57 



was abstracted as soon as formed by the alkaline 

 solution, while in the other it remained in contact 

 with the plants ; but neither the presence nor the 

 absence of this acid caused any difference in the 

 growth or appearance of the plants ; and their de- 

 cline is referable only to that complete abstraction 

 of oxygen gas, which has been before shewn to be 

 fatal to the vegetative process : hence then carbonic 

 acid, applied to the leaves of plants, is not essential 

 to vegetation, neither is it destructive to the continu- 

 ance of that process. 



45. All that has been hitherto said, applies to 

 the circumstances of plants growing naturally in air : 

 when they are placed in water, other phenomena arise, 

 from which have been drawn arguments in favour 

 of an absorption and emission of gases by leaves. 

 Dr Priestley first advanced the opinion, that plants, 

 in certain circumstances, emitted oxygen gas; and In- 

 genhousz soon after discovered, that the leaves of 

 plants, when immersed in water, and exposed to the 

 light of day, produced an air which he announced as 

 oxygen gas. But whatever may be the results of 

 these experiments on plants immersed in water, they 

 are not necessarily to be received as proof of the 

 same actions being performed by the leaves when 

 growing naturally in the air : on the contrary, it has 

 been shewn by direct experiment (S3.), that when 

 plants are confined in a given bulk of atmospheric 

 air, they gradually and completely destroy its oxy- 

 genous portion, which could not possibly happen if 

 they possessed the power of emitting oxygen gas. 

 Moreover, the experiments of Dr Ingenhousz him- 

 self teach us, that this supposed emission of oxygen 



