LEAVES ON THE ATMOSPHERE. 215 



deeply into all the various chemical exaini- 

 aatioiiS respecting it than suits our purpose. 

 It is only necessary to add a short view of 

 Dr. Darwin's hypothesis which Dr. Thomson 

 has not mentioned, probahly on account of 

 its insufficiency. That lively writer thou2,ht 

 the watery perspiration of leaves, acted upon 

 by light, gave out oxygen for the use of the 

 plant itself, such oxygen being immediately 

 absorbed by the air-vessels. This is by no 

 means adequate to explain any of the phe- 

 nomena, but rather contradictory to most 

 of them, and is totally superseded by the ob- 

 servations and experiments of other writers. 



There can be no question of the general 

 purpose ansv/ered to the vegetable constitu- 

 tion by these functions of leaves. They 

 confirm Mr. Knight's theory of vegetation, 

 who has proved that very little alburnum or 

 new v.'ood is secreted when light is kept from 

 the leaves. They also help us to understand 

 how essential oils may be produced, w\hch 

 are known, as weU as sugar, to be composed 

 of oxygen, hydrogen and carbon in different 

 proportions. We can now liave a general 

 idea how the nutritious sap, acted upon by 



