Water tot 



the pine they do not consider worthy the name 

 of leaves. 



Leaf-trees are continually cooling the air by the 

 moisture which they give up to it ; but the pine- 

 needles have so few pores, and are so very much 

 protected, that the little water they part with is not 

 enough to produce any appreciable effect upon the air. 



It is, perhaps, hardly necessary to do more than 

 remind our readers that the evaporation of water 

 is always accompanied by the absorption of heat, or, 

 in other words, that water cannot be converted into 

 gas or vapour, which it is when evaporated, without 

 using up heat. Whether it be the heat of a fire or the 

 heat of the sun, it is all the same. A certain amount 

 of heat is required to make water pass from the liquid 

 to the gaseous state, and if this heat be taken from the 

 air, the air is necessarily by so much the cooler. 



And this brings us to another part of the subject, 

 the question, namely, as to the amount of water given 

 off by trees and other plants, notwithstanding the 

 various ways in which, as we have seen, they are 

 protected. 



We have distinguished hitherto between the two 

 processes of evaporation and transpiration, because 

 they are distinct ; the one being due to the action of 

 the air, and the other to the action, so to say, of the 

 plant. Evaporation takes place whenever air comes 

 in contact with anything moister than itself; whether 

 it be animal or vegetable, whether it be wet earth or 

 damp clothes, from all it draws water, and by its own 

 heat converts this water into vapour. The other 

 process, transpiration, is that by which, through the 

 pores the openings left in the skin of stem and leaf 



