33° Mr. Hatchett's Analysis of 
difference, 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 two have any effect on the sul- 
phurated molybdaena, and that these two 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 com- 
municates oxygen to both, so as to convert the one into sul- 
phuric and the other into molybdic acid. 
The rapidity with which nitric acid oxygenates molybdaena, 
even to supersaturation, resembles the effects produced by the 
same acid on some other metallic substances, particularly tin ; 
for in both cases the acid ceases to acUas soon as the supersa- 
turation with oxygen is effected ; and on this account the nitric 
acid is unable to dissolve the molybdic acid. 
Before I proceed, I must observe that whenever a solution of 
the molybdic acid becomes blue, or tending towards that co- 
lour, 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 dissolve a considerable quantity of mo- 
lybdic acid; and the solution is always more or less of a blue 
colour according to the quantity which is 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 co- 
lour disappears, and returns again when the liquor becomes 
cold.* 
* If lead or any other metal is present, the blue colour is permanent. 
