VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 387 



a yellowish-brown, with a purple hue. In the experiment with the oxide of silver, 

 the inside of the phial, in the region where the oil reposed on the aqueous solu- 

 tion, was beautifully silvered, the revived metal forming a narrow metallic ring, ex- 

 tending quite round the phial ; and in both experiments small detached pellicles of 

 revived metal were visible in the oil, and adhered in several places to the inside of 

 the phial, forming bright spots, in which the colour of the metal, and its peculiar 

 splendour, were perfectly conspicuous. 



Eocper. 15. As carbon is one of the constituent principles of spirit of wine, as 

 well as of essential oils and sulphuric ether, I thought it possible that I might 

 succeed in the reduction of the oxide of gold, by mixing alcohol, with an aqueous 

 solution of nitro-muriate of gold, and exposing the mixture, in a phial well closed, 

 to the heat of boiling water ; but the experiment did not succeed. By pouring 

 upon this mixture a small quantity of oil of olives, and exposing it again to the 

 heat of boiling water, the gold was revived. 



Is it not probable, that the reason why the oxide was not reduced by alcohol, is 

 the mobility of those elements, which ought to act on each other, in order that 

 the effect in question may be produced ? I have no doubt but the oxide would be 

 reduced, could the alcohol be made to rest on the surface of the aqueous solution, 

 without mixing with it. I wished to have been able to have collected and examined 

 the elastic fluids, which probably were formed in most of the preceding experiments; 

 but my time was so much taken up with other matters, that I had not leisure to 

 pursue these investigations farther. In order to see what effects would be produced 

 by the heat generated at the surface of an opaque body, of a nature different from 

 those hitherto used in the reduction of the metallic oxides, and one that is little 

 disposed to form a chemical union with oxygen, (magnesia alba,) when, being im- 

 mersed in an aqueous solution of the oxide of gold, the rays of the sun were made 

 to impinge on it, I contrived the following experiment. 



Exper. l6. I took 4 small thin phials, a, b, c, and d, of very fine glass, and 

 putting into each of them about 5 grains of dry magnesia alba, I filled the phial a 

 nearly full with a saturated aqueous solution of the oxide of gold. I filled the 

 phial b, in like manner, with some of the same solution, diluted with an equal 

 quantity of distilled water ; and the phials c and d were filled with the solution still 

 further diluted. These phials, open or without stoppers, were exposed one whole 

 day to the action of the direct rays of a bright sun, their contents being often well 

 mixed together, during that time, by shaking. 



The contents of all these phials changed colour, more or less, but they acquired 

 very different hues. The contents of the phial a became of a very deep rich gold 

 colour, approaching to orange, the earthy sediment being throughout of the same 

 tint. The contents of the phial b, which were at first of a light straw colour, 

 first changed to a light green, and then to a greenish blue. The phial having been 

 suffered to stand quiet several days in an uninhabited room, in a retired part of the 

 house, the solution became nearly colourless, and the sediment was found to be of 



3 D 2 



