Dr Daubeny on separating Lime from Magnesia. 117 
sulphate of magnesia in the latter solution prevented the de- 
composition of the last portions of sulphate of lime. 
At all events, the whole tenor of these experiments contro- 
verts the notion of sulphate of lime becoming more soluble in 
consequence of the presence of sulphate of magnesia, and there- 
fore removes that objection to Mr Phillips’ method. 
II. I proceed now to consider another of the methods used 
for separating the two earths, namely, that in which we employ 
the joint agency of carbonate of ammonia * and phosphate of 
soda,— a process which has received a sort of authority among 
chemists, from the distinguished name of Dr Wollaston, who 
first recommended it to public notice. Had that philosopher 
brought it forward as a precise method of determining the pro- 
portion of magnesia in a mineral body conjointly with lime, in 
those cases in which it enters as a principal ingredient into the 
composition of the mass, it might have seemed almost a super- 
fluous task to have undertaken to confirm his results, and a piece 
of presumption for me to controvert them. As, however, I am 
not aware that this method was proposed by him, except as a 
very delicate test to indicate the presence of minute quantities 
of magnesia, I thought it not unnecessary to inquire, whether it 
could also be employed with ease and certainty, for determining 
the precise proportions which this earth might bear to lime, pre- 
sent with it in the same solution. 
It is evident, that, for the latter purpose, it is as necessary 
that the carbonate of ammonia should be a complete precipitant 
of lime, as that the phosphate of soda should be so of magnesia, 
although, for the former intent, it may be sufficient, if we can 
depend merely upon the phosphate of soda. Now, with respect 
to the carbonate of ammonia, I have found, in all my experi- 
ments where this substance was employed, that a portion of 
lime remained unprecipitated, whether that earth was pre- 
— ; — ^ — ^ 
To avoid misapprehension, it may be as well to mention, that in this paper I 
have called by the name of Subcarbonate of Ammonia the salt sublimed by heat- 
ing together muriate of ammonia and carbonate of lime ; and by the term of Car- 
honate, the same salt after it has lost nearly all its pungency, by exposure to the air 
from twelve to twenty-four hours, in which time it seems to part with nearly half 
its weight. Neither salt, perhaps, is strictly speaking a definite compound. 
