pharmacopoeia process, one atom, or 123 parts, ot 
crystallized sulphate of magnesia was boiled with one atom, 
or 144 parts, of crystallized carbonate of soda, the whole 
was evaporated to dryness, and a carbonate of magnesia, 
of nearly equal density with that formed by the sesquicar- 
bonate of ammonia, was left. The residual salts were then 
calcined, and afterwards allowed to remain in water until 
all the sulphate of soda was dissolved out. The magnesia, 
when washed, was found to be of the required density. 
In the directions of the Pharmacopoeia, the carbonate of 
magnesia is ordered to be calcined for two hours. I found 
that half an hour, at a good bright heat, all the carbonic 
acid was expelled. The sulphate of magnesia of commerce 
sometimes contains a very small quantity of a persalt of 
iron; which quantity, however small, will give a reddish 
tinge to the calcined magnesia. This may be got rid of by 
dissolving the sulphate of magnesia in water, and adding 
lime-water until the color of reddened litmus paper is re- 
stored; the solution must then be allowed to stand for 
twenty-four hours, when peroxide of iron, and any excess 
of lime which may have been employed, will be precipi- 
tated. The most delicate test that can be employed for the 
presence of iron is the ferrocyanide of potassium. I may, 
perhaps, also state, that I have found the sulphate of mag- 
nesia, which is sold in the shops, generally adulterated with 
sulphate of soda, and that, if pure, 100 grains of it ought to 
yield 16.2 of calcined magnesia. 
Pharmaceutical Journal. 
