Chemical Properties attributed to Light. 81 



hour, there were here and there some faint appear- 

 ances of a change of its colour ; but it showed no dis- 

 position to take that deep purple hue which the ribbon 

 had always acquired, when exposed to the light in the 

 preceding experiments. 



On taking the ribbon 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 

 instantaneously began to change colour, and soon be- 

 came of a deep purple tint ; but, though I examined 

 the surface of the ribbon with the utmost care and 

 with a good lens, both during the experiment 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 necessarily, 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 of the oxide of gold dissolved 

 in water are of such dimensions ; and it is very re- 

 markable that the colours produced in some of my 

 experiments on white ribbons, 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 enamel- 

 lers, in the intense heat of their furnaces. 



VOL. IV. 6 



