CHROMATE OF LEAD. 191 



it is far more soluble. In the solution, either the sul- 

 phuric acid is precipitated; by a baryta-salt, or the lime 

 by an oxalate. 



The sulphate of lead is extracted from the residue 

 by digestion with tartrate of ammonia containing free 

 ammonia, and the lead precipitated from the solution by 

 sulphuretted hydrogen or chromate of potassa. 



The mixture of clay and sulphate of baryta which 

 remains at last, is heated with concentrated sulphuric 

 acid until the greater excess of the latter has been 

 expelled, when the sulphate of alumina is extracted 

 with water, and the alumina precipitated by ammonia. 



In order to extract the silica from the residue, 

 which contains also sulphate of baryta, it is boiled in 

 a concentrated solution of carbonate of soda, in which 

 the silica is dissolved and may be reprecipitated by 

 sal-ammoniac. The residue consists of carbonate of 

 baryta mixed with more or less undecomposed sul- 

 phate of baryta. (See No. 15.) 



A good method of determining the amount of 



Fig. 20. 



chromic acid (and therefore of chromate of lead con- 

 tained in a specimen of commercial chrome-yellow. 



