338 Mr. Hatchett's Analysis of 
blue, and on the sixth day it was changed to a beautiful deep 
blue. 
The solution continued all the time to be transparent ; and 
although the vessel remained four weeks in the same situation, 
the blue colour suffered no further change. 
This solution much resembles that which Scheele disco- 
vered in the course of his experiments on manganese; but the 
apparent similar effects, I believe, are produced by opposite 
causes ; for the changes of colour in the alkaline solution of 
manganese appear to be effected by the absorption of oxygen, 
but those of the molybdic solution are caused by a diminution 
of the inherent quantity. 
The contrast of the causes which operate on the two solu- 
tions becomes the more evident when the effects which acids 
produce on them are considered ; for when a few drops of an 
acid are added to the solution of manganese, the changes of 
colour are accelerated, not merely by the neutralization of the 
alkali and consequent precipitation of the manganese, but (as 
I conceive) by the accession of oxygen either immediately 
from the acid, or from the atmosphere, which the manganese 
is better able to absorb when the disengagement of it from 
the alkali is thus assisted by the addition of the acid.* On the 
contrary, if nitric acid is dropped into the molybdic solution, 
the colour is immediately destroyed, in the same manner as 
in all the other blue solutions of molybdaena when oxygen is 
thus presented to them. 
The facility with which molybdaena parts with oxygen is 
evinced not merely in the humid way, for M. Pelletier 
found that molybdic acid yielded oxygen to arsenic when 
* Scheele’s Essays, p. 108. 
