Transportation on Land 307 



the temperature of one pound of water from 32 F. to 212 F., 

 or through 180 degrees is, therefore, 180 British Thermal Units. 

 In your experiment with steam and boiling water (page 304), 

 you find that steam gives up about six times as much 

 heat as boiling water does. By very carefully conducted 

 experiments it has been found that the quantity of heat re- 

 quired to change one pound of boiling water into steam is 972 

 B.T.U., or a little more than five times the quantity utilized in 

 raising the temperature of water from the freezing to the boil- 

 ing point. The facts are shown graphically in Fig. 96. 



The practical significance of this difference in the heat 

 capacities of steam and boiling water may be made clear by 



i 82F. 



FIG. 96. Graphic representation of heat units in converting one pound 

 of ice into one pound of steam. 



expressing the facts in the following manner. Water in cooling 

 from the boiling temperature of 212 F. to 32 F. gives up heat 

 energy to the amount of 180 B.T.U. per pound. Steam in 

 merely changing from vapor at 212 F. to water at the same 

 temperature gives up heat energy to the relatively enormous 

 amount of 972 B.T.U. per pound. Thus it is evident that the 

 heat of steam, which is not shown by the thermometer, is a 

 store of energy readily utilized in work of great value to the 

 whole world. 



135. What steam bubbles suggest. In your observation 

 of the behavior of the steam bubbles or volumes in boiling 

 water, attention was directed to the fact that the bubbles, 

 small at the start, expanded rapidly on their way to the surface. 

 This behavior of the steam bubbles suggests two important 

 points. First, the volume of steam produced from a given 



