LATENT HEAT 



85 



two cups? Continue to stir and note the changes until the ice is 

 melted. Do your notes show that like amounts of heat have pro- 

 duced like changes of temperature in the two cups? Continue to 

 heat, stirring and noting the temperatures occasionally. Is there 

 now an approximately equal rise of the temperatures of the water 

 in the cups? 



When the water in one cup begins to boil, does its temperature 

 continue to rise as fast as that of the water in the other cup? What 

 apparently became of the 

 heat delivered to the ice- 

 water before the ice melted ? 

 What apparently became 

 of the heat delivered -to 

 the water while it is boil- 

 in g? 



The preceding experi- A 



ment shows that heat is 

 absorbed in melting ice, 

 and that the heat so 

 absorbed does not raise 

 the temperature of the 



ice. It also shows that heat changes water into steam, and 

 that although very much heat was applied none of it was 

 used in raising the temperature of the boiling water but all 

 of it in changing the condition of the water. 



Carefully performed experiments show that it takes 80 

 times as much heat to change a gram of ice at C. into 

 water at C. ; and about 536 times as much to change 

 a gram of water at 100 C. into steam at 100 C. as it does to 

 raise the temperature of the same mass of water one degree 

 C. The heat absorbed in changes of this kind is called 

 latent heat. It is all given out again when the water freezes 

 or the steam condenses. 



This explains why ice melting in a refrigerator takes so 



FIGURE 39 



