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 

 6.4 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 G4 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. 



