140 SCIENCE OF COMMON THINGS. 



Heat caused by friction. Use of greasing cart wheels. 



Fig. ST. 



Fig. 37 represents an Indian explaining the method of kindling a fire 

 by the friction of two pieces of wood. 



895 Why do wheels catch fire in such cases ? 



Because the friction of the wheels against the axle- 

 tree disturbs their latent heat, and produces ignition. 



896 What is the use of greasing cart wheels f 



Grease lessens the friction ; and, because there is less 

 friction, the latent heat of the wheels is less disturbed. 



89*7 Does a body ever cease to give out heat by friction f 



No j however long the operation may be continued. 



898 What conclusions respecting the nature of heat have philosophers 

 drawn from this fact ? 



That heat cannot be a material substance, but merely 

 & property of matter. 



899 Why is it easier to produce heat from the friction of rough sur- 

 faces than smooth ones f 



Because in the friction of rough surfaces certain par- 

 ticles are rubbed off, which, being small, are readily con- 

 densed, and made to evolve their latent caloric. 



900 Why, when you rub a smooth metallic surface, as a button, for 

 example, against apiece qf plank, does the metal become more heated than the 

 wood? 



Because the caloric is forced out of the wood, as it 



