Dr Daubeny on separating Lime from Magnesia. S03 
present, I might have drawn a conclusion unfavourable to the 
use of phosphate of soda as a precipitant of magnesia, had it not 
been for the experiments before detailed ; but as these leave no 
doubt on this head, I am disposed to attribute the apparent er- 
ror to causes which it may be worth while briefly to mention. 
The first of these was the presence of a portion of carbona- 
ceous matter in the carbonate of ammonia employed, which 
being retained in chemical combination by the triple phosphate, 
or at least carried down along with it, proved the means of de- 
composing a portion of the phosphoric acid, when the precipi- 
tate was heated with the view of reducing it to the state of a 
simple phosphate of magnesia. 
The second was, I apprehend, my applying the phosphate in 
too diluted a condition to act readily on the ammoniaco-magne- 
sian carbonate, owing to which circumstance, a small portion of 
magnesia remained in solution, as was proved by the addition 
of pure potash, which rendered the liquor turbid, after the phos- 
phate of soda; had ceased to act, although the same re-agent, in 
a solution to which phosphate of soda had been applied in a 
more concentrated form, produced, as we have seen, no, effect 
whatever. 
But the principal cause of this apparent variation in the re- 
sults, is the uncertain composition of carbonate of magnesia, 
which, as Berzelius has shewn, retains a portion of its water, 
even after it has been exposed to an high temperature for a con- 
siderable time-j-. Hence, the real quantity of magnesia em- 
ployed, was, in these cases, always below that at which I had 
estimated it. 
*** Carbonate of ammonia has such an affinity for carbonaceous matter, in cer- 
tain states, that, even when repeatedly sublimed, it seems to carry up some along 
with it. Hence, in my experiments, whenever an impure carbonate of ammonia 
was employed, I always found a blackish matter in my solutions ; and Dr Kidd 
informs me, that he has found, that, in passing nitrous acid gas through a solution 
of ammonia, which he had considered tolerably well purified, a discoloration was 
produced, which he can only attribute to the actfon of the nitrous acid upon the 
bituminous or carbonaceous matter disengaged during the neutralisation of the 
ammonia. 
-|- See Berzelius’s paper “ On some Compounds which depend on Weak Affi- 
nities,’’ in this Journal^ vol. i. p. 63, &c. 
