On the Chemical Prtperties aitrihuted to Light, 403 



moiftened with the aqueous folution of the oxide of gold, and thoroughly dried in a dark 

 clofet, was now expofed, in this dryjiate, to the aftion of the dire<5t rays of a very bright 

 ■fun. 



It had been expofed to this ftrong light above half an hour, before its colour began to 

 hcfcnftbly changed \ and, at the end of three hours, it had acquired only a very faint violet 



hue. 



Being now thoroughly wetted with diftilled water, it changed colour very rapidly, and 

 foon came to be of a deep purple tint, approacliing to crimfon. 



Experiment No. 8. A piece of white taffeta riband, which had been wetted with the 

 folution, and thoroughly dried in the dark, was fufpended in a clean dry phial of very fine 

 tranfparent glafs ; and the phial, being well flopped with a dry cork, was expofed to the 

 ftrong light of a bright fun. 



After the riband had been expofed, in this manner, to the aflion of the fun's direft 

 rays about half an hour, there were here and there fome faint appearances of a change of 

 its colour ; but it fliowed no difpofitlon to take that deep purple hue which the riband had 

 always acquired, when expofed to the light in the preceding experiments. 



On taking the riband out of the phial, and wetting it thoroughly with diftilled water, 

 and expofing it again, while thus wetted, to the fun's rays, it almoft inftantly began to 

 change colour, and foon became of a deep purple tint ; but, though I examined the 

 furface of the riband with the utmoft care, and with a good lens, both during the 

 experiment and after it, I could not perceive the fmalleft particle of revived gold, nor did 

 I fee any veflige remaining that appeared to Indicate that any had in fa£1: been revived. 



This experiment was repeated feveral times, and always with refults which led me to 

 conclude, (what indeed was reafonable to expeft,) that light has little efFeil in changing 

 the colour of metallic oxides, as long as they are in a J] ale of cryjlalli-zation. 



The heat which is generated by the abforption of the rays of light muft neceflarily, at the 

 moment of its generation at leaflr, exift in almoft infinitely fmall fpaces ; and confequently, 

 it is only in bodies that are inconceivally fmall that it can produce durable effedls, in any 

 degree indicative of its extreme intenfity. 



Perhaps the particles of the oxide of gold diffblved in water are of fuch dimenfions ; 

 and it Is very remarkable, that the colours produced, in fome of my experiments on white 

 ribands, by means of an aqueous folution of the oxide of gold, are preclfely the fame as 

 are produced from the oxide of that metal, by enamellers, in the intenfe heat of their 

 furnaces. 



As the colouring fubft:ance is the fame, and as the colours produced are the fame, why 

 fhould we not conclude that the effe£i:s are produced in both thcfe cafes by the fame means, 

 that is to fay, by the agency of heat ? or, in other words, and to be more explicit, by 

 expofing the oxide in a certain temperature, at which it becomes difpofed to vitrify, or ta 

 undergo a change in regard to the quantity of oxygen with which it is combined ? 



But the refults of the following experiments afford ftill more fatisfa£lory information,, 

 refpefting the intenfity of the heat generated in all cai'es where light is abforbed, and the 

 ftriking effedts which, under certain circuraftances, it is capable of producing. 



The facility with which moft of the metallic oxides are reduced, in the dry way, by 



means 



