40 OTHER FACTS ABOUT HEAT 



Heat must be supplied to ice to melt it. On the other hand, 

 water, in freezing, loses heat, and the amount of heat lost by 

 freezing water is exactly equal to the amount of heat absorbed 

 by melting ice. 



The number of units of heat required to melt a unit mass 

 of ice is called the licat of fusion of water. 



31. Climate. Water, in freezing, loses heat, even though its 

 temperature remains at o C. Because water loses heat when 

 it freezes, the presence of large streams of water greatly in- 

 fluences the climate of a region. In winter the heat from 

 the freezing water keeps the temperature of the surrounding 

 air higher than it would naturally be, and consequently the 

 cold weather is less severe. In summer water evaporates, 

 heat is taken from the air, and consequently the warm weather 

 is less intense. 



32. Molding of Glass and Forging of Iron. The fire which 

 is hot enough to melt a lump of ice may not be hot enough 

 to melt an iron poker ; on the other hand, it may be suffi- 

 ciently hot to melt a tin spoon. Different substances melt, 

 or liquefy, at different temperatures ; for example, ice melts 

 at o C, and tin at 233 C., while iron requires the relatively 

 high temperature of 1200 C. Most substances have a definite 

 melting or freezing point which never changes so long as the 

 surrounding conditions remain the same. 



But while most substances have a definite melting point, 

 some substances do not. If a glass rod is held in a Bunsen 

 burner, it will gradually grow softer and softer, and finally a 

 drop of molten glass will fall from the end of the rod into 

 the fire. The glass did not suddenly become a liquid at a 

 definite temperature ; instead it softened gradually, and then 

 melted. While glass is in the soft, yielding, pliable state, it 

 is molded into dishes, bottles, and other useful objects, such as 

 lamp shades, globes, etc. (Fig. 20). If glass melted at a definite 



