the Carinthian Molybdate of Lead. 337 
degree is the yellow acid, or that which is supersaturated with 
oxygen. 
The affinity between molybdasna and oxygen is but weak, 
at least in respect to that portion which is required to consti- 
tute molybdic acid ; for it has been proved, in the course of 
these experiments, that considerable changes are produced by 
a very small difference in the proportions of the acids or al- 
kalies, and even by the degrees of heat. Scheele and Mr. 
Islmann have observed, that all of the metals, excepting gold 
and platina, are able in the humid way to deprive the molybdic 
acid of oxygen, so as to cause it to become blue; but here 
their effects appear to cease. M. Pelletier found that a 
solution of molybdic acid was turned blue when hydrogenous 
gas was passed through it. Mr. Klaproth has also remarked 
that light, under certain circumstances, changed the colour of 
molybdic acid to blue. And the effects of light appear in 
some measure to be connected with the following experiment. 
I made a solution of the molybdic acid, by digesting sul- 
phuric acid on molybdate of lead, and diluted it with an 
equal quantity of water. The solution was filtrated, and I 
then added a solution of hepar suiphuris until the brownish- 
red precipitate which was produced began to fall much paler. 
After this the liquor was filtrated, and was of a pale beer 
colour. 
I placed it accidentally in an open glass jar, on a shelf near 
a window, on which the sun shone during a great part of the 
day, and was surprised to observe that in about two days it 
began to assume a greenish tinge, which gradually became 
deeper ; on the third day it was of a full green, on the fourth 
it had a tinge of blue, on the fifth the colour was greenish- 
X x 2 
