Water in Organic Substance 31 



all the material things with which man comes in contact 

 in his daily life are made up of more or less water, or are 

 associated with it. Sometimes this is very evident, as 

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

 with straw and cornmeal. 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 vapor- 

 ized, we learn that the decrease in weight is nearly all 

 accounted for by the water thus collected. 



This fact suggests to us the chemist's way of deter- 

 mining 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 por- 

 tion of the material which is left behind after the water 

 is evaporated, we call the dry substance. 



Water is associated with plant and animal tissues 

 in two ways, hygroscopically and physiologically. It 

 is easy to illustrate the former way by an object lesson. 

 If an ounce of cornmeal 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 return 

 quite or nearly to its original weight. The loss would 



