ACTION OF LEAVES ON THE ATMOSPHERE. l6l 



down, either equably or by jerks, without any unifor- 

 mity or cooperation among themselves. It is difficult 

 to guess at the purpose which this singular action is 

 designed to answer to the plant itself; its effect on a 

 rational beholder cannot be indifferent. 



The chemical actions of light, heat, and the compo- 

 nent parts of the atmospheric air, upon leaves, and., 

 where the latter are wanting, on the green stems of 

 plants, are now, as far as concerns all plants in com- 

 mon, tolerably well understood. The observations and 

 experiments of Priestley and Ingenhousz have been 

 confirmed, extended in a variety of ways, or explained- 

 on the principles of improved chemistry, by Dr. Per- 

 cival and Mr. Henry in England, Dr. Woodhouse in. 

 America, and M. Sennebier and M. Theodore de 

 Saussure, as well as various other philosophers, on the 

 continent of Europe. It is agreed that in the day-time 

 plants imbibe from the atmosphere carbonic acid gas, 

 (which was formerly called fixed air, and is an union 

 of oxygen and carbon,) that they decompose it, absorb 

 the carbon as matter of nourishment which is added 

 to the sap, and emit the oxygen. So they absorb the 

 same gas from water, when it is separated from that 

 fluid by the action of light. The burning of a candle, 

 or the breathing of animals, in confined air, produces 

 so much of this gas, that neither of these operations 

 can go on beyond a certain time ; but the air so con- 

 taminated serves as food for vegetables, whose leaves, 

 assisted by light, soon restore the oxygen, or, in other 



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