63 
afterwards exposed to the sun for two hours, the air 
was brought back to its original purity. He also 
found, that mustard plants, when confined in a ves- 
sel of common air through the night., rendered it im- 
pure; but that this same air, together with the plants, 
being the next morning exposed to the sun, was re- 
stored, in an hour and a half, to its former purity *. 
When, likewise, similar portions of the same respired 
air were confined in vessels with similar plants, and 
respectively placed in sunshine and in the shade, the 
air, exposed to the sun, recovered its purity in a few- 
hours, while that, placed in the shade, was rendered 
more impure than before. He repeated these com- 
parative experiments a great many times, and varied 
them in different ways, until there remained not the 
least doubt, but that it was to the light of the sun 
that the purification of the air was owing, and to the 
absence of this light, that the opposite effect was pro- 
duced f. 
291. Dr Ingenhousz next made experiments on 
air that had been vitiated by the burning of a com- 
mon candle, suffering the carbonic acid which it con- 
tained to remain in it. With this air he filled three 
bell glasses of the same size, and introduced into 
each glass some plants of young mustard. One of 
the glasses he exposed to the sun at mid- day; 
another he placed in the shade ; and the third was 
removed to a place of perfect darkness. The next 
morning, before the sun had again shone on the 
* Exper. torn. i. p. <263. 265. 
f Exper &c. torn, ii. pref. p. 31. 
