COMPOUNDS OF NUTRITION 29 



associated with it. Sometimes this is very evident, as 

 with green plants or juicy fruits. It is not so evident 

 with straw and corn meal. If, however, we submit 

 almost any substance, no matter how dry it may appear, 

 except, perhaps, glass and metals, to the heat of an oven 

 at 212° F., we find that a material loss of weight occurs; 

 and if we so arrange that whatever is driven off is first 

 drawn through some substance that entirely absorbs the 

 water which has been vaporized, we learn that the decrease 

 in weight is nearly all accoimted for by the water thus 

 collected. 



34. Measurement of water-content. — ^This suggests 

 to us the chemist's way of determining the proportion of 

 water which any particular material contains. He weighs 

 out a certain amount of the substance and then keeps it 

 in an oven at 212° F. for five hours perhaps, after which it 

 is re-weighed. The difference in the two weights, or the 

 loss, is assumed to be all water, and the percentage in the 

 original substance is easily calculated. That portion of 

 the material which is left behind after the water is evap- 

 orated is called the dry substance. 



35. Hygroscopic water. — ^Water is associated with 

 plant and animal substances and tissues in two ways, 

 hygroscopicaUy and physiologically. It is easy to illus- 

 trate the former way. If an ounce of corn meal were to 

 be dried in an oven as described, it would, as stated, lose 

 in weight. If it were subsequently allowed to remain 

 exposed in the open air in a barn or out-of-doors, it would 

 regain part or all of its original weight. The loss would 

 be due to water driven away by heat, and the gain to 

 water absorbed from the atmosphere, which we call 

 hygroscopic moisture. 



All solids attract moisture up to a certain proportion 



