332 LATENT AND SPECIFIC HEAT. 



salt itself, the double liquefaction requires a deal of heat 

 which is generally furnished by the cream in the freezer. 

 The freezing mixture most commonly used consists of one 

 weight of salt and two weights of snow or pounded ice. 

 The mixture assumes a temperature of 18 C., which 

 furnished the zero adopted by Fahrenheit. 



(a.) By mixing, at the free/ing temperature, three weights of 

 snow with two weights of dilute sulphuric acid, the temperature 

 may be reduced to about 20 F., a diminution of over 50 Fahren- 

 heit degrees. If equal weights of snow and dilute sulphuric acid 

 be thus reduced to a temperature of 20 F. and then mixed, the 

 temperature will fall to about 60 F. By mixing equal weights 

 of sodium sulphate crystals (Glauber's salt), ammonium nitrate and 

 water, all at the ordinary temperature, and stirring the mixture 

 with a thermometer, the temperature will be seen to fall from about 

 65 F. to about 10 F., which is considerably below the freezing point 

 of pure water. Glauber's salt and chlorhydric (muriatic) acid form 

 a good freezing mixture. 



522. Solidification. Solidification signifies the 

 passage from the liquid to the solid condition. During 

 solidification there is an increase of temperature. 

 This may seem paradoxical in certain cases, but, even in 

 the case of water, it is true that solidification is a warming 

 process. 



(a.) The sensible heat that disappeared as latent heat during 

 liquefaction, being no longer employed in doing the work of main- 

 taining liquidity, is reconverted into sensible heat and immediately 

 employed in increasing the molecular vibrations. The molecular 

 potential energy is transmuted into molecular kinetic energy. This 

 is frequently illustrated by the precaution taken in winter to place 

 tubs of water in vegetable cellars that the latent heat of the freez- 

 ing water may be changed into sensible heat and thus protect the 

 vegetables. 



523. Temperature of Solidification. The 



melting point is the highest temperature at which solidi- 



