VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 1 3 



molybdic acids combined with ammonia. It was first filtrated, and then gradually 

 evaporated. When evaporated to half the original quantity, the colour was green, 

 but towards the end of the operation it again became blue, and when evaporated 

 to dryness, the residuum was a whitish salt, tinged in some parts with blue. 



g. This salt was reduced to powder, and was put into a small glass retort, to 

 which a receiver was fitted. I then placed the retort in a small open furnace, and 

 gradually raised the fire till the bottom of it began to melt. The retort was now 

 removed, and the contents examined. The receiver contained some water, and a 

 small quantity of muriatic acid. Near the extremity of the beak of the retort 

 was some muriate of ammonia, with some fuming muriatic acid, and the re- 

 mainder of the tube was filled with a hard greyish-blue salt. In the retort was a 

 black pulverulent residuum. I collected all of the blue salt, and again sublimed 

 it, and again obtained muriatic acid, blue salt, and some of the black powder. 

 The blue salt was composed of muriate of ammonia combined with the acid, or 

 rather with a blue oxyde of molybdaena. 



h. The black residuum was put into a glass retort, and some nitric acid being 

 poured on it, it was exposed to a moderate heat. Nitrous fumes were discharged, 

 and when the distillation had been repeated, I found the whole of this black pow- 

 der converted into the citron-coloured molybdic acid. I had evident proof that in 

 this experiment a portion of the muriate of ammonia was decomposed by each 

 sublimation, and also that part of the molybdic acid was deprived of oxygen, and 

 remained in the retort, if not in the state of metal, at least combined with so small 

 a quantity of oxygen as to be nearly approaching to it.* 



Molybdate of lead with muriatic acid. — Exper. 1. 1 dr. of the ore was digested 

 with muriatic acid, and distilled water was added till the whole was dissolved, ex- 

 cepting a small residuum of siliceous earth. The solution was filtrated, and re- 

 peatedly evaporated, till muriate of lead was no longer separated. The muriate of 

 lead, when edulcorated, I found to be perfectly free from any other substance. 



I now saturated the acid solution, from which the muriate of lead had been 

 separated, with solution of carbonate of ammonia, and obtained a pale yellow 

 flocculent precipitate, which was well edulcorated. This precipitate immediately 

 dissolved in very dilute nitric acid, and with sulphuric acid I precipitated a small 

 portion of lead, after which, with prussiate of pot-ash, I separated a quantity of 

 iron. The solution, when saturated with the ammonia, was deep blue, and was 

 composed of muriatic acid, ammonia, and the blue oxyde of molybdasna, like 

 that mentioned in the former experiment. 



In the course of these experiments, I have observed that the full blue colour only 

 takes place at the precise moment of saturation, and if the alkali is even added to 

 a considerable excess, the colour does not sutler any further change; but if much 

 water is first added, the blue colour does not appear ; or if water is added after- 



* A black powder of a similar nature appears to have been obtained hy Scheele, when he distilled the 

 white molybdic acid with a small quantity of olive oil. Essays, p. 238. — Orig. 



