PRACTICAL APPLICATION 33 



We find by experiment that every liquid has its own spe- 

 cific boiling point ; for example, alcohol boils at 87 C. and 

 brine at 103 C. Both specific heat and the heat of vaporiza- 

 tion vary with the liquid used. 



23- Condensation. If one holds a cold lid in the steam of 

 boiling water, drops of water gather on the lid ; the steam is 

 cooled by contact with the cold lid and condenses into water. 

 Bottles of water brought from a cold cellar into a warm room 

 become covered with a mist of fine drops of water, because 

 the moisture in the air, chilled by contact with the cold 

 bottles, immediately condenses into drops of water. Glasses 

 filled with ice water show a similar mist. 



In Section 21, we saw that 536 calories are required to 

 change I gram of water into steam ; if, now, the steam in 

 turn condenses into water, it is natural to expect a release of 

 the heat used in transforming water into steam. Experiment 

 shows not only that vapor gives out heat during condensation, 

 but that the amount of heat thus set free is exactly equal to 

 the amount absorbed during vaporization. (See Laboratory 

 Manual.) 



We learn that the heat of vaporization is the same whether 

 it is considered as the heat absorbed by I gram of water in 

 its change to steam, or as the heat given out by I gram of 

 steam during its condensation into water. 



24. Practical Application. We understand now the value of 

 steam as a heating agent. Water is heated in a boiler in the 

 cellar, and the steam passes through pipes which run to the 

 various rooms ; there the steam condenses into water in the radi- 

 ators, each gram of steam setting free 536 calories of heat. 

 When we consider the size of the radiators and the large num- 

 ber of grams of steam which they contain, and consider fur- 

 ther that each gram in condensing sets free 536 calories, we 

 understand the ease with which buildings are heated by steam. 



CL. GKN. SCI. 3 



