WATER, AND ITS USES 95 



and to use, and its general adoption in business and in 

 schools is desirable. Its heat unit is called the calorie, 

 and its value is the heat involved in changing the temperature 

 of one gram of water one degree of the Centigrade 

 thermometer. 



It has been found by repeated experiments, carefully 

 conducted in physics laboratories, that when ice melts 

 about 79 calories of heat disappear for every one gram 

 melted. In other words the heat of fusion of ice is 

 79 calories per gram. In similar experiments the heat 

 of vaporization is found to be about 536 calories per 

 gram of water. 



The meaning of these values is somewhat difficult to 

 grasp at first. Let it be illustrated in this way. To cool 

 boiling hot water (100 C.) down to the freezing temperature 

 (o C.) involves the liberation of 100 calories of heat per gram. 

 If that same one gram of water at o C. is changed to ice at 

 o C., involving a change of state from liquid to solid but no 

 change in temperature, almost 80 calories of additional heat 

 are liberated the heat that was required to keep the gram of 

 ice in the liquid state. 



Even more striking is the meaning of the 536 calories of 

 heat of vaporization of water. In the condensation of 

 invisible water vapor 1 whose temperature is 100 C. to water 

 that is boiling hot (100 C.), involving as it does a change 

 of state without change of thermometer readings, there is 

 5.36 times as much heat set free as when that same water 

 is cooled from its boiling temperature to the freezing 

 temperature. 



A common use of the great heat value of this " latent heat" 

 of steam is in the warming of. rooms and buildings by the 

 condensation of relatively small weights of steam in radia- 



1 The so-called steam that is visible consists of a cloud of minute water 

 particles resulting from the condensation of invisible water vapor. 



