CHAPTER VIII 
THE ATMOSPHERE 
In the Introduction we pointed out that the air was a 
mixture of gases, and we referred briefly to the part played 
by these gases in the life of plants. This was dealing 
with the atmosphere from the chemical side. It remains 
now to say something of the physical effects of air upon 
plants—that is to say, of air in motion—wind. 
Wind.—The effect of wind upon vegetation is twofold : 
(1) It promotes evaporation, and therefore increases 
transpiration ; and (2) it lowers the temperature of the 
bodies over which it blows. The second effect is really 
a consequence of the first; for a body which is losing 
water by evaporation is at the same time losing heat and 
becoming cooler. Heat is required to change any liquid 
into vapour ; where the heat is not imparted from without, 
the body from which the water is evaporating provides it, 
and its temperature falls. Common experience testifies 
in a very simple way to the truth of this. A person, 
exerting himself greatly, becomes hot and perspires. If 
he takes off his hat, a pleasant coolness accompanies the 
disappearance of the perspiration from his head. Again, 
if ether is poured upon the hand, it evaporates away in 
a few seconds, but heat for the purpose is drawn from 
the hand, which therefore experiences a sensation of 
cold. 
Wind promotes evaporation by constantly and rapidly 
renewing the air in contact with the evaporating surfaces. 
Wet clothes dry quicker when a breeze is blowing than in 
a calm. Plants suffer in the same way, and when the 
wind is strong, they are in danger of losing more water 
than they can afford. This danger is increased by the 
drying and the cooling of the soil, and the decrease in 
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