8o An Inquiry concerning the 



same window; 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 teacup, turned 

 upside down, which excluded all light. 



The Tnagnesia alba in the plate A, which was ex- 

 posed 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 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, with- 

 out the smallest appearance of change. 



Experiment No. 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 strong light above half 

 an hour, before its colour began to be sensibly changed; 

 and at the end of three 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. 



Experimejit No. 8. — A piece of white taffeta ribbon, 

 which had been wetted with the solution, and thor- 

 oughly 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 exposed to 

 the strong light of a bright sun. 



After the ribbon had been exposed in this manner 

 to the action of the sun's direct rays about half an 



