68 HEAT. 



fecting the senses and the thermometer. In the same 

 manner, our bodies are cooled in summer by perspira- 

 tion, and the evaporation which accompanies it. All 

 vapors may, indeed, be regarded as combinations of 

 heat with the liquids from which they are formed. 

 And in this case, also, the heat which becomes latent 

 in thus combining, is called latent heat. 



155. FREEZING BY EVAPORATION. The 



How can ether 



be made to more rapidly a substance evaporates, the 

 ^Explain* ^its niore heat does it require for the evapora- 

 action. tioii. This it obtains from objects in con- 



tact with it. Ether may be made to evaporate so 

 rapidly as to freeze water, even in summer. This is 

 best accomplished by covering the bottom of a test- 

 tube with a cotton rag, or several layers of porous f p 

 paper, as represented in the figure, dipping it into 

 ether, and then waving it to and fro in the 

 air, or spinning it between the palms of the hands. 

 By repeating this process several times, a few 

 drops of water, previously placed in the tube, may 

 be frozen. A mixture of liquefied carbonic acid and 

 nitrous oxide gases, previously liquefied, produce on 

 evaporation a temperature of 220 degrees below zero. 

 156. PROTECTION FROM HEAT BY EVA- 



How does eva- 

 poration pro- PORATION. By previously moistening the 



tect from heat? fingerS; they m&y bu dipped unharmedj f or 



an instant, into molten lead, or passed through a stream 

 of white-hot iron as it flows from the furnace. A 

 cloak of comparatively cool vapor is formed from the 

 moisture upon the fingers, and keeps them from con- 

 tact with the molten metal. 



