the Carinthian Molybdate of Lead. 331 
The cause of this I believe to be a change produced by heat 
in the respective degrees 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 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 a gmentation of affinity ceases, and the acid 
menstruum receives again the portion of oxygen from the mo- 
lybdic acid, which then returns to the state of a blue oxyde ; 
or if the heat is continued until the solution is distilled to dry- 
ness, the residuum is the molybdic acid exactly in the same 
state as it was before the solution was made, for the continua- 
tion of the heat enables it to retain the portion of oxygen re- 
quisite 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 two acidifiable 
bases, sulphur and molybdaena, 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 oxy- 
gen is greater than that of azote, and also under certain cir- 
cumstances superior to molybdaena, the latter requires the 
assistance of heat to be able to retain a full portion of oxygen, 
and this increase of affinity lasts no longer than during the 
continuation of the heat. 
