GASES 67 



they freeze. Water is an exception: 100 c.c. of water at 

 C. become 109 c.c. of ice at C. On this account water 

 pipes often burst in winter, and water that freezes in the 

 cracks of rocks breaks the rocks in pieces (cf. 289). 



Metals that have a larger volume when solid than 

 when liquid can be used to make castings ; for the casting 

 will then fill the mold completely. Cast iron, type metal, 

 and brass are metals of this sort. But gold and silver, 

 which shrink when they become solid, cannot be cast; 

 they must be stamped, or " minted. " 



When a liquid becomes a gas, or vapor, an enormous increase in 

 volume takes place. Thus, 1 c.c. of water at 100 C. becomes about 

 1,200 c.c. of steam at 100 C. Engineers express this by saying, "A 

 cubic inch of water gives a cubic foot of steam." 



The heat taken up in changing a gram of water at 100 C. to steam 

 at 100 C. is 536 calories (cf. 64). The same amount of heat is set 

 free when a gram of steam at 100 C. is condensed to water at 100 C. 

 Hence a scalding from steam is much more painful than one from hot 

 water. 



When a liquid is evaporating rapidly, it is always a little colder than 

 the surrounding air. In tropical countries people take advantage of 

 this fact to cool their drinking water. The water is put into porous 

 jars. A little of the water goes through the jar, and by its evaporation 

 on the outside cools the jar and the water in it. 



71. Gases. Gases do not have either a definite shape 

 or a definite volume. The volume of a gas depends upon 

 its temperature (cf. 61) and its pressure (cf. 41). 

 Cohesion between the molecules of a gas is slight, because 

 the molecules are so far apart. Because of the motion of 

 its molecules, a gas placed in a vacuum expands, until 

 it fills the vacuum. 



If we force the molecules of a gas closer together, we 



