24 PHILOSOPHICAL TRANSACTIONS. [ANNO 179$. 



striated yellowish mass, which dissolved in water without leaving any residuum. 

 This solution resembled the former in every respect.. 



§ J 3. Molybdic acid with, sulphur. — To remove every doubt concerning the nature 

 of the yellow acid obtained by the analysis, I made the following experiment. I 

 put 20 grs. of the yellow acid and 100 grs. of sulphur into a small glass retort, and 

 continued the distillation till the bottom began to melt. The residuum was a black 

 substance, which was greasy to the touch, stained the fingers black, communicated 

 to them a shining metallic lustre, and had all the other properties of the mineral 

 known by the name of Molybdaena. I afterwards distilled this black matter with 

 nitric acid, which converted it into a yellow powder, similar in appearance and pro- 

 perties to the molybdic acid which had been originally employed. 



§ 14. General Observations. — It has been proved in the course of this paper, that 

 molybdate of lead can be decomposed in the humid way by the fixed alkalies, though 

 these have no effect when boiled with molybdaena mineralized by sulphur.* The 

 state of the molybdaena in the 2 substances appears to be the cause of this differ- 

 ence, for in the former it is oxygenated, but in the latter I conceive it to be in the 

 state of metal. From the experiments of Scheele it also appears, that of all the 

 known acids only 2 have any effect on the sulphurated molybdaena, and that these 

 2 are the nitric acid, and that of arsenic. The latter however seems rather to act 

 on the sulphur than on the molybdaena ; but the former communicates oxygen to 

 both, so to convert the one into sulphuric and the other into molybdic acid. 



The rapidity with which nitric acid oxygenates molybdaena, even to super- 

 saturation, resembles the effects produced by the same acid on some other metallic 

 substances, particularly tin ; for in both cases the acid ceases to act as soon as the 

 supersaturation with oxygen is effected; and on this account the nitric acid is unable 

 to dissolve the molybdic acid. Before proceeding I must observe that whenever a 

 solution of the molybdic acid becomes blue, or tending towards that colour, it is 

 a certain sign that the molybdic acid has suffered a diminution of oxygen. A variety 

 of facts which prove this, have been already brought forward in the different experi- 

 ments contained in this paper ; and I shall soon have occasion to mention others. 

 Sulphuric acid can disolve a considerable quantity of molybdic acid ; and the solution 

 is always more or less of a blue colour according to the quantity dissolved ; and the 

 blue colour proves that the molybdic acid has parted with a portion of oxygen ; but 

 if the solution be heated, the blue colour disappears, and returns again when the 

 liquor becomes cold.-f~ 



The cause of this I believe to be a change produced by heat in the respective de- 

 grees of affinity which prevail between the metallic base and oxygen, and between 

 the base of the acid menstruum and oxygen ; so that when the solution is heated, 

 the affinity between the blue oxyde of molybdaena and oxygen is increased, and a 



* Scheele's Essays, p. 230 j and Mem. sur la Molybdene, par M. Pelletier. Journ. de Phys. 1785, 

 D. 437. — Orig. 



+ When lead or any other metal is present, the blue colour is permanent. — Orig. 



