SPINEL. 37 



phoric acid is precipitated by ammonia and sulphate of 

 magnesia. 



The silicate of alumina is decomposed by concen- 

 trated hydrochloric acid, the mass evaporated to dry- 

 ness on the water-bath, the residue moistened with 

 hydrochloric acid, the alumina-salt extracted with water, 

 and the alumina precipitated by carbonate of ammonia. 



III. The weighed alumina containing phosphoric 

 acid is dissolved in concentrated nitric acid, and the 

 solution heated with about the same quantity (accu- 

 rately weighed) of pure tin (tin-foil). The mixture is 

 diluted with water, heated until boiling, and the bin- 

 oxide of tin which has combined with the whole of the 

 phosphoric acid is filtered off, washed, and ignited. 

 The difference between the weight of this precipitate 

 and that of the binoxide of tin which should have been 

 furnished by the metal employed represents the phos- 

 phoric acid. The alumina is then precipitated from 

 the solution by sulphide of ammonium. 



Or chloride of tin is added to the solution of phos- 

 phate of alumina, heated to boiling, and the oxide of 

 tin and all the phosphoric acid precipitated by sul- 

 phate of soda. If sesquioxide of iron is present, a 

 portion of it is thrown down. The accuracy of this 

 method is not yet determined. 



20. SPINEL. (ALUMINA AND MAGNESIA IN 

 GENERAL.) 



MgO,Al 2 3 . 



The mineral is very finely pulverized in a steel 

 mortar separated from the iron with hydrochloric acid, 

 and then fused with at least six times its weight of 

 4 



