Mr. J. Brown on the Molybdate of Lead. 257 



the quantity of which in 26*84< grs. was thus found to be 16*20 

 grs., which is equivalent to 60'35 per cent, of oxide of lead. 



The next thing to be obtained was the rest of the molybdic 

 acid. This was contained in the washings from the sulphuret 

 of lead in the form of tersulphuret of molybdenum. When 

 the solution was sufficiently concentrated, it was made slightly 

 acid by means of nitric acid : a brownish-coloured precipitate 

 fell down, which is tersulphuret of molybdenum. This was 

 then thrown on a filter and washed. It was then dried at 212° 

 and weighed : its weight was 3*37 grs. From this and the 

 previous quantity of molybdic acid the quantity per cent, was 

 calculated, which was 39*30 grs. 



According to this analysis, the composition of molybdate of 

 lead is — 



Molybdic acid . . 39*30 

 Protoxide of lead . 60*35 



99*65 



V. In the course of the preceding analysis it was observed 

 that the sulphohydret of ammonia exercised a powerful solvent 

 action on the mineral itself. The following new method of 

 successfully analysing this mineral was therefore adopted. 



23*0 grs., after being reduced to a very fine powder, were 

 digested with the aid of heat in sulphohydret of ammonia. 

 The solution became immediately of a deep red colour, 

 owing to the tersulphuret of molybdenum which was held in 

 solution by the sulphohydret of ammonia, while the lead was 

 precipitated as sulphuret, and fell to the bottom in the form 

 of a black powder. The clear supernatant liquor was then 

 drawn off, and a fresh portion of sulphohydret of ammonia 

 was added. This, after standing for some time, was thrown on 

 a filter, and washed with water containing sulphohydret of 

 ammonia. The tersulphuret of molybdenum passed through 

 in solution, while the sulphuret of lead remained on the filter. 

 When this was completely washed it was dissolved in dilute 

 muriatic acid, which took up the sulphuret of lead and left 

 the undecomposed matter along with the sulphur. These 

 were then thrown on a filter and washed: the whole jivas then 

 burnt. The sulphur was thus driven off^ while the insoluble 

 matter remained. The insoluble matter in 23 grs. amounted 

 to '24 gr., whilst in the former analysis it amounted to 1*15 

 in 23 grs. 



When the washings from the sulphur were sufficiently con- 

 centrated, the lead was precipitated by means of ammonia and 

 oxalate of ammonia. The oxalate of lead was then thrown 

 on a filter and weighed. The quantity of oxide of lead in 



Phil. Map. S. 3. Vol. 31 . No. 208. Oct. 1 847. S 



