CERITE. 159 



which must be concentrated by evaporation if too 

 largely diluted by the washings, is mixed with a boil- 

 ing saturated solution of sulphate of potassa, and 

 allowed to cool. Cerium, lanthanum and didymium 

 are thus precipitated as double sulphates, while iron, 

 &c., remain dissolved. The precipitate is filtered 

 offj and washed with a saturated solution of sulphate of 

 potassa. Crystalline crusts of sulphate of potassa are 

 placed in the filtrate, and in this way the remainder of 

 the double salt is precipitated. 



The precipitated salt is dissolved in the necessary 

 quantity of boiling water, with the addition of some 

 hydrochloric acid, and the bases precipitated from 

 the hot solution by an excess of caustic potassa. 

 (Ammonia precipitates basic salts.) Or the double 

 salt is mixed with pure lamp-black and starch 

 paste, covered with coarsely pulverized charcoal and 

 heated to redness for one hour. 



The sulphide of potassium formed is completely 

 washed out with water, the sulphide of cerium dis- 

 solved in nitric acid, evaporated to dryness and ignited. 



After ignition, they appear as a cinnamon-brown 

 powder. When converted into sulphates by diges- 

 tion with concentrated sulphuric acid, they give, 

 with water, a yellow solution, from which sulphate 

 of potassa precipitates a lemon-yellow mixture of 

 double salts. 



Another method of obtaining the oxide of cerium con- 

 sists in precipitating the original sulphuric solution, 

 while hot, with an excess of hydrate of potassa, wash- 

 ing the precipitate, and digesting it with an excess of 

 solution of oxalic acid, when the iron and lime are 

 dissolved, and the oxide of cerium, &o., left as white 

 oxalates, which are filtered off and washed. By igni- 

 tion in air, they are converted into the brown oxides. 



There is at present no method of accurately sepa- 

 rating these three oxides from each other. 



