68 HEAT AND ENERGY 



pour a little sulphuric acid upon water in a test tube. In each 

 case feel of the glass around the liquid. What do you discover 

 about chemical action? 



Heat is very coAimonly caused by combustion or burn- 

 ing. This is a sort of chemical action and is treated in 

 260. Electricity is also a common source of heat; 

 its heating effects are shown in electric lights and are 

 used in furnaces and heaters. Its action is explained 

 in 192. 



75. Theory of Heat. In studying the molecular 

 theory ( 11) we learned that the molecules of all matter 

 are thought to be in a state of constant vibrating motion. 

 Naturally we may suppose that in some bodies the vibra- 

 tion is more rapid than in others ; also that in the same 

 body the motion may be greater or less at different 

 times. The heat of any body is believed to vary with 

 this vibration of its molecules, as stated in the theory 

 of heat as follows : The heat of a body is the energy of 

 vibration of its molecules ; the faster they move, the warmer 

 is the body. 



With this theory in mind, the results obtained in 

 Experiments 58 and 59 may be easily understood. 

 Rubbing, in the one case, and pounding, in the other, 

 simply caused the motion of the molecules to become 

 more rapid, and the masses became warmer. The theory 

 applies also in the other cases. 



Within certain limits we can discover differences in 

 the heat of things about us ; we say that a body feels 

 more or less " warm." It must be carefully noted, how- 

 ever, that this is only the effect that heat produces upon 



