48 HEAT. 



all the rays of light converge is thus found. The heat 

 focus is a little beyond, but so near that the difference 

 need not be taken into account. 



Section 3. Changes effected by Heat. 



99. EXPANSION, MELTING, AND VAPORIZA- 

 TION are the principal changes effected by 



heat? heat, while contraction, freezing, and con- 



densation of vapor are produced by its withdrawal. 

 But before these changes are explained, it will be well 

 to consider certain remarkable differences in the heat- 

 ing effects of heat, in the case of different substances. 



100. THE HEATING EFFECT OF HEAT IS 



Are the effects DIFFERENT FOR DIFFERENT SUBSTANCES. 



of neat equal 



in different It might naturally be supposed that the 

 same quantity of heat actually imparted 

 to different substances would make them equally hot ; 

 but this is not the case. If two cannon balls of the 

 same size, and at the same temperature, are quenched, 

 the one in mercury and the other in water, the mer- 

 cury will be made much hotter than the water, by 

 the reception of the same amount of heat. It does 

 not simply feel hotter, as it might do if it were not 

 really so, from the superior conducting power of the 

 mercury, but it is actually so, as may be ascertained 

 by testing the temperature by the thermometer. 

 What is sped- 101- SPECIFIC HEAT. If the above ex- 

 fic heat ? periment were varied, by quenching in 

 mercury a bullet of one-thirtieth the size of that used 



