46 COMMON PHENOMENA DUE TO HEAT 



cold weather is less severe. In summer water evaporates, heat 

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

 less intense. 



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 sufficiently 

 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 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. When a glass rod is held in a Bunsen 

 burner, it gradually grows softer and softer, and finally a drop 

 of molten glass falls from the end of the rod into the fire. The 

 glass did not suddenly become a liquid at a definite tempera- 

 ture; instead it softened and gradually changed to a liquid. 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. 24). If glass melted suddenly at a definite 

 temperature, it could not be molded in this way. Iron acts 

 similarly as it begins to melt, and because of this property the 

 blacksmith can shape his horseshoes, and the workman can 

 make his engines and other articles of service to man. 



Strange behavior of water. One has but to remember 

 that bottles of water burst when they freeze, and that ice 

 floats on water like wood, to know that water expands on 

 freezing or on solidifying. A quantity of water which occupies 

 100 cubic feet of space will, on becoming ice, need 109 cubic 

 feet of space. On a cold winter night the water sometimes 

 freezes in the water pipes, and the pipes burst. Water is very 

 peculiar in expanding on solidification, because most sub- 



