182 SCIENCE OF COMMON THINGS. 



Why -water freezes first at the surface. Why running water freezes slowly. 



If ice were heavier than water it would sink, and a 

 river w^ould soon become a solid Hock of ice, which 

 could never be dissolved. 



The general rule is, that all substances become heavier from condensa- 

 tion ; but ice is lighter than water. 



1199 Why does not the ice on the surface of a river chill the water 

 beneath, and make it freeze f 



1. Because water is a very bad conductor ', and is 

 heated or chilled by convection only ; 



2. If the ice on the surface were to communicate its 

 coldness to the water beneath, the water beneath would 

 communicate its heat to the, ice. and the ice would in- 

 stantly melt / and 



3. The ice on the surface acts as a shield, to prevent 

 the cold from penetrating through the river to freeze the 

 water below the surface. 



1500 Why does water freeze at the surface first? 



Because the surface is in contact with the air, and the 

 ftir carries away its heat. 



1501 Why does the coat of ice grow tiiicker and thicker if the frost 

 continues f 



Because the heat of the water (immediately below 

 the frozen surface) passes through the ice into the cold 

 air. 



1SOS Why are not whole rivers frozen (layer ly layer) till they be- 

 come solid icef 



Because water is so slow a conductor, that our frosts 

 never continue long enough to convert a whole river 

 into a solid mass of ice. 



12O3 Why does not running water freeze as fast as still water f 



1. Because the motion of the current disturbs the 

 crystals, and prevents their 'forming into a continuous 

 surface ; and 



2. The heat of the under surface is communicated to 

 the upper surface by the motion of the water. 



1SO4 When running water is frozen, why is the ice generally very 

 rough f 



Because little flakes of ice are first formed and carried 

 down the stream, till they meet some obstacle to stop 



