L 227 J 
number are perhaps not to be 
traced by human fagacity; and, in- 
deed, if the air of a vegetable is 
from the. nature of common air, or 
air approaching it, changed into 
true poifonous air, by applying only 
heat. to it, as I have faid already, © 
and that dome more or lefs light 
to which a plant is expofed changes 
its natural air into the moit falu~ 
brious. or the moft poifenous air, 
may it not be fufpected, with fome 
degree of reafon, that a great Va- 
riety. of caufes, which have been 
till now overlooked, and which vary 
themfelves continually, may bring 
on avery material alteration in our 
atmoiphere, fuch as, for inftance, 
heat and cold, drynefs and. moifture, 
light -and_-obfcurity, which I haye 
already demoniftrated to. affect the 
Operation of vegetables upon the 
6 air, 
