30 Introduction. 



in doing the work of melting. The amount of heat re- 

 quired to melt one pound of ice is 142 units when ex- 

 pressed in round 'numbers ; or if the work done is expressed 

 in foot-pounds it will be 



142 X 778.3 = 110,518.6 foot-pounds 



and the time required for one horse power to do the work 

 would be 



When crushed ice and salt are mixed in the ice-cream 

 freezer the changing of the two solids to a liquid requires 

 so much energy, and it is used so rapidly, that the cream is 

 quickly frozen, its molecular motion being used in doing 

 the work. 



When water has been brought to the boiling tempera- 

 ture it ceases to become warmer so long as boiling contin- 

 ues, all of the heat energy entering from the fire being re- 

 quired to do the work of changing liquid water into steam. 

 The amount of heat required to change one pound of water 

 at 212 F. into steam at the same temperature is 966.6 

 hoat units. When expressed in foot-pounds it becomes 



778.3X966.6 = 752,305 



and the time required for one horse-power to do this work 

 is 



When a pound of water at 32 F. becomes ice at 32 F. 

 there reappears as heat the 142 heat units which were re- 

 quired to melt it, and so too when one pound of steam con- 

 denses into water there reappears 966.6 heat units. Be- 

 fore the nature of these changes were as well understood as 



