454< Count Rumford’s Inquiry concerning the 
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 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, 
without 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 solu- 
tion 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. 
Experiment No. 8. A piece of white taffeta riband, which 
had been wetted with the solution, and thoroughly dried in the 
dark, was suspended in a clean dry phial of very fine trans- 
parent glass ; and the phial, being well stopped with a dry cork, 
was exposed to the strong light of a bright sun. 
After the riband had been exposed, in this manner, to, the 
action of the sun’s direct rays about half an hour, there were 
here and there some faint appearances of a change of its colour ; 
but it showed no disposition to take that deep purple hue which 
