14 PHYSIOLOGY 



farther apart. They do not change in size or in number 

 but the spaces between them become larger and the sub- 

 stance occupies more space than it did before. (Fig. 6). 



A B 



Fig. 6.— A diagram illustrating expansion. A bit of matter, A, before ; B, after 

 the application of heat. The particles have not changed, but as they are far- 

 ther apart they occupy more space. 



When butter melts, it expands until it becomes a liquid. 

 When water evaporates, it expands until it becomes a 

 gas. In both cases the increased motion of the particles 

 represents an increase in heat, and the increase is the 

 same whether the process takes place quickly or slowly. 

 The increase of heat, or motion, on the part of one sub- 

 stance means a corresponding loss of heat, or motion, on 

 the part of some other substance, thus there can be no 

 evaporation unless something is cooled in the process. 

 If a hot substance is brought in contact with a cold sub- 

 stance, the more rapidly moving particles transfer their 

 motion to the more slowly moving ones as a bat does to 

 a ball, and as the one substance grows warmer the other 

 grows colder until the temperature of both is the same. 

 If a substance evaporates from the surface of the body, 

 it takes heat from the body in so doing and we feel 

 cooler. 



Diffusion. — Water apparently disappears when it 



