106 THE NATURE STUDY COURSE. 



Put some lumps of ice in water and set the dish on the 

 stove. Test the heat with a thermometer. Test it again 

 when half the ice is melted. Why does the thermometer show 

 the same reading as before? Because all the heat that the 

 water received has been used to convert the ice into water. 



Water in going from ice to liquid and from liquid to vapor 

 takes up heat, and when it is coming back from vapor to 

 liquid and from liquid to ice it gives out just as much heat as 

 it took up. 



Connect this with the fact that when clouds are forming for 

 rain, temperature rises ; when water is passing off into vapor, 

 temperature falls. 



Why does it cool the school-room to sprinkle the floor with 

 water ? Would sprinkling it with hot water cool the air in 

 the room ? Is a feverish brow cooled by bathing it with 

 warm water and allowing the water to evaporate ? 



Get the advanced classes to see that the water in the 

 atmosphere is the great equalizer of heat. When the air gets 

 cold its intensity is arrested by condensation ; when it gets 

 hot the intensity of the heat is checked by evaporation. 

 Were it not for this equalization the extremes of temperature 

 would render the globe uninhabitable. 



Sources of Fresh Water. — How does the atmosphere become 

 the great distributer of water ? 



What are the sources of wells, springs, rivers and lakes ? 



Hard and Soft Water. — Where do we get soft water? 

 Where, hard water? 



But we saw that rain was the original source of well-water. 

 What has made it hard ? 



Here is a basin of water. How can you tell whether it is 

 soft or hard ? By testing it with soap. If it will not make a 



