140 RELATION OF TREES TO THE ATMOSPHERE. 



and that nature has no other means of composing it ; that 

 it is not simply a chemical, but a vital product ; and that 

 its production, like its preservation, depends entirely on 

 plants, and would be impossible without their agency. 

 But as all plants united are not equal in bulk to the 

 trees, it may be truly averred that any series of opera- 

 tions or accidents that should deprive the earth entirely 

 of its forests would leave the atmosphere without a source 

 for its regeneration. 



The use of the foliage of trees in renovating the at- 

 mosphere is not, I believe, denied by any man of science. 

 This theory has been proved to be true by experiments in 

 vital chemistry. The same chemical appropriation of 

 gases and transpiration of oxygen is performed by all 

 classes of vegetables ; but any work in the economy of 

 nature assigned to vegetation is the most effectually ac- 

 complished by trees. The property of foliage that requires 

 carbonic-acid gas for its breathing purposes, and causes it 

 to give out oxygen, is of vital importance ; and it is hardly 

 to be doubted that a close room well lighted by the sun 

 would sustain its healthful atmosphere a longer time, if 

 it were filled with plants in leaf, but not in flower, and 

 occupied by breathing animals, than if the animals occu- 

 pied it without the plants. 



But there is another function performed by the foliage 

 of trees and herbs in which no chemical process is in- 

 volved, that of exhaling moisture into the atmosphere 

 after it has been absorbed by the roots. Hence the 

 humidity of this element is greatly dependent on foliage. 

 A few simple experiments will show how much more 

 rapidly and abundantly this evaporation takes place when 

 the soil is covered with growing plants than when the 

 surface is bare. Take two teacups of equal size and fill 

 them with, water. Place them on a table, and insert into 

 one of them cuttings of growing plants with their leaves, 



