HEAT AND CHANGE OF STATE 63 



occurs, though in none of them are such large quantities of 

 heat involved. The heat which is absorbed or given out 

 with a change of state- is usually spoken of as latent heat. 

 The scientists call it the heat of fusion when solids turn to 

 liquids, and the heat of vaporization when liquids turn to gases. 

 Though water at zero Centigrade ordinarily turns to ice when 

 80 calories of heat per gram are withdrawn from it, it is possible 

 to cool it still more and have it remain liquid df it is not agi- 

 tated. As soon as it is stirred, however, part of it turns to ice 

 and the remainder returns to the temperature of zero. This 

 is the way in which "anchor ice" is formed at the bottom of 

 rivers. The water becomes super-cooled in the stretches of 

 quiet water above a rapid and when the rapid is reached the 

 movement of the water causes the formation of anchor ice 

 which may frequently be seen clinging to the stones beneath 

 the water. 



TABLE OF LATENT HEATS 



In boiling In melting 



Air 47 Copper 30 



Alcohol 206 Ice 80 



Ether 91 Iron 35 



Mercury 62 Platinum 27 



Sulphur 262 Tin 14 



Water 536 



60. Practical Applications. In steam-heating systems, the 

 water is turned to steam in a boiler located in the basement or 

 other convenient place and is then carried in pipes to the rooms 

 to be heated. In the radiators, it gives up its latent heat and 

 becomes liquid again, then runs back to the boiler and is re- 

 heated. Something of the kind is provided by nature on a 

 much larger scale in autumn evenings when the moisture in 

 the air turns to fog and, in giving up its latent heat, protects 

 tender vegetation from the cold. Farmers sometimes utilize 

 the latent heat in water by placing tubs of the liquid in cellars 



