SECT. XVII. PHYSIOLOGY. 137 
tion, partly from natural difficulties, and partly 
from the theories with which it is beset. I 
shall consider the facts, the theories are of minor 
consequence. 
cccxxx1t. The atmosphere is a gaseous com- 
pound of about twenty-one parts of oxygen, 
seventy-eight of azote, and one of carbonic acid. 
It is transparent and colourless, and envelopes the 
globe in a stratum computed at about forty-five 
miles. 
ceexxxt. The weight of the atmosphere 
varies according to the state of the weather, and 
the height at which the barometrical estimate is 
taken; at the level of the sea, the pressure of 
the air raises a column of mercury about thirty 
inches in vacuo. 
cccxxxiv. Above the temperature of 60°, every 
‘degree of heat, according to Sir G. Shuckburgh, 
increases the volume of atmospheric air ;, part; 
the expansion is, however, probably much above 
this valuation, as it is rated higher by several 
French Chemists. 
cccxxxv. Of the air received into the lungs, 
the azote is expired unaltered, and the oxygen is 
converted into carbonic acid. 
cccxxxvi. When any given quantity of oxy- 
gen combines with carbon, the volume of gas is 
not changed, but its weight is increased one-half 
by the addition of the carbon. The quantity of 
Ss 
