cane 3 
SECTION XXvVI. 
Air is one of the moft changeable fub- 
Siances in nature, and appearing 
under very different forms and 
qualities from a variety of caufes. 
THE air of our atmofphere is 
feldom during a whole day of the 
fame quality. Its degree of whole- 
fomenefs is perhaps not lefs fubject 
to variations than its weight and its — 
degree of heat and cold. The ba- 
rometer indicates the firft, and the 
thermometer the other. But thofe 
two inftruments feem to have no 
relation to the more or lefs purity 
of the atmofphere, or the more or 
lefs fitnefs of the air for the ufe of 
refpiration. 
plant during the night, when the plants yield but a 
The 
