258 Mr. J. Brown on the Molybdate of Lead. 



22*76 grs. amounted to 13*71 grs., which is equivalent to 60*35 

 grs. per cent. 



The next point was to precipitate the tersulphuret of mo- 

 lybdenum. This was done by making the sohition in sulpho- 

 hydret of ammonia slightly acid by means of muriatic acid. 

 The tersulphuret went down in the form of a brownish-coloured 

 precipitate. This was then thrown on a filter, dried, ignited 

 and weighed. The quantity in 22*76 grs. was thus found to 

 be 991 grs., which is equivalent to 39*13 per cent, of molyb- 

 dic acid. 



The constituents therefore of molybdate of lead according 

 to this analysis are — 



Molybdic acid . . 39*19 

 Lead protoxide . . 60*23 



99*42 

 Phosphates and arseniates of lead were decomposed in the 

 same manner ; and it is evident this process would also answer 

 with antimoniates, vanadiates and seleniets. 



Note by Dr. R, D. Thomson. 



Test for Arseniates^ Sfc. — I may notice a simple and quick 

 method of testing minerals containing arsenic in its various 

 forms, phosphates, molybdates, vanadiates, &c. A few grains 

 of the mineral to be examined are to be finely pulverized in an 

 agate mortar and introduced into a test-tube, and boiled with 

 bisulphohydret of ammonia for a few minutes. The mineral is 

 partially decomposed ; the sulphuret of lead precipitates, while 

 sulphuret of arsenic, &c. is dissolved by the excess of the re- 

 agent. The tube is then allowed to stand at rest, and the 

 supernatant liquor poured off or filtered. The excess of bi- 

 sulphohydret of ammonia being removed by evaporation, the 

 yellow sulphuret of arsenic precipitates. A molybdate is 



• In these analyses the lead only was ascertained, and the deficiency 

 was taken as molybdic acid. 



The two last analyses were made by means of sulphohydret of ammonia, 

 the tln-ee preceding by nitric acid. 



