62 
the apparatus which they employed being placed in- 
differently in sunshine or in the shade ; or to their 
having suffered the plants to remain in confinement 
until they had lost their vigour or their life *. It must, 
however, be observed, in justice to those philoso- 
phers, that they were? both duly apprized of these 
circumstances, and conducted many of their experi- 
ments with an immediate regard to them. While 
therefore we admit, that the variation in the results of 
some of Priestley's experiments may have arisen from 
inattention to these particulars, yet the discordance 
between him and Scheele is, more probably, to be as- 
cribed to the different nature of the gases which 
they employed, in consequence of the precaution 
which the latter always adopted, of first removing the 
carbonic acid from the foul air which he submitted 
to experiment. 
290. But certainly the experiments of M. Ingen- 
housz, both by their number and variety, have con- 
tributed, more than those of any other philosopher, 
to elucidate the subject of the purification of our at- 
mosphere by plants. In air that had been so far de- 
praved by respiration as to extinguish a lighted can- 
dle, he placed a plant of peppermint, and then ex- 
posed the vessel, for three hours, to the sun, at the 
end of which time the air again supported flame. 
When, however, the common nettle was placed in a 
similar portion of respired air during the night, the 
air was not at all improved ; but the apparatus being 
* Exper. sur les Veget. torn. ii. pref. p. 31, 
