54 HEAT 



We have learned that water boils at 212 Fahren- 

 heit, and we have always thought of "boiling hot" as being 

 very hot. Of course we have known that the fire is much 

 hotter but we have never thought much about it. Ice 

 water (32F.) is changed to boiling water (212F.) by an 

 increase of 180, yet the lowest temperature at which 

 there is a dull red light is about 950 Fahrenheit. The 

 temperature of bright red light is much higher, yellow 

 light is caused by a still higher temperature, while daz- 

 zling white light is caused by the extremely high tem- 

 perature of 2200 F. The electric light which we have in 

 our houses is a good example of light from heat. 



21. Heat from Friction. 



When two objects are rubbed together we say there 

 is friction between the objects. This friction makes it 

 hard for the objects to move past each other and it re- 

 quires more strength where there is much friction than 

 where the friction is less. Oil has the power to reduce 

 the friction between two objects and for that reason we 

 oil the bearings of machinery, also the curves of railroad 

 tracks, or we grease the places where we want one part 

 to slip easily over another part. Another name for 

 oiling is lubricating. 



Try rubbing your hands together, pressing them 

 firmly against each other and moving them rapidly. How 

 do they feel? Why do persons rub their hands together 

 in cold weather? Rub a coin on your coat sleeve or on 

 the carpet and tell what happens. Feel of a gimlet after 

 boring a hole, and explain the results. Sometimes the 

 bearings of railroad car wheels become so hot, even if 



