34 HOW CROPS FEED. 
§ 4, 
RELATIONS OF ATMOSPHERIC WATER TO VEGETABLE 
NUTRITION. 
Occurrence of Water in the Atmosphere,—If water be 
exposed to the air in a shallow, open vessel for some time, 
it is seen to decrease in quantity, and finally disappears en- 
tirely ; it evaporates, vaporizes, or volatilizes. It is con- 
verted into vapor. It assumes the form of air, and becomes 
a part of the atmosphere. 
The rapidity of evaporation is greater the more eleva- 
ted the temperature of the water, and the drier the atmos- 
phere that is over it. Even snow and ice slowly suffer 
loss of weight in a dry day though it be frosty. 
In this manner evaporation is almost constantly going 
on from the surface of the ocean and all other bodies of 
water, so that the air always carries a portion of aqueous 
vapor. 
On the other hand, a body or mixture whose tempera- 
ture is far lower than that of the atmosphere, condenses 
vapor from the air and makes it manifest in the form of 
water. Thus a glass of ice-water in a warm summer’s day 
becomes externally bedewed with moisture. In a similar 
manner, dew deposits in clear and calm summer nights 
upon the surface of the ground, upon grass, and upon all 
exposed objects, whose temperature rapidly falls when 
they cease to be warmed by the sun. Again, when the 
invisible vapor which fills a hot tea-kettle or steam-boiler 
issues into cold air, a visible cloud is immediately formed, 
which consists of minute droplets of water. In like man- 
ner, fogs and the clouds of the sky are produced by the 
cooling of air charged with vapor. When the cooling is 
sufficiently great and sudden, the droplets acquire such 
size as to fall directly to the ground; the water assumes 
the form of rain. 
