CHROMATE OF LEAD. 189 



washed with a dilute solution of that salt and ignited. 

 The following method is more certain. 



The mineral, which should be powdered as finely as 

 possible and weighed, is fused, in a platinum crucible, 

 with four times its weight of bisulphate of potassa, care 

 being taken that the mass, which froths up at first, 

 may not run over the side of the crucible. Ultimately, 

 it is heated to redness, and retained in fusion, at a red 

 heat, for a considerable time. The salts formed are 

 sparingly soluble in water and acids; the sesquioxide 

 of chromium must therefore be converted into alkaline 

 chromates, for which purpose there is added to the 

 cooled mass, in the crucible, about twice its volume of 

 a mixture of equal parts of nitre and carbonate of soda. 

 The mass is then heated to complete fusion. 



On cooling, the chromate of potassa is extracted 

 with hot water, the residue of sesquioxide of iron, alu- 

 mina and magnesia, thoroughly washed, dissolved in 

 concentrated hydrochloric acid, and analyzed as in No. 

 81. 



The solution of chromate of potassa is acidified 

 with hydrochloric acid, heated to ebullition, and al- 

 cohol added to the boiling solution until it has acquired 

 an emerald-green color, when the sesquioxide of chro- 

 mium is precipitated by ammonia, ignited and weighed. 



103. CHROMATE OF LEAD. 



PbO, CrO 3 . 



(Chrome-yellow, often adulterated with white clav, 

 with BaO, S0 3 , CaO, CO 2 , CaO, SO 3 , or with PbO, 

 SO,.) 



I. Pure chromate of lead should give the quantities 

 of oxide of lead and chromic acid calculated from the 

 formula. 



