40 
254. We have been drawn into these detailed re- 
marks, not from any desire to depreciate Dr Priest- 
ley's labours, but from the circumstance of their 
having first given origin to the opinion, that plants, 
by their vegetation, at all times purify the air, and 
from a consideration of the importance which has 
ever since been attached to them. In the experi- 
mental sciences, it is chiefly by the successive detec- 
tion of each other's errors that we gradually advance 
to truth ; for rarely, indeed, does it happen, that hu- 
man sagacity can at once foresee and appreciate all 
the possible circumstances in an experiment, which 
may influence and controul its result. There is, 
therefore, no cause to wonder that this illustrious 
philosopher did not discover those sources of fallacy, 
which the more advanced state of science has alone 
enabled his successors to point out : and the reflec- 
tion, that our apparently more correct views may, at 
no distant day, undergo a similar revision, ought not 
only to teach us becoming diffidence in our own 
opinions, but may serve to check that rising triumph, 
which little minds are sometimes apt to feel, when 
they see thus exposed the mistakes of superior men. 
255. The accurate and decisive experiments of 
M. Scheele, on the germination of seeds, have been 
already detailed (20?.), and his experiments on the 
vegetation of plants seem to have afforded precisely 
the same results. Dr Priestley, he observes, found 
that vegetables make foul air salubrious j but, in his 
own experiments, they always injured the air. He 
kept vegetables in a closed matrass, filled with foul 
air, both in darkness, and in the light of the sun ? 
