34 THE VITAL FUNCTIONS. 



with oxygen, and then being precipitated from 

 its combination in the manner above described. 

 It is not the whole of the carbon introduced into 

 the vegetable system, in the form of carbonic 

 acid, which has to undergo the first of these 

 changes, a part of that carbon being already in 

 the condition to which that operation would re- 

 duce it, and consequently in a state fit to receive 

 the decomposing action of the leaves. The whole 

 of these chemical changes may be included under 

 the general term Aeration. 



Thus the great object to be answered by this 

 vegetable aeration is exactly the converse of 

 that which we shall afterwards see is effected by 

 the respiration of animals : in the former it is 

 that of adding carbon, in an assimilated state, to 

 the vegetable organization ; in the latter, it is 

 that of discharging the superfluous quantity of 

 carbon from the animal system. The absorption 

 of oxygen, and the partial disengagement of 

 carbonic acid, which constitute the nocturnal 

 changes effected by plants, must have a tendency 

 to deteriorate the atmosphere with respect to its 

 capability of supporting animal life ; but this 

 effect is much more than compensated by the 

 greater quantity of oxygen given out by the same 

 plants during the day. On the whole, therefore, 

 the atmosphere is continually receiving from the 

 vegetable kingdom a large accession of oxygen, 

 and is, at the same time, freed from an equal 

 portion of carbonic acid gas ; both of which 



