Mr. J. Brown on the Molyhdate of Lead, 255 



of which remained in the retort: the molybdic acid then 

 weighed 95 grs. The sulphate of lead formerly obtained was 

 then treated in the following manner ; — It was boiled with four 

 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 gr. 

 The acid was then exactly neutralized with caustic potash, 

 which precipitated the lead as oxide : this, when washed and 

 dried, weighed H'6'00 grs. 



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 

 NH3 : after standing for some time a small quantity of per- 

 oxide of iron was precipitated, which when filtered and dried 

 weighed 1*0 gr. This, when added to the former quantity of 

 peroxide of iron, makes the quantity 5*2 grs., and the quantity 

 of oxide of lead 145 grs. 



The composition of 250 grs. of molybdate of lead is there- 

 fore — 



24-5-9 98-36 



If the iron and silica be subtracted as impurities, this ana- 

 lysis is very correct; but the method is very tedious and in- 

 convenient, and requires very great care. 



III. The next person who turned his attention to this 

 mineral was Gobel*. 



100 grs. of the mineral were digested with dilute hydro- 

 chloric 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 then found 

 to be 72" 5 grs., which is equivalent to 59 grs. of oxide of lead. 

 The solution freed from lead was evaporated to dryness : when 

 perfectly dry a small quantity of nitric acid was added, and 

 the solution was again dried. It was then heated to redness 

 in a close vessel and weighed : its weight was thus found to 

 be 40'5 grs. 



J 00 grains contain therefore — 



♦ Schweigger's Journal fur Chemie und Phi/sik, vol, xxxvii, 71 ♦ 



