VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 27 



or an alkali. The portion of molybdic acid which I detected in the nitric acid em- 

 ployed to purify the ore, and Mr. Klaproth's experiments made with the same acid, 

 prove the first, and the experiment mentioned in ^ 8 is a proof of the latter. The 

 phenomena which appeared in the last experiment throw some light on the effects 

 produced by nitric acid on molybdaena ; for when the sulphuric and muriatic solu- 

 tions of the molybdic acid were saturated with pot-ash or soda, they gradually 

 changed to yellowish green, and so on to blue, in proportion as the alkali was added; 

 but when nitric acid was added to the alkaline solution, the change of colour was 

 exactly the reverse of the former, for the changes were then blue, green, and yel- 

 low, in proportion to the quantity of nitric acid. 



The cause of these effects I conceive to be the different degrees of oxygenation of 

 the molybdaena ; for when the first portion of nitric acid was added, it rather com- 

 bined with the alkali than with the molybdic acid, and the latter was therefore in 

 some degree separated with a diminution of the original quantity of oxygen, and 

 consequently appeared as the blue oxyde in solution. After this, the 2d portion of 

 nitric acid began to oxygenate the blue oxyde, and therefore changed the colour of 

 the solution to green ; but the 3d addition of nitric acid acted immediately on the 

 oxyde, turned the solution yellow, and when assisted by heat, caused a quantity of 

 the yellow molybdic acid to be precipitated. The alkali however appears to have 

 impeded the complete separation of the molybdic acid, and retained a part of it to- 

 gether with the nitric acid, so as to form a yellow triple salt. When the sulphuric 

 and muriatic solutions of the molybdic acid are saturated with ammonia, triple salts 

 are formed, which are different in their properties from those which have been de- 

 scribed ; for the triple salts produced by ammonia are permanent, and do not 

 appear to be decomposed by evaporation. 



When iron is present in the acid solution, sufficiently diluted, it is precipitated 

 by ammonia free from molybdic acid, especially when the menstruum is the sul- 

 phuric acid. The affinity of the molybdic acid with muriate of ammonia is so great, 

 that by sublimation it even in part quits lead to unite with it, and then forms the 

 blue triple salt, from which the blue oxyde does not separate, but in proportion as 

 it is deprived of oxygen by the gradual decomposition of the ammonia caused by re- 

 peated sublimations. 



When the sulphuric solution saturated with ammonia is evaporated to a proper 

 degree, the triple salt crystallizes in the usual figure of the sulphate of ammonia, 

 but the colour is bluish green. When however the evaporation is continued to dry- 

 ness, a pale greyish-blue salt is left, and by distillation this salt is decomposed, after 

 the manner of the decomposition noticed in the sulphate of ammonia, and the 

 molybdaena remains in the form of a black powder, deprived of oxygen. 



I think it necessary here to observe, that when molybdaena is not in the metallic 

 state, it appears to suffer 4 degrees of oxygenation. The first is the black oxyde ; 

 the 2d is the blue oxyde ; the 3d is the green oxyde which, as it seems to be inter- 



e 2 



