245 
look for those causes of purification, by the opera- 
tion of which, the uniformity of composition in the 
atmosphere is, at all times, and in all situations, 
maintained? To this most interesting, but difficult 
question, no satisfactory answer can, we think, be 
returned, in the present state of chemical knowledge. 
The mades in which the atmosphere is depraved 
by the living functions of animals and vegetables, 
by combustion, and by various other processes, in 
which its oxygen is withdrawn and made to enter in- 
to new combinations, are pretty well known, and, 
to a certain extent, may be appreciated with tolerable 
accuracy ; but the various means by which this oxy- 
gen is released from its combinations, in the diversi- 
fied modes of decomposition which are perpetually 
taking place, have been much less regarded, and 
cannot, therefore, with equal accuracy, be traced. 
Until this department of chemistry attain to greater 
perfection, it is, therefore, impossible to present a 
tolerably accurate view of this subject. We may, 
however, be certain of the general fact, that, as oxy- 
gen is withdrawn from our atmosphere, in order to 
enter into new combinations, so it can again be re- 
stored to it only by decompositions which shall set it 
free ; and these decompositions must be as numerous, 
and to an extent as great, as the combinations to 
which they succeed. To follow, however, this circle 
of actions through all its round, may demand the 
persevering industry of ages ; and it is only when 
this shall be accomplished, that chemistry will have 
advanced our knowledge of the individual relations 
of our globe, in a degree corresponding with that to 
