66 THE SUN'S GIFT OF HEAT 



so as to allow them to expand at the same rate throughout 

 and thus to prevent their breaking. This explains why it 

 is unwise to pour boiling water slowly into a cold glass, or 

 cold water slowly into a hot glass. 



The experiment with the ball-and-ring apparatus easily 

 makes clear the meaning of the terms mass, volume, density, 

 and weight, which we shall have occasion to use from time 

 to time. After the iron ball was heated, it contained no 

 more iron than before it was heated. The amount of matter 

 in it, its mass, remained the same. But under heat the iron 

 expanded and occupied more space; that is, its volume 

 was greater. Heat increased the volume, 

 but not the mass, of each of the sub- 

 stances we experimented upon. 



We all know that some substances are 

 heavier than others. A cubic inch of 



CORK 



LEAD 



FIGURE 28. EQUAL lead, for example, is heavier than a 



MASSES OF CORK cub j c j nch Q f CQrk We that th 



AND LEAD , . , 



lead has greater density than the cork ; 

 that is, a piece of lead has more matter in it than a piece of 

 cork of the same volume. (Figure 28.) 



Weight is simply the measure of attraction between the 

 earth and the body weighed. The greater the amount 

 of matter, the greater is the attraction between it and the 

 earth; that is, the greater its weight. Weight, however, 

 must not be confused with density. The farther away a 

 substance is from the center of the earth, the less it weighs. 

 (Page 47.) A cubic inch of lead would weigh appreciably 

 less at the top of a high mountain than at the level of the 

 sea. But the density of the lead would not be affected by 

 its distance from the earth's center. 



When the iron ball was heated, its volume was increased, 



