ACTION OF PLANTS ON THE AIR. 283 



remain subject to its influence till they are wholly 

 decomposed, or nothing but a little mould remains. 



744. Disease is occasioned by many causes ; but 

 independent of numerous minor sources, such as the 

 attacks of insects, &c., the most important are defi- 

 ciency of light, deficiency of vital energy, and defi- 

 ciency of heat ; any of these alone, or a combination 

 of them, induces disease in plants, and often leads to 

 death, by enabling the elements of organic matter to 

 commence those changes which constitute decay and 

 decomposition. 



746. As has already been observed, plants are 

 constantly employed in preserving the purity of the 

 atmosphere : they feed on the foul or vitiated air 

 produced by respiration, combustion, and decay ; 

 and their vigor and luxuriance are always in propor- 

 tion to the impurity of the air. It must, however, 

 be clearly understood, that there is a limit to the 

 quantity of carbonic acid plants are able to decom- 

 pose ; an excess would kill them ; whilst, if there 

 were none of it in the air, they could not live. The 

 vegetation of the whole globe is just enough to keep 

 the air in a uniform state of purity (106, 123, 710). 



iiiMiiittiiii' 



