L *4* J 



mofl checked by the cold. Now 

 this is juft what happens. As foon 

 as in the advanced autumn the 

 leaves begin to wither and to fall, 

 and to contribute even fomewhat 

 themfelves to contaminate the air 

 by their corruption, the degree of 

 purity of the atmofphere is really 

 lefs than it w r as during the time of 

 the fummer ; and this atmofphere 

 does not return to its former good 

 quality till the winter is fet in, and 

 till the remaining tendency to cor- 

 ruption is checked by the increafe 

 of cold. In the fpring, when the 

 fun begins to promote fomewhat 

 the general tendency to corruption, 

 without having yet influence enough 

 upon the vegetables to make them 

 difplay their leaves, the common 

 air begins to be lefs fit for refpira- 

 tion, till it returns again to its for- 

 mer 



