QUANTITY AND TRANSMISSION OF HEAT 139 



To change a gram of water at 100 to steam at the same 

 temperature requires 536 calories of heat, steam being said 

 to have a heat of vaporization of 536. 



Experiment 41. Place 200 grams of finely cracked ice in a tin 

 cup. Upon this pour 200 grams of water at 80 and note the result- 

 ing temperature. Why is the resulting temperature not 40? 



Experiment 42. Pass steam through a steam trap (Figure 119) 

 into a vessel containing 300 grams of water at about 15? Take 



FIG. 119. A Steam Trap. 



the temperature of the water and then weigh carefully. From the 

 data calculate the heat of vaporization. 

 300 (t - 15) 



W - 300 

 t = final temperature, w = final weight, H = heat of vaporization. 



The condensation of steam is a great source of heat and 

 is much used in systems of heating for buildings. The 

 condensation point is the point at which the most heat 

 is given off. For example, a gram of water gives up 536 

 calories of heat in simply changing from steam at 100 

 to water at 100. 



We are now better able to understand why evaporation 

 is a cooling process. It is simply that, since only the 



