48 
checks to their growth, that, in our own climate at 
least, we could scarcely ever hope to see many of 
them attain to a state of maturity. 
266. Should we even allow, to the fullest extent, 
that plants, vegetating under direct exposure to the 
sun, produce changes in the air different from those 
which they produce in the shade, yet this, surely, 
does not disprove the correctness of our former ob- 
servation. It only proves, that, by a change of cir- 
cumstances, a change in effect is induced ; and as this 
effect is not essential to the end we desire to ob- 
tain, it must be considered as extrinsic and acciden- 
tal ; for that condition can, at no time, be held ne- 
cessary to the production of a given effect, which 
may, at any time, with impunity be excluded. 
267. After what has just been observed concern- 
ing the changes induced on the air by vegetables 
growing in the shade, and in perfect darkness, it is 
almost superfluous to add, that similar effects must 
be produced by plants which vegetate through 
the night. Dr Priestley very early observed, that a 
fresh cabbage leaf greatly deteriorated the air in 
which it was confined for a single night, although it 
had not acquired the least smell of putrefaction * ; and 
Ingenhousz likewise remarked, that the more vigo- 
rous the plants were, the more was the air affected. 
He found that a withered plant, confined in air 
through the night, produced in it scarcely any change; 
while a similar plant, in a state of active vegetation. 
* Obs. on air abridged, vol. iii. p. 251 
