CHAPTER V 

 HEAT: ITS DISTRIBUTION AND MEASUREMENT 



73. Sources of Heat. The principal sources of heat 

 are: 



Chemical action, as in the case of burning. 

 Friction, as when a match is rubbed on a rough surface. 

 Compression, as when the tube of a bicycle pump becomes 

 hot because the air in it has been repeatedly compressed. 



74. Effects of Heat. In the study of the physical states 

 of matter, it was shown that heat changes the physical con- 

 dition from solid to liquid, and from liquid to gas; but the 

 first effect of heat is to raise the temperature, the next to 

 increase the volume. Heat is recognized as a form of energy 

 because it overcomes the force which holds particles of mat- 

 ter together, and separates the molecules, causing expansion. 



Many compounds, on being heated to a high temperature, 

 separate into two or more substances (some of which are 

 gases), without first melting. This is the case with wood, 

 paper, and similar compounds. Charcoal is made by heat- 

 ing wood in a partly covered space, without burning it. 

 During the process of heating, a heavy yellowish smoke 

 comes from the wood, and a sticky dark substance, known as 

 wood tar, collects on the inside of the charcoal kiln. The 

 solid which remains is charcoal. (See Fig. 29, p. 72.) 



75. The Thermometer. Change of temperature is some- 

 times known by the sense of feeling, but it is more accurately 

 shown by means of a thermometer. The sense of feeling 

 often reports comparative temperature only. For example, 

 if a person puts one hand into hot water and the other into 

 cold water for a time, and then puts both hands into warm 



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