380 PHILOSOPHICAL TRANSACTIONS. [ANNO l/^B. 



ment in several ways, sometimes using paper, sometimes fine linen, and some- 

 times fine cotton cloth, instead of the silk ribband; 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 

 ribband, fine linen, cotton, paper, &c. 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 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. 



Exper. 4. A piece of ribband was wetted with the aqueous solution of the oxide 

 of gold, and held vertically by the side of the clear flame of a burning wax candle, 

 at the distance of less than half an inch from the flame. The ribband was dried, 

 but its colour was not in the smallest degree changed. When it was held a few 

 seconds within about -f 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. The heat 

 which existed at that distance from the flame, on the side of it, where this co- 

 loured stripe was produced, was sufficiently intense, as I found by experiment, to 

 melt very fine silver wire, flatted, such as is used in making silver lace. 



Exper. 5. Two like pieces of ribband were wetted at the same time in the solu- 

 tion, and suspended, while wet, in 2 thin phials, a and b, of very transparent and 

 colourless glass ; the mouths of the phials being left open. Both these phials were 

 placed in a window which fronted the south ; that distinguished by the letter a 

 being exposed naked to the direct rays of a bright sun ; while b was inclosed in a 

 cylinder of paste-board, painted black within and without, and closed with a fit co- 

 ver, and consequently remained in perfect darkness. In a very few minutes, the 

 ribband in the phial a began sensibly to change its colour, and to take a purple hue; 

 and at the end of 5 hours it had acquired a deep crimson tint throughout. The 

 phial b was exposed in the window, in its dark cylindrical cover, 3 days ; but there 

 was not the smallest appearance of any change of colour in the silk. 



Exper. 6. Two small parcels of magnesia alba, in an impalpable powder, about 

 half as much in each as could be made to lie on a shilling, were placed in heaps, 

 in 1 china plates, a and b, and thoroughly moistened with the before-mentioned 

 aqueous solution of the oxide of gold. Both plates were placed in the same win- 

 dow ; the moistened earth in the plate a being exposed naked to the sun's rays ; 

 while that in the plate b was exactly covered with a tea-cup, turned upside down, 

 which excluded all light. The magnesia alba in the plate a, which was exposed to 

 the strong light of the sun, began almost immediately to change colour, taking a 

 faint violet hue, which by degrees became more and more intense, and in a few 



