166 



PURIFICATION OP AIR 



Next in order of time to those of Hales follow the ex- 

 periments of Bonnet. We have already detailed his ob- 

 servations on the power of leaves to imbibe moisture ; 

 whence it is ascertained that plants are furnished with a 

 system of cuticular absorbents, which carr}- fluids into 

 their sap-vessels, so as to enable them in some degree to 

 dispense with supplies from the root. With respect to 

 the effects of air upon leaves, this ingenious philosopher 

 has not been equally successful. He is recorded as the 

 discoverer of the expiration of plants, but it appears from 

 his work that he merely observed the bubbles of air 

 which cling to leaves, dead as well as living, and indeed 

 to any other body, when immersed in water and exposed 

 to the light of the sun. He found these bubbles disap- 

 peared in the evening, and returned again when the sun 

 shone, and he faithfully reports that by their attachment 

 to the surfaces of leaves, the latter were rendered more 

 buoyant, and rose in the water ; a sure proof that the 

 air had not previously existed, in the same volume at 

 least, in the substance of those leaves. Accordingly, 

 Bonnet concluded that the latter, in imbibing the sur- 

 rounding water, left the air which had been contained in 

 the water, and that this liberated air became visible from 

 being warmed and rarefied by the sun. This was as 

 near the truth as Bonnet could come, it not being then 

 known that light has a power of separating air of a pecu- 

 liar kind, carbonic acid gas, from water. I find no indi- 

 cations in his work of his having had any idea of leaves 

 absorbing air and giving it out again ; still less of their 

 affecting any change in its properties. 



