PYROMORPHITE. 63 



hind on treatment with nitric acid. After washing 

 and igniting, it is weighed and analyzed as in No. 15. 



Gypsum would also be in great measure left behind 

 on dissolving in nitric acid. It may, however, be en- 

 tirely dissolved and separated from any barite pre- 

 sent at the same time, by boiling with a large quan- 

 tity of dilute nitric acid. The amount of gypsum 

 present may be inferred from that of the sulphate of 

 baryta obtained by precipitating the solution with 

 chloride of barium. 



Sulphate of lead would also be left undissolved by 

 dilute nitric apid. After washing, it becomes black 

 when treated with sulphide of ammonium ; it is soluble 

 in tartrate of ammonia mixed with free ammonia. In 

 a mixture of sulphate of lead and sulphate of baryta, 

 the former may be converted, by digestion with sul- 

 phide of ammonium, into sulphide of lead, which can 

 be transformed into chloride by treatment with con- 

 centrated hydrochloric acid, and may then be dissolved 

 out by water. 



39. PYROMORPBITB. 

 3(3PbO,P0 5 )-r-PbCl* 



In many varieties, the chloride of lead is replaced 

 by chloride of calcium, in others, a part of the phos- 

 phoric acid is replaced by arsenic acid. The green 

 varieties contain traces of sesquioxide of iron and ses- 

 quioxide of chromium or chromic acid. 



Those specimens which are free from lime are finely 



* May be artificially obtained in crystals, by fusing in a porce- 

 lain crucible an intimate mixture of 1 part of fused phosphate of 

 soda, and 7 parts of chloride of lead ; the mass is very gradually 

 heated to about the fusing-pointof the latter ; it is then allowed to 

 cool, and the liquid portion decanted from the crystals. 



