Dr Daubeny on separating Lime from Magnesia. 305 
thrown down with the former ; so that the quantity of precipi- 
tate afterwards afforded by phosphate of soda or ammonia, ne- 
cessarily falls short of that which would otherwise have been 
obtained. 
But we have no proof that the lime which has resisted the 
action of the carbonate of ammonia, and remains in the liquor, 
interferes with the operation of the alkaline phosphate on the 
remaining portion of the magnesia ; so that in this respect, the 
method appears to be as rigorous as can well be devised. 
Having said so much on the operation of the alkaline phos- 
phates upon magnesian salts, it might be expected that I should 
conclude my account by a statement of the composition of phos- 
phate of magnesia, as well as that of the triple phosphate, de- 
duced from my own experiments, as this has in either case been 
inferred hitherto rather from synthesis than analysis. Having 
met, however, with difficulties in my first attempts (which, I 
believe, are acknowledged by all who have been engaged on this 
genus of salts), I must decline for the present entering upon 
this part of the subject, only regretting that the same philoso- 
pher ^ who, by so ingenious a mode of operating, appears to 
have at length set at rest the long-agitated question with respect 
to the true composition of phosphoric acid, did not at the same 
time extend his analytical researches to that of the salts which 
it forms -f-. 
• See Sir H. Davy’s paper on Phosphoric Acid, in the Phil. Trans, for 1818. 
*f* * It may be as well to mention the method I pursued in attempting to ana,, 
lyse Phosphate of Magnesia, Which was as follows : 
Phosphate of Magnesia <as is well known) may be dissolved by the sulphuric, 
nitric, and muriatic acids, but chemists are divided as to whether any decomposi^ 
tion of the salt takes place or not. Dalton, in the Manchester Memoirs^ vol. v. 
(New Series), maintains that this is the case ; but the majority of chemists believe 
that the sulphuric acid decomposes, and that the others merely dissolve it. How- 
ever this may be, to effect a solution of this salt in one or other of the acids ap- 
pears the best preliminary to its analysis. I therefore attempted to precipitate the 
magnesia from its solution in acids by various re-agents. 
The fixed alkalies do not appear, however, to succeed, and ammonia precipi- 
tates anew the triple phosphate. By adding a solution of nitrate of lead to one 
confining 10 gr. of phosphate of magnesia dissolved in nitric acid, I obtained, af- 
ter some time, very regular tetraedral crystals of a salt, which, when its water of 
crystallization was driven off, was found to weigh 20.2 gr. On examination, they 
were found to correspond in chemical characters, as well as in crystalline form. 
