CHAPTER VIII 

 HEAT 



HEAT is a form of energy with which we are well ac- 

 quainted, since there are so many effects of heat that are 

 very important in our daily lives. Heat was supposed 

 to be a fluid until the beginning of the nineteenth century. 

 This fluid was called " caloric " and its mysterious passage 

 from one body to another was supposed to result in 

 changes of temperature. 



Heat results from the motion of molecules. As we have 

 learned, matter is made up of minute particles which, 

 although closely crowded together, still have spaces 

 between them. These particles are in constant motion,' 

 striking each other at every turn. The velocity with 

 which these little particles move to and fro in their short 

 excursions determines the temperature of the body - 

 the faster the motion the higher the temperature. When 

 the activity of the particles is lessened, the body becomes 

 cooler. If the vibrations are sufficiently rapid, both heat 

 and light are produced. 



Sources of Heat. - - There are a number of sources of 

 heat, but the principal sources are friction, compression, 

 chemical action, the sun, body heat, and electrical resist- 

 ance. 



Friction. The resistance offered by the sliding of one 

 body on another is friction, and it is a well-known fact 

 that friction produces heat. Most of us have observed 

 this in numerous instances. The journals of cars sometimes 



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