ANALYSIS OP MINERAL WATERS. 267 



The decomposition of the magnesian salt by ammonia would 

 have the former advantage, as the muriate of ammonia would 

 be expelled at the end of the process by heat ; but this de- 

 composition, it is well known, is only partial. Sub-carbonate 

 of ammonia causes a more abundant precipitation of magnesia, 

 but still its action is likewise partial, a ternary soluble salt be- 

 ing formed after a certain quantity has been added. It seem- 

 ed probable, that this might be obviated, by adding the sub- 

 carbonate of ammonia as long as it occasioned any precipita- 

 tion, then evaporating the clear liquor to dryness, expelling 

 the muriate of ammonia, and any excess of ammonia, by heat, 

 re-dissolving, and again adding the sub-carbonate of ammonia 

 to decompose the remaining magnesian salt. Proceeding in 

 this way, I found that a copious precipitation took place on the 

 second addition, and even at the fourth a small quantity of 

 precipitate was thrown down. But the decomposition, after 

 all, was not perfect, for the quantity of magnesia obtained was 

 not equal to what was procured by other methods. 



Sub-carbonate of soda or potash has been usually employed 

 to precipitate magnesia from its saline combinations. The 

 precipitation, however, is only partial, unless an excess of the 

 precipitant be employed (and even then, perhaps, is not alto- 

 gether complete) ; and as this exces cannot easily be estima- 

 ted, it introduces a source of error in estimating the quantity 

 of muriate of soda at the end of the operation, against which it 

 is not easy to guard. 



The method proposed by Dr Wollaston, of precipitating 

 magnesia from its solution, by first adding carbonate of am- 

 monia, and then phosphate of soda, so as to form the insoluble 

 phosphate of ammonia and magnesia, is one much more per- 

 fect ; the whole of the magnesia appears to be precipitated, 

 and as a method, therefore, of determining the quantity of this 



L 1 2 base* 



