^3 
the same time part of the hydrogen, proportional to its pres- 
sure, becomes absorbed by the water ; hence the volume of 
air in the vessel is not much altered, but its quality approaches 
that of the atmosphere. The same process takes place with 
oxygen, or any other gas or foul air : for water exposed to 
the pressure of the atmosphere, will contain a certain quantity 
of the gases composing that atmosphere, and will therefore 
restore or reduce any gas confined over an adequate quantity 
of it, to a certain degree of purity, varying with the nature 
of the gas so confined. 
This principle, then, appears to be one of the chief 
means employed by nature to preserve the atmosphere 
of an uniform quality. Permit us further to observe, that 
wind seems to be another of those means. Oxygen gas, by 
the sun's influence upon vegetation, &c., may be produced 
in abundance at some particular period or place ; this, how- 
ever, is immediately dispersed by the wind, whose velocity 
when scarcely -perceptible^ according to Dr. Hutton is about 
one mile per hour^ but which, at a medium velocity, may travel 
round the earth in a month's time ; it therefore must pass 
both large barren tracts of land, and an extensive portion of 
the sea; to either of which it will impart of its excess of 
oxygen gas. When it is considered that this dispersion is a 
daily process, and also that in countries where cultivation, 
and of course vegetation is in the greatest perfection, popula- 
tion, with its consequent respiration, combustion, and other 
destructive processes proportionally abounds; we think it 
scarcely possible to be seriously conjectured that any con- 
siderable variation can exist in the quality of the atmosphere ; 
at least so far as the two gases of oxygen and nitrogen are con- 
cerned in its composition, which we now come to consider. 
It is acknowledged that the atmosphere is composed 
principally of the tv/o gases oxygen and nitrogen, which 
by different philosophers have been stated in various pro- 
portions, from 25 per cent, oxygen, and 75 nitrogen^ to 
