28 PHILOSOPHICAL TRANSACTIONS. |~ANNO I796. 



mediate between an oxyde and an acid, I am inclined to call molybdous acid, ac- 

 cording to the distinction made by the new Nomenclature ; the last and 4th degree 

 is the yellow acid, or that which is supersaturated with oxygen. 



The affinity between molybdaena and oxygen is but weak, at least in respect to 

 that portion which is required to constitute 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 alkalies, 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 hydro- 

 genous 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 sulphuric 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 sulphuris till 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 surprized to observe that in about 2 days it began to assume a greenish tinge, 

 which gradually became deeper ; on the 3d day it was of a full green, on the 4th it 

 had a tinge of blue, on the 5th the colour was greenish-blue, and on the Oth day it 

 was changed to a beautiful deep blue. The solution continued all the time to be 

 transparent ; and though the vessel remained 4 weeks in the same situation, the 

 blue colour suffered no further change. This solution much resembles that which 

 Scheele discovered in the course of his experiments on manganese ; but the apparent 

 similar effects, I believe, are produced by opposite causes ; for the changes of co- 

 lour 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 in- 

 herent quantity. 



The contrast of the causes which operate on the 2 solutions 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 pre- 

 cipitation of the manganese, but, as I conceive, by the accession of oxygen either 

 mmediately 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, when 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 



