978 Ik Colquhoun oH tJie Assay 



be saturated with muriatic acid, then mixed with a httle phos- 

 phate of soda and an excess of carbonate of ammonia, and next 

 evaporated to dryness by the appHcation of a very gentle heat. 

 The whole of the magnesia combines, in this process, with 

 phosphoric acid and ammonia, and forms with them a double 

 subsalt ; which, being insoluble or very nearly so, may be se- 

 parated by treating the saline residue with water. By calcina- 

 tion in a low red heat, it is converted into neutral phosphate of 

 magnesia, 100 parts of which contain about 35.7 parts of mag- 

 nesia. 



When the quantity of oxide of manganese contained by the 

 ore is inconsiderable, it is a preferable system of manipulation 

 to precipitate it along with the magnesia by carbonate of pot- 

 ash. Let the total weight of this mixed precipitate of magne- 

 sia and the oxide, after calcination, be determined : then re- 

 dissolve it in muriatic acid, and precipitate the manganese in 

 the usual manner by hydrosulphuret of ammonia. The weight 

 of the oxide, deducted from that of the original precipitate, in- 

 dicates accurately the quantity of the magnesia. 



The portion of the ore which remains undissolved after di- 

 gestion in muriatic acid is next to be examined : in general, it 

 will be found to consist of a mixture of carbonaceous and earthy 

 matter. To ascertain the quantity of the carbonaceous ingre- 

 dient, the residue should be dried in a strong sand-bath heat, 

 in a platinum crucible ; then, after being weighed, it should be 

 calcined under free access of the atmospheric air, until the 

 whole of the combustible matter is burnt off. Its amount will 

 of course be indicated by the loss of weight sustained in the 

 calcination. 



The residue is to be analyzed by the methods which are cus- 

 tomarily employed in the examination of an earthy mineral. 

 Thus, it may be decomposed by fusion in the furnace in a pla- 

 tinum crucible along with thrice its weight of dry carbonate of 

 soda, the fused mass may then be dissolved in very dilute mu- 

 riatic or nitric acid, the solution evaporated to dryness, and the 

 residue digested in acidulated water, which will take up the 

 earths in the state of saline combinations, while the silica will 

 be left undissolved. Separate the solution from the silica by 

 rne^ns of a filter, and mix it with a rather smaller quantity of 



