Mr. Brown on the Analysis of Molybdate of Lead. 181 



and the filtered liquid neutralised by caustic ammonia. After standing for 

 twenty-four hours, a pale yellowish coloured precipitate fell down, which 

 was collected on a filter, washed, and dried. Its weight was then 4*20 

 grains. It had a yellowish colour, and when dissolved in hydrochloric 

 acid, gave a blue precipitate with yellow prussiate of potash. 



Part of the clear blue solution, which was composed of sulphate and 

 molybdate of ammonia, was then put into a retort and evaporated down, 

 the rest of the solution being added as the liquid in the retort evaporated. 

 The whole was then dried and strongly heated. In this manner all the 

 sulphate of ammonia was driven off, while the molybdate of ammonia 

 was decomposed into molybdic acid and ammonia — the former of which 

 remained in the retort. The molybdic acid then weighed 95 grains. 

 The sulphate of lead formerly obtained was then treated in the following 

 manner: — It was boiled with 4 ounces of carbonate of soda in solution; 

 the powder was then washed, and nitric acid, much diluted, was poured 

 on it. The whole dissolved, except a small quantity of silica, which was 

 thrown on a filter; this, when washed and dried, weighed *7 grain. The 

 acid was then exactly neutralised with caustic potash, which precipitated 

 the lead as oxide. This, when washed and dried, weighed 14600 grains. 



The oxide of lead was then dissolved in nitric acid, and sulphuric acid 

 was added. After standing for some time, the solution was filtered, and 

 the filtered liquor saturated with caustic ammonia; after standing, a 

 small quantity of peroxide of iron was precipitated, which, when filtered 

 and dried, weighed 1*0 grain. This, when added to the former quantity 

 of peroxide of iron, makes the quantity 52 grains, and the quantity of 

 oxide of lead 145* grains. 



The composition of 250 grains of molybdate of lead is therefore — 



Oxide of lead, 145*0 58-00, per cent. 



Molybdic acid, 95*0 38*00 — 



Peroxide of iron, 5'2., 2-08 — 



Silica, -7 '28 — 



2459 98-36 



If the iron and silica be subtracted as impurities, this analysis is very 

 correct. But the method is very tedious and inconvenient, and requires 

 very great care. 



III. The next person who turned his attention to this mineral was 

 Gobel.* 



100 grains of the mineral were digested with dilute hydrochloric acid, 

 with the assistance of heat. Upon cooling, the lead was deposited in the 

 form of chloride. These crystals were then collected together and dried. 

 The weight was found to be 72'5 grains, which is equivalent to 59 

 grains of oxide of lead. The solution, freed from lead, was evaporated to 



* Schweigger's Journal fur Chemie und Physik, xxxvii. 71. 



