VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 25 



portion of oxygen therefore quits the acid menstruum, and combines with the blue 

 oxyde, which then becomes molybdic acid ; but as soon as the heat is dissipated, the 

 cause of this augmentation of affinity ceases, and the acid menstruum receives again 

 the portion of oxygen from the molybdic acid, which then returns to the state of a 

 blue oxyde ; or if the heat is continued till the solution is distilled to dryness, the 

 residuum is the molybdic acid exactly in the same state as it was before the solution 

 was made; for the continuation of the heat enables it to retain the portion of oxygen 

 requisite to constitute a metallic acid. I do not therefore believe that the total 

 quantity of oxygen in the solution suffers alteration any further than that it is dis- 

 tributed in different proportions between the 2 acidifiable bases, sulphur and molyb - 

 daena, according to the temperature of the solution. 



As the affinity between azote and oxygen is comparatively weak, the metal 

 molybdaena effects a decomposition of the nitric acid, and acquires a sufficiency of 

 oxygen to become molybdic acid. But as the affinity between sulphur and oxygen 

 is greater than that of azote, and also under certain circumstances superior to 

 molybdaena, the latter requires the assistance of heat to be able to retain a full por- 

 tion of oxygen, and this increase of affinity lasts no longer than during the con- 

 tinuation of the heat. To corroborate this assertion, it will be proper to consider 

 the effects of muriatic acid on that of molybdaena, especially as the affinity between 

 the radical principle, or base of the muriatic acid, and oxygen, is known to be so 

 great, that no chemist has as yet been able to effect a separation of the constituent 

 principles. 



It has been mentioned, that molybdic acid when dissolved in muriatic acid, also 

 parts with some oxygen, and tinges the menstruum with a green colour. But heat 

 does not enable it to take back the oxygen, for it augments the effects of the mu- 

 riatic acid, which, when distilled, passes oxygenated into the receiver, and the 

 molybdic acid is converted into a bluish grey oxyde.* These effects clearly prove, 

 that heat in this case acts inversely to what it did when the nitric and sulphuric acids 

 were the menstrua. For then the increase of affinity was between molybdaena and 

 oxygen, but here it is in favour of the base of muriatic acid ; so that by the con- 

 tinuation of heat, the muriatic acid carries with it into the receiver a certain portion 

 of oxygen, which it has taken from the molybdic acid, and the latter is left in the 

 state of an oxyde. 



From this it appears that muriatic acid uniformly tends to deprive the molybdic 

 acid of a certain quantity of oxygen, and that heat produces a contrary effect on 

 this solution to that which it did on the one made with sulphuric acid ; and heat 

 and cold do not therefore produce a change of colour. I do not however believe 

 that muriatic acid acts thus constantly on all those metals which can be dissolved by 

 it ; on the contrary, there is a muriatic solution much resembling the sulphuric so- 

 lution of molybdic acid in the vicissitudes of colour which it exhibits by heat and 

 cold. 



* Elemens d'Hist. Nat. et de Chimie, par M. de Fourcroy, tome 2, p. 439. — Orig. 



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