78 An Inquiry concerning the 



vapour which arose from the burning candle: the 

 purple stain was produced as before, which extended as 

 far as the ribbon had been wetted with the solution, 

 but no farther. 



I afterwards varied this experiment in several ways, 

 sometimes using paper, sometimes fine linen, and 

 sometimes fine cotton cloths, instead of the silk ribbon ; 

 but nearly the same tinge was produced, whatever the 

 substance was that was made to imbibe the aqueous 

 solution of the metallic oxide. 



Similar experiments and with similar results were 

 likewise made with pieces of ribbon, fine linen, cotton, 

 paper, etc., wetted in an aqueous solution of nitrate of 

 silver: with this difference, however, that the tinge 

 produced by this metallic oxide, instead of being of a 

 deep purple inclining to a crimson, was of a very dark 

 orange colour or rather of a yellowish brown. 



In order to discover whether the purple tinge, in the 

 experiments with the oxide of gold, was occasioned by 

 the heat communicated by the ascending current of 

 hot vapour or by the light of the candle, I made the 

 following experiment, the result of which I conceive to 

 have been decisive : — 



Experiment No. 4. — A piece of ribbon was wetted 

 with the aqueous solution of the oxide of gold, and 

 held vertically by the side of the clear flame of a burn- 

 ins: wax candle, at the distance of less than half an 

 inch from the flame. 



The ribbon was dried, but its colour was not in the 

 smallest degree changed. 



When it was held a few seconds within about \ of 

 an inch of the flame, a tinge of a most beautiful crim- 

 son colour, in the form of a narrow vertical stripe, was 

 produced. 



