382 PHILOSOPHICAL TRANSACTIONS. [ANNO 1798. 



becomes disposed to vitrify, or to undergo a change in regard to the quantity of 

 oxygen with which it is combined ? 



But the results of the following experiments afford still more satisfactory infor- 

 mation, respecting the intensity of the heat generated in all cases where light is 

 absorbed, and the striking effects which, under certain circumstances, it is capable 

 of producing. The facility with which most of the metallic oxides are reduced, in 

 the dry way, by means of charcoal, shows that, at a certain high temperature, 

 oxygen is disposed to quit those metals, in order to form a chemical union with the 

 charcoal, or at least with some one of its constituent principles, if it be a compound 

 substance ; and hence I concluded, that gold might be revived, in the moist way, 

 by means of charcoal, from a solution of its oxide in water, were it possible, under 

 such circumstances, to comnwnicate to the charcoal, and to the oxide, at the same 

 time, a degree of heat sufficient for that purpose. To see if this might not be 

 done by means of light, I made, or rather repeated, the following very interesting 

 experiment. 



Exper. 9. Into a thin tube of very fine colourless glass, 10 inches long, and -^ 

 of an inch in diameter, closed hermetically at its lower end, I put as many pieces 

 of charcoal, about the size of large peas, as filled the tube to the height of 2 

 inches ; and, having poured on them as much of the aqueous solution of nitro- 

 muriate of gold as nearly covered them, exposed the tube, with its contents, to the 

 action of the direct rays of a very bright sun. In less than half an hour, small 

 specks of revived gold, in all its metallic splendour, began to make their appearance 

 here and there on the surface of the charcoal ; and in 6 hours the solution, which 

 at first was of a bright yellow colour, became perfectly colourless, and as clear and 

 transparent as the purest water. The surface of the charcoal was in several places 

 nearly covered with small particles of revived gold ; and the inside of the glass tube, 

 in that part where it was in contact with the upper surface of the contained liquid, 

 was most beautifully gilded. This gilding of the tube was very splendid, when 

 viewed by reflected light ; but when the tube was placed between the light and the 

 eye, it appeared like a thin cloud, of a greenish blue colour, without the smallest 

 appearance of any metallic splendour. From the colour, and apparent density of 

 this cloud, I was induced to conclude, that the gilding on the glass was less than 

 one millionth part of an inch in thickness. 



This interesting experiment was repeated 6 times, and always with nearly the 

 same result. The gold was completely revived in each of them, and the solution 

 left perfectly colourless ; in most of the experiments however the sides of the glass 

 were not gilded, all the revived gold remaining attached to the surface of the char- 

 coal. In two of these experiments, I made use of pieces of charcoal which had 

 been previously boiled several hours in a large quantity of distilled water, and which 

 were introduced wet, and hot, into the tube, and immediately covered by the solu- 

 tion, to prevent them from imbibing any air ; and, in different experiments, the 

 solution was used of different degrees of strength. I plainly perceived that the 



