59 



completely suspended, but the plants must be en- 

 tirely cut off from the contact of atmospheric air, 

 which has been shewn to be essential to their life ; 

 and if the separated leaf speedily dies when its upper 

 surface only (23.) is laid upon water, what reason 

 have we to conclude that its life can be preserved by 

 immersing it completely in that fluid. Even grant- 

 ing that the production of oxygen gas, in these ex- 

 periments, was effected by a natural function of the 

 leaves, no sensible advantage, depending on this 

 power, can be supposed to arise to those plants which 

 flourish in the open air ; for the carbonic acid, from 

 the decomposition of which alone the oxygen gas is 

 derived, exists in the atmosphere in a quantity in- 

 finitely too small to be productive of any good effect: 

 and we have shewn, that instead of absorbing car- 

 bonic acid from the atmosphere, plants, by their ve- 

 getation, are constantly producing it ; and instead 

 of emitting oxygen gas, they are at all times con- 

 verting it into this very acid. Neither, after all, is 

 it clear that the oxygen gas, which is thus obtained 

 by the decomposition of carbonic acid, depends on 

 any peculiar operation of the leaves ; " for light is 

 an essential agent in the decomposition, and it is pro- 

 bably by its agency, or by its entering into combi- 

 nation with the oxygen, that this substance is enabled 

 to assume the gaseous form, and to separate from 

 the carbon *." 



47. That the organized structure of the leaves is 

 not at all necessary to effect the separation of air from 



* Thomson's Chemistry, vol. iv. p. 285. 



