OF CARBON. 47 



plants and explains the means nature employs 

 for purifying tne atmosphere of such an amount 

 of noxious matter. The proofs might here be 

 multiplied that such is the case, but the one 

 stated is so easy of comprehension, and so 

 readily exhibited, that it is useless to do so. 



It will be explained farther that the roots 

 also absorb a portion of carbon, which is 

 equally assimilated by the plant, and thus per- 

 form their part in this attribute of the vegeta- 

 ble tribe, but as the carbon so absorbed must 

 be dissolved in water, its consideration will be 

 considered under that head. 



Another source of carbon to plants arises 

 from vegetable matter in a state of decay. 

 Decay is only another word in chemistry, for 

 the slow process of combustion, where in the 

 plants are acted on by the oxygen of the at- 

 mosphere, and give off the carbon they had 

 assimilated during life, in union with a por- 

 tion of the oxygen in the state indeed of car- 

 bonic acid. 



All decaying vegetable matter under favour- 

 able circumstances as to moisture, tempera- 

 ture, &c. is surrounded by an atmosphere of 

 this carbonic acid gas, formed at the expense 

 of the oxygen of the atmosphere, and as this 

 process is carried on equally well in a soil 

 permeable to the air, as in the air itself, it is a 

 continued source of carbonic acid gas, which 

 it emits very slowly, and which is thus en- 

 abled to be absorbed by plants growing near it. 



In the early stages of vegetable life, before 

 any leaves are formed, and when consequently 



