Notices respecting New Books. 273 



and chemists seem, in this instance, to have satisfied them- 

 selves with contemplating at a distance the beauty of the 

 Jinal cause, instead of approaching to a nearer examination 

 of the facts on which the opinion has been maintained." 

 He argues thus : 



" Against the opinion of the absorption and emission of 

 gases by the leaves of plants when growing naturally in sir, 

 we have already, both on physiological and on chemical 

 grounds, been induced to enter our protest. That the same 

 substance, carbonic acid, should dufnig the day be absorbed 

 by the leaf, and decomposed within it as salutary, and 

 during the night should be formed within the same leaf, 

 and emitted from it as noxious, seems to be not only incon- 

 sistent but absurd. Where would be the advantage in the 

 carbon of the acid being retained for twelve hours as food, 

 if for the next twelve it must again be given out as excre^ 

 mentitious ? Or where is there an instance, in the whole 

 circle of existence, of a living agent not only first forming 

 its own food, but feeding on its own excretions? If this 

 carbon were during the day retained as food, whence comes 

 that composing the acid which plants, when confined in a 

 given bulk of air, are constantly forming ? If oxygen gas, 

 as these chemists suppose, be during the day constantly 

 emitted, why does that gas gradually disappear as the pro- 

 cess of vegetation proceeds ? And why at last is none to 

 be met with, although there is present an abundance of car- 

 bonic acid, out of which it is supposed to be formed ? It 

 has been proved that during the day carbonic acid, by the 

 act of vegetation, is constantly forming; but if, at the same 

 time, it be as constantly absorbed by the leaves, how can its 

 presence be manifested in such quantity and in such pro- 

 gression as experiment evinces that it is ? All these observa- 

 tions apply Ui the circumstances of plants growing naturally 

 in air; when they are placed in water, other phaenomena 

 arise, from which have been drawn arouments in favour of 

 an absorption and emission of gases by leaves. It has, how- 

 ever, been shown by direct experiment, that v.-hen plants are 

 confined in a given bulk^of atmospheric air, they gradually 



Vol. 'J8. iSo. 111. Aug. I807. S and 



