278 NATURE-STUDY REVIEW [12:6-Sept., 1916 



When I asked where this principle was to be observed many men- 

 tioned the freezing of ice cream while others explained the "salt- 

 ing" of the points and frogs of railway switches. I insisted on a 

 wider and commoner example, and one boy mentioned his moth- 

 er's ice-box. 



EXPERIMENT TEN 



To establish the very common principle that evaporation cools, 

 we used two thermometers and a few drops of alcohol. I used 

 the school thermometer and a common ten cent thermometer. I 

 placed them close together and noticed that both registered 72 . 

 Then I saturated a small cloth with alcohol and poured a few drops 

 of the alcohol on the bulb of the ten cent thermometer. I placed 

 the cloth about the ten cent thermometer and proceeded to fan 

 both thermometers with a fan of folded newspaper. I ceased fan- 

 ning after a minute and noticed that the temperature of the school 

 thermometer was 72 while that of the other was 62 . The tem- 

 perature of the ten cent thermometer had dropped io°. 



We then asked for common applications of this principle. By 

 a discussion, we found that our sprinkling wagons only lay the 

 dust for a time although they do cool the air. We mentioned that 

 it was usually cooler after a shower in summer. The commonest 

 proof of this is of course our perspiration — which is only nature's 

 way of cooling our overheated bodies. The breeze and the fan 

 only accelerate evaporation. I told the class that New Orleans 

 could make all the ice it chose by simply evaporating liquid am- 

 monia. Thus by an apparatus costing only a few cents some of 

 the most fundamental and interesting laws of nature can be made 

 comprehensible to the twelve-year-old boy or girl. 



EXPERIMENT ELEVEN 



When^ I announced to the class that I would make water boil 

 with ice the next time we met, they were suspicious of some trick. 

 They wondered how it could be done. Since wonder is the be- 

 ginning of knowledge, I thought it better to arouse their curiosity 

 than to tell them that I would prove that the boiling point of water 

 depended upon the force of the pressure of the atmosphere. 



For this experiment you need a round-bottomed Florence flask, 

 a tight cork, a Bunsen burner and some ice or cold water. We 

 rilled the flask one-fourth full of water and heated it till it began 



