LATENT AND SPECIFIC HEAT. 329 



EGT1ON III. 



LATENT AND SPECIFIC HEAT. 



514. Thermal Units. -In 473 it was stated that 

 heat is measurable; but that we may measure it, a standard 

 or unit of measure is necessary. A thermal or heat 

 unit is the amount of heat necessary to warm a 

 weight unit of water one degree above the freezing 

 point. The weight unit generally used is the kilogram or 

 pound; any other weight unit may be used with equal 

 propriety. The degree may be centigrade or Fahrenheit. 



(a.) We therefore have at least four units in common use. They 

 are the amounts of heat necessary to warm 



(1.) A kilogram of water from C. to 1 C. 

 (2.) A kilogram of water from 32 F. to 33 P. 

 (3.) A pound of water from C. to 1 C. 

 (4.) A pound of water from 32 F. to 33 F. 



It makes no practical difference which unit is used, excepting so 

 far as convenience is concerned, but the unit must not be changed 

 during any problem. The first of these units is called a calorie. 



515. Two Fruitful Questions. We have already seen 

 that heat melts ice, and that during the melting the temperature i& 

 constant ; that heat boils water, and that durisg the boiling the 

 temperature is constant. One feature of this change of condition 

 remains to be noticed more fully. Take a block of ice with a tem- 

 perature of 10 C. (14 F.) and warm it. A thermometer placed in 

 it rises to C. The ice begins to melt, but the mercury no longer 

 rises. Heat is still applied, but there is no increase of temperature ; 

 the mercury in the thermometer remains stationary until the last 

 particle of ice has been liquefied. Then, and not till then, does the 

 temperature begin to rise. It continues to do so until the ther- 

 mometer marks 100 C. The liquid then begins to boil, and the 

 temperature a second time becomes fixed. But during all the time 

 that the thermometer stood at C. , or while the ice was melting, 

 heat was given by the lamp and received by the ice. Why then did 

 not the temperature rise during that time, instead of remaining the 



