48 



attract carbonic acid, retaining the carbon, and gi- 

 ving out the oxygen gas ; for this would not only 

 be making an attraction and expulsion of oxygen 

 gas to go on at the same time, but would be doing 

 the same by the carbonic acid also : and how could 

 that acid be ever formed and emitted at all, if the 

 oxygen gas which enters into it is first singly ex- 

 pelled, and the carbon, which is its other constitu- 

 ent, is permanently retained ? And if we suppose, 

 that, during the day, carbonic acid is received and 

 oxygen gas expelled by the leaves, and that during 

 the night the exact contrary takes place * ; it seems 

 very difficult to account for the death of plants (3O.) 

 confined in a given bulk of air at all, since they 

 would, in fact, be weaving Penelope's web, undoing 

 by night what they had done by day, and vice versa. 

 All these difficulties are greatly increased by the sup- 

 position, that the air enters into the vessels of plants 

 in its undccomfioscd state ; for since the nitrogen gas 

 remains unaffected (29.), what useful purpose could 

 its reception answer ? Or by what means, when se- 

 parated from its oxygen gas, could it again be ex- 

 pelled from the plant in the exact bulk which it be- 

 fore occupied ? On these grounds, we deny the en- 

 trance of aeriform fluids into the vascular system of 

 plants, by any power analogous to chemical affinity. 

 38. If from the experiments already related 

 (32.), it be granted, that, during vegetation, the 

 oxygen gas of the air gradually and completely dis- 

 appears, and that a bulk of carbonic acid, nearly 



* Thomson's System of Chemistry, vol. iv. p. 29L 



