RELATIONS BETWEEN WATER AND LIFE 9 



supply is one of the first problems men must solve, whether 

 they live in city or country. 



In physiology you learned about digestion. You know 

 that digestion is the changing of food into liquid form. 

 All food must be so changed before our bodies can use it, 

 md the basis of this liquid form of food is water. Nearly 

 all of the weight of blood is water. Plants, in proportion 

 to their weight, need even more water than do animals, 

 especially when they are growing. Their roots are always 

 taking in water from the soil, and from their leaves water 

 is always going off into the air. Water evaporates from 

 leaves just as it evaporates from the surface of lakes and 

 ponds. 



When we say "water" we think of a liquid. But we 

 know that water below a certain temperature changes to a 

 solid that we call ice. And above a certain temperature it 

 changes to a gas that we call steam. It may also change 

 into an invisible gas that we call water vapor. This change 

 of water into ice when it is cold, and into steam when it is 

 hot, suggests that heat determines the form in which sub- 

 stances exist. Thus you see you can hardly begin to study 

 one of these topics before you begin to study another. We 

 have just begun to study water and already we need to 

 know something about heat. Just what is heat? That 

 is certainly a question we shall have to answer before we 

 get very far in our study of water. 



There are three principal forms in which substances exist: 

 ihe solid form, the liquid form, and the gaseous form. Scien- 

 tists refer to all substances taken together as matter, and 

 they call solid, liquid, and gas three states of matter. 

 The "solid earth" on which we walk is the most familiar 

 solid, water the most familiar liquid, and air the most 



