, 



FREEZING AND MELTING. 



higher temperature than freezing-point. To 

 make it possible, either the temperature of the 

 ice-heat must be raised or the melting and 

 freezing point of ice must be lowered . Ice can 

 have its melting point lowered by mixing it 

 with salt. A mixture of water and salt re- 

 quires thirty-two degrees of frost to freeze it ; 

 therefore a mixture of ice and salt can be 

 melted by the heat of anything at a higher 

 temperature than this. Such a thing is pure 

 ice, which has been exposed for some time to 

 the air : it remains at freezing or melting 

 point, 32 F., or zero on the Centigrade scale. 

 At this temperature, then, it has heat enough 

 to melt a part of its own substance when it has 

 been made meltable at this and lower tempera- 

 tures by being mixed with salt. It proceeds to 

 melt a part of itself, devoting a part of its 

 heat-energy to this work. Having so used up 

 some of its heat, it has less heat left it is 

 colder. 



How strongly ice is made to cool itself in this 

 way is shown by the formation of a thick coat 

 of hoar-frost on the outside of the vessel up to 

 the level of the ice and salt inside. When a 

 tumbler or other vessel contains ice or ice-water, 

 dew can be seen collecting on the outside. This 

 is the moisture of the surrounding air, con- 

 densed by the cold. But ice can never freeze 

 this moisture into hoar-frost. Our mixture of 

 ice and salt easily does so, for it loses so 



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