194 
gene, and the charcoal is deposited in the form of 
a black powder. 
The principal consumption of the carbonic acid 
in the atmosphere, seems to be in affording nourish- 
ment to plants ; and some of them appear to be 
supplied with carbon chiefly from this source. 
Carbonic acid gas is formed during fermentation, 
combustion, putrefaction, respiration, and a num- 
ber of operations taking place upon the surface of 
the earth ; and there is no other process known 
in nature by which it can be destroyed but by 
vegetation. 
After a given portion of air has been deprived 
of aqueous vapour and carbonic acid gas, it appears 
little altered in its properties ; it supports combus- 
tion and animal life. There are many modes of 
separating its principal constituents, oxygene and 
azote, from each other. A simple one is by burn- 
ing phosphorus in a confined volume of air : this 
absorbs the oxygene and leaves the azote ; and 
100 parts in volume of air, in which phosphorus 
has been burnt, yield 79 parts of azote ; and by 
mixing this azote with SI parts of fresh oxygene 
gas artificially procured, a substance having the 
original characters of air is produced. To procure 
pure oxygene from air, quicksilver may be kept 
heated in it, at about 600°, till it becomes a red 
powder; this powder, when ignited, will be re- 
stored to the state of quicksilver by giving off 
oxygene. 
Oxygene is necessary to some functions of vege- 
tables, but its great importance in nature is in its 
relation to the economy of animals. It is abso- 
