HEAVY SPAR, CELESTJNE, AND GYPSUM. 31 



nia, these will remain behind on igniting this preci- 

 pitate. From the solution, which should be filtered 

 rapidly, and with as little exposure to air as possible, 

 the lime is precipitated by oxalate of ammonia. 



The filter with the mercury-residue, which contains 

 the phosphoric acid, is well dried, and the contents 

 thrown into a platinum crucible in which they are 

 mixed with carbonate of potassa and soda ; the filter 

 is rolled up and buried in a mixture. The crucible 

 is now heated (but not to redness) under a chimney 

 with a good draught, until the mercury is volatilized, 

 after which the mass may be heated to redness and 

 fused. It is then dissolved in water, an excess of hy- 

 drochloric acid added, and the phosphoric acid precipi- 

 tated by ammonia and sulphate of magnesia. 



15. BARITE, CELESTITE, AND GYPSUM, 

 BaO, S0 3 . SrO, S0 3 . CaO, S0 3 + 2 HO. 



The water in gypsum is determined by ignition. 

 The salts of strontia and lime are converted into car- 

 bonates by action of a solution of carbonate of ammo- 

 nia at ordinary temperature, while the sulphate of 

 baryta remains unaltered. 



At a boiling heat or with carbonate of soda the 

 decomposition is not so complete. 



The mixed salts must be finely powdered and well 

 washed with cold water. Nitric acid dissolves the 

 strontia and the lime, but does not act upon the sulphate 

 of baryta. 



The latter can be decomposed by fusing with four 

 times its weight of carbonate of potassa and soda. 



The mass is then treated with boiling water, the 

 carbonate of baryta filtered off' while hot, and washed 

 with boiling water. 



