SECT. VI. ELABORATION OF OXYGENE. 153 



SECTION VI. 



Elaboration of Oxygene. 



IN treating of the utility of the gases as a vege- 

 table food it has been already shown that the leaves 

 of plants abstract oxygene from confined atmos- 

 pheres, at least when placed in the shade, though 

 they do not inhale all the oxygene that disappears ; 

 but it has been further proved from experiment, 

 that the leaves of plants do also evolve a gas in the The leaves 

 sun. This phenomenon was first observed by disengage 



Bonnet, who gave indeed a wrong explanation of it ; 

 belivingr it to be the extrication of the air that might shown by 



Bonnet, 



have entered the plant along with the sap, or be- 

 lieving it to come directly from the water. His 

 method was to expose the leaves to the sun, in an 

 inverted glass vessel filled with water; air bubbles 

 began immediately to disengage themselves from 

 the surface of the leaves, and to ascend to the sum- 

 mit of the water. 



The next experiments on this subject are those of And 

 Priestley, who discovered that the leaves of plants 

 in a state of vegetation have the property of ameli- 

 orating vitiated air. On the 17th of August 1771> 

 he put a sprig of Mint into a quantity of atmo- 

 spheric air in which a candle had burnt out, and 

 found after confining it till the 27th of the same 

 month, that the air was again ameliorated and ca- 



