81 
in the air is remarked, likewise, by Dr Woodhcuse, 
who says that a plant in perfect health, confined in 
atmospheric air, will live in it for a long time with- 
out producing any sensible change. In his experi- 
ments, many vegetables did not affect the air in five 
days ; some diminished its purity in three hours ; 
and others altered it in a most slow and gradual 
manner, causing little change in it in twenty days *. 
316. If, therefore, we compare this slow consump- 
tion of oxygen gas by plants, with its more rapid 
conversion by seeds, we must either suppose that 
plants require the aid of those advantages, derived 
from the air, in a degree infinitely less than seeds; or, 
what is more probable, we must believe that the ne- 
cessary changes, which their vegetation produces in 
the air, are, at all times, more or less counteracted 
by some other office which they perform ; in other 
words, that the oxygen gas, which is converted into 
carbonic acid by the act of vegetation, is again re- 
stored, more or less completely, to the atmosphere, 
by some other process. This, restoration we have 
seen to be actually made, and it now only remains 
that we endeavour to discover the manner in which 
it is carried on, and try to ascertain whether it be, or 
be not, executed by the proper vegetative powers of 
the plant. 
* Nicholson's Journal, vol. ii. p. 152. 
