VOL. LXXXV1II.] PHILOSOPHICAL TRANSACTIONS. 381 



hours ended in a deep purple ; while that in the plate b, which was kept in the dark, 

 retained the yellowish cast it had acquired from the solution, without the smallest 

 appearance of change. 



Exper. 7. A small parcel of magnesia alba, placed on a china plate, having been 

 moistened with the aqueous solution of the oxide of gold, and thoroughly dried in 

 a dark closet, was now exposed in this dry state, to the action of the direct rays 

 of a very bright sun. It had been exposed to this strongs* <ght above half an hour, 

 before its colour began to be sensibly changed ; and at uie end of 3 hours it had 

 acquired only a very faint violet hue. Being now thoroughly wetted with distilled 

 water, it changed colour very rapidly, and soon came to be of a deep purple tint, 

 approaching to crimson. 



Exper. 8. A piece of white taffeta ribband, which had been wetted with the solu- 

 tion, and thoroughly dried in the dark, was suspended in a clean dry phial of very 

 fine transparent glass ; and the phial, being well stopped with a dry cork, was ex - 

 posed to the strong light of a bright sun. After the ribband had been exposed, in 

 this manner, to the action of the sun's direct rays for about half an hour, there 

 were here and there some faint appearances of a change of its colour ; but it 

 showed no disposition to take that deep purple hue which the ribband had always 

 acquired, when exposed to the light in the preceding experiments. On taking the 

 ribband out of the phial, and wetting it thoroughly with distilled water, and exposing 

 it again, while thus wetted, to the sun's rays, it almost instantly began to change 

 colour, and soon became of a deep purple tint ; but though I examined the sur- 

 face of the ribband with the utmost care, and with a good lens, both during the ex- 

 periment and after it, I could not perceive the smallest particle of revived gold, nor 

 did I see any vestige remaining that appeared to indicate that any had in fact been 

 revived. This experiment was repeated several times, and always with results 

 which led me to conclude, what indeed was reasonable to expect, that light has 

 little effect in changing the colour of metallic oxides, as long as they are in a state 

 of crystallization. 



The heat which is generated by the absorption of the rays of light must neces- 

 sarily, at the moment of its generation at least, exist in almost infinitely small 

 spaces ; and consequently, it is only in bodies that are inconceivably small that it 

 can produce durable effects, in any degree indicative of its extreme intensity. 

 Perhaps the particles cf the oxide of gold dissolved in water, are of such dimen- 

 sions ; and it is very remarkable, that the colours produced, in some of my experi- 

 ments on white ribbands, by means of an aqueous solution of the oxide of gold, are 

 precisely the same as are produced from the oxide of that metal, by enamellers, in 

 the intense heat of their furnaces. 



As the colouring substance is the same, and as the colours produced are the 

 same, why should we not conclude that the effects are produced in both these cases 

 by the same means, that is to say, by the agency of heat ? or, in other words, and 

 to be more explicit, by exposing the oxide in a certain temperature, at which it 



