32 The Feeding of Animals 



be due to water driven out, and the gain to water ab- 

 sorbed from the atmosphere, which we call hygroscopic 

 moisture. 



All solids attract moisture up to a certain propor- 

 tion, which varies with the substance and with the 

 conditions that prevail. The surfaces of the particles 

 of matter are ordinarily covered with a thin film of 

 water, which is thicker on a cold, wet day than on a 

 warm, dry day, and so the same quantity of hay or 

 grain weighs less at one time than at another, because 

 the percentage of hygroscopic water varies. An equi- 

 librium will always be established between the attrac- 

 tion of a substance for moisture and the tension of the 

 vapor of water in the surrounding air, which accounts 

 for the effect of temperature and of the degree to 

 which the air is saturated with water vapor. As 

 all substances do not have the same attraction for 

 .moisture, therefore, under similar atmospheric condi- 

 tions, one feeding stuff may retain more water than 

 another. 



Water that is held physiologically is that which is 

 a constant and essential part of living organisms, in 

 which relation it is necessary to life and performs 

 certain important functions. These functions are of 

 three kinds: (1) The presence of water in the tis- 

 sues of plants and animals gives them more or less 

 firmness or rigidity combined with elasticity ; ( 2 ) 

 water acts as a food solvent; (3) water is the great 

 carrier of food materials and of waste products from 

 one part to another of the vegetable or animal or- 

 ganism, 



