464 ACTION OF PLANTS ON THE AIE. 



sulphates, and phosphates, those bodies are formed, we are as yet alto- 

 gether ignorant. 



By some chemists it has been supposed that the decomposition of car- 

 The decompo- bpnic acid by plants in the sunshine is not instantaneously 

 sition of car- complete, but that a gradual process of reduction takes place, 



bonic acid is , , , , .,, i v,,! , -, 



not partial, but the carbon losing by little and little its oxygen, but never, 

 total. perhaps, losing it all. My own experiments, previously al- 



luded to, which show that the quantity of oxygen set free is never quite 

 equal to that of the carbonic acid consumed, have been used in support 

 of this view. But this, I think, is an interpretation which they will 

 scarcely bear. There are many facts connected with the chemical action 

 of light which might be cited as offering abundant proof that the decom- 

 position in question is, on the contrary, instantaneous and complete, and 

 in that I am led to believe really consists the primary function of the 

 light, the carbon thus obtained being subsequently employed in accom- 

 plishing the decomposition of water, and other processes of reduction 

 known to go on in the vegetable organism, but with which, under the 

 circumstances of the case, it is impossible that the sunlight should be di- 

 rectly concerned. I separate as distinct factors in the life of a plant 

 the obtaining of carbon from the air, which is accomplished by the influ- 

 ence of an external agent, and the moulding or modifying it with other 

 ingredients into organized material, which we have thus far imputed to a 

 plastic power in the plant itself, and respecting which more will be pres- 

 ently said. Free carbon once obtained, we- can easily conceive that all 

 other operations of reduction may follow, and that this division of the 

 action of plants into two distinct stages or factors, as we have just term- 

 ed them, is not a mere speculation, but represents what in reality occurs, 

 will perhaps be admitted on recalling what has been remarked on growth 

 in the sunshine and in darkness respectively. 



As a summary of the action of vegetation on the air, it is on all hands 

 Summary of admitted that plants tend, by the removal of carbonic acid 

 SLfcforTthe therefrom and the return of oxygen thereto, to compensate for 

 atmosphere, the disturbance occasioned by animals, which is to the oppo- 

 site effect. In this way, through very many centuries, the same percent- 

 age constitution of the atmosphere is maintained, the sum total of veg- 

 etable being automatically adjusted to the sum total of animal life ; auto- 

 matically, and not by any interference of Providence ; for if we admit, 

 what has been conclusively established by direct experiment, that plants 

 would grow more luxuriantly in an atmosphere somewhat richer in car- 

 bonic acid than the existing one, we may see how upon this condition de- 

 pends a principle of conservation, which must forever retain the air at its 

 present constitution, no matter how animal life may vary. The proofs 

 that are sometimes offered that there has been no change in this respect 



