54 



when light is withdrawn, this process of decomposing car- 

 bonic acid cannot go on. Plants, therefore, can only derive 

 nourishment when, in consequence of the influence of light, 

 they are able to effect its decomposition. When trees are too 

 crowded, and so close to each other as not to receive any 

 light, do they not live in perpetual night, never able to 

 decompose and receive any carbon, without which a tree 

 cannot prosper? A branch from an apple tree was sepa- 

 rated, and introduced into water connected with a mercurial 

 gauge when the leaves were upon it ; it raised the mercury 

 by means of the ascending juices to four inches ; but a 

 similar branch, from which the leaves were removed, scarcely 

 raised it a quarter of an inch. The use of carbonic acid in 

 the atmosphere seems to be in affording nourishment to 

 plants. It is formed during fermentation, combustion, putre- 

 faction, and respiration, and a number of other operations 

 taking place on the surface of the earth, and there is no 

 process known in nature, by which it can be destroyed, but 

 vegetation. 



The food on which vegetables live enters by the roots and 

 leaves of plants ; the leaves under the influence of the sun 

 decompose the carbonic acid, and give off its oxygen and 

 retain its carbon, and the carbon unites with the elements 

 of water in the sap, and forms those several compounds of 

 which plants chiefly consist. Plants, it would appear, are 

 destined by Providence to purify the air, which is loaded with 

 carbonic acid from the lungs of animals, and give out a fresh 

 supply of oxygen to replace what is taken up by the lungs of 

 animals. The influence which the evaporation of the leaves 

 has on the whole atmosphere, as well as on the whole earth, 

 produces very extensive effects. The French complain that 

 with the diminution of their forests, their vineyards suffer. 



When the leaves of plants are accidentally wanting at a 

 period when their presence is necessary, as for example, if, 



