[ 127 3 



number are perhaps not to bq 

 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 fome more or lefs light 

 to which a plant is expofed changes 

 its natural air into the mod falu- 

 brious or the moft poifonous air$ 

 may it not be fufpedted, with fome 

 degree of reafon, that a great va-* 

 riety of cauies, which have been 

 till now overlooked, and which vary 

 themfelves continually, may bring 

 on a very material alteration in our 

 atmofphere, fuch as, for inftance^ 

 heat and cold, drynefs and moifture^ 

 light and obfcurity, which I have 

 already demonftrated to affect the 

 operation of vegetables upon the 

 6 air, 



