E 142 ] 
moft 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 was 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 yetinfluence enough 
upon the vegetables to make them 
difplay their leaves, the common 
air begins to be lefs fit for re{pira- 
tion, till it returns again to its for- 
mer 
