WHY ICE FORMS ONLY AT THE SURFACE 83 



86. Exercises. 



1. Hard cookies placed in a box containing fresh bread become soft 

 and mellow; why? Why do crackers lose their crispness when taken 

 out of their box? 



2. Tell why a potato loses weight when baked. 



3. Why does candy become sticky, and why does salt "cake," in 

 damp weather? 



4. The Jordan River flows into Lake Utah, and Lake Utah empties 

 into Great Salt Lake. Would you expect Lake Utah to have fresh 

 water or salty? Why? 



5. Tell how the crew of a ship can prepare fresh water out of sea 

 water? How does nature do it? 



6. Name some of the ways in which dirty water thrown into the 

 yard is purified by nature. Why should such water never be thrown 

 near a well or cistern? 



7. Why does moisture condense on the windows of the house on a 

 cold wash day? 



8. Why should bottles of alcohol, gasoline, and turpentine be stop- 

 pered? 



87. Why Ice Forms Only at the Surface. Since ice 

 is lighter than water (cf. 70), it floats, and the ice cover- 

 ing usually prevents the water beneath from freezing. But 

 the chief reason why lakes and streams do not freeze to 

 a great depth is that water is most dense, or heavy (cf. 

 33), not at C., the freezing temperature, but at 4 C. 

 Let us see the results of this fact : 



A lake or stream is cooled chiefly from the top, where it 

 touches the cold air. Now, when a lake having water at, 

 let us say, 10 C., is cooled by air at C., or below, the 

 surface layer becomes colder than 10 C., and heavier, 

 and so sinks to the bottom, while a warmer layer takes its 

 place. This movement of water goes on until all the 

 water is cooled to 4 C. But as the upper layer of water, 



