I, 



IIOAV CROPS FEED. 



RELATIONS OF ATMOSPHERIC WATER TO VEGETABLE 

 NUTRITION. 



Occurrence of Water in the Atmosphere.— If w ator be 

 exposed to the uir in a sliallow, open vessel for some time, 

 it is seen to decrease in quantity, and finally disapi)ears en- 

 tirely ; it evaporates, vaporizes, or volatilizes. It is con- 

 verted into vapor. It assmnes the form of air, and becomes 

 a part of the atmosphere. 



The rapidity of evaporation is gi'eater the more eleva- 

 ted the temperature of the water, and the drier the atmos- 

 phere that is over it. Even snow and ice slowly suiFer 

 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 thnn 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 mhiute 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. 



