68 THE SUN'S GIFT OF HEAT 



nite shape and melts down into water. If now we apply suffi- 

 cient heat to the water, the motion of the molecules becomes 

 so violent that they fly off from one another in steam. But 

 while this effect of heat in changing ice to water and water to 

 steam is familiar to us all, it is not so generally known that 

 the application of sufficient heat will change other substances 

 from a solid to a liquid and from a liquid to a gaseous state. 



Iron, for instance, may be solid as we ordinarily see it, 

 or liquid as it comes from the blast furnace, or gas as it 

 exists in the indescribably hot atmosphere of the sun. 

 When heat is withdrawn, the processes are reversed, from 

 gas to liquid and then to solid. 



Some substances, such as camphor, pass from a solid state 

 directly to a gaseous state. Even ice may do this under 

 certain conditions. Housewives in cold climates know, 

 for example, that clothes on the line will " freeze dry " 

 in zero weather. 



Substances usually expand as they change from the solid 

 state to the liquid state, and contract when the process is 

 reversed. Ice is a notable exception to this general rule, 

 since when water freezes its volume increases. If it were 

 not for this, ice would not float. Certain metals such as 

 cast iron also have the property of expanding at the moment 

 of solidifying. Type metal is a mixture of metals that 

 possesses this property. It is poured into the molds in 

 a molten condition. When it solidifies it expands and 

 forces itself into every available crevice, thus taking on 

 the sharp outlines that type must have. 



Substances always increase in volume as they change 

 from a liquid to a gaseous state. Engineers roughly esti- 

 mate, for example, that a cubic inch of water makes a 

 cubic foot of steam. 



