^4 Mr. J. Brown on the Molybdate of Lead. 



then added to the solution, which Immediately became of a 

 fine blue colour. When sufficiently concentrated, a quantity 

 of molybdic acid separated. The solution was then evapo- 

 rated to dryness, and the molybdic acid remained in the form 

 of a fine citron-yellow powder, which when completely dried 

 weighed 34<-25 grs. 



The constituents therefore of 100 parts of the purest cry- 

 stals of Carinthian molybdate of lead are — 



Oxide of lead . . 64<*42 59'59\ corrected from the 



Molybdic acid . . 34*25 34--25J chloride. 



As Klaproth did not know the true composition of chloride 

 of lead, the quantity of PbO given above is wrong. Calcula- 

 ting the quantity of oxide from the quantity of chloride which 

 he obtained, we get 59*59 per cent, of oxide of lead, which is 

 very near the theoretical quantity, or 60'57. But the great 

 error is in the molybdic acid. What Klaproth considered as 

 silica, was very probably molybdic acid, as that acid is not 

 entirely soluble in hydrochloric acid; and as he apparently 

 deducted this as impurity, he gets too little molybdic acid. 

 He also does not mention how he washed out the molybdic 

 acid from the chloride of lead. It could not well have been 

 done with water, for chloride of lead is soluble to a great ex- 

 tent. This is a great point of imperfection in the analysis. 



II. This mineral was next subjected to a close examination 

 by Charles Hatchett, Esq., whose analysis is recorded in the 

 Philosophical Transactions (vol. xviii.), of which the following 

 is a summary : — 



250 grs. of the ore, freed from as much impurity as pos- 

 sible, were put into a glass flask, and digested with sulphuric 

 acid for some time under a strong heat. When the solution 

 cooled, the clear liquor was drawn off", and the residual sul- 

 phate of lead washed by subsidence. This process was repeated 

 several times. The acid solutions were then filtered, and the 

 filtered liquid neutralized 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 grs. 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 sul- 

 phate 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 am- 

 monia was driven off, whilst the molybdate of ammonia was 

 decomposed into molybdic acid and ammonia, the former 



