270 A GENERAL FORMULA FOR THE 
In both experiments, the quantity of muriate of soda is ac- 
curately obtained, or as nearly so as can be expected. They 
correspond, too, as nearly as can be looked for, even in a repe- 
tition of the same experiment, in the quantity of magnesia 
which they indicate. To ascertain how far this corresponded 
with the real quantity, I converted 10 grains of the crystallised 
muriate of magnesia into sulphate by the addition of sulphuric 
acid, and exposed it to a low red heat; the product weighed 
G.4 grains, equivalent to 2.13 of magnesia. This may be re- 
garded as a perfect coincidence, and as establishing the accu- 
racy of the other results *. 
It thus appears, that phosphoric acid with an excess of am- 
monia may be employed to precipitate magnesia from its sa- 
line combinations ; and in a process such as the present, it has 
the advantage, that the muriate of ammonia formed, can be af- 
terwards volatilised by heat, and the quantity of any residual 
ingredient can of course be easily ascertained. Neutral phos- 
phate of ammonia would also have this advantage ; but it does 
not succeed, phosphate of magnesia not being sufficiently inso- 
luble. On adding a solution of phosphate of ammonia to a so- 
lution of sulphate of magnesia, the mixture became turbid in 
a minute or two, and in a short time a precipitate in crystal- 
line grains formed at the bottom and sides; but it was not 
considerable, and did not increase. Phosphate of ammonia, 
however, with an excess of ammonia, or with the previous ad- 
dition of carbonate of ammonia, may be employed with the 
same 
* According to the result of this last experiment, 100 grains of crystallised 
muriate of magnesia would give 64 of real sulphate of magnesia, composed of 
21.3 of magnesia, and 42.7 of sulphuric acid. This quantity of sulphuric acid is 
equivalent to 29.4 of muriatic acid. Hence 100 grains of this salt crystallised 
(in which state its composition, I believe, has not been determined) consist. of 
21.3 magnesia, 29.4 muriatic acid, and 49.3 of water.. 
