144 



THE WATERS OF THE EARTH 



stances in solution cannot be removed in this way. When 

 dirty surface water seeps down through thick enough beds 

 of sand and porous rock, it is cleansed of its dirt; but it 

 does not lose by this filtering process any of the substances 

 it held in solution. On the contrary, it may have dissolved 

 substances from the rocks through which it filtered. In 



this way " soft " 

 rain water may be- 

 come hard water or 

 mineral water before 

 it reaches the surface 

 again in springs or 

 wells. 



When water has 

 absorbed carbon di- 

 oxide it is able to 

 dissolve limestone 

 and it then becomes 

 hard. When water 

 of this kind is boiled 

 or evaporated the 

 carbon dioxide 

 escapes and the lime 

 deposits, thus ren- 

 dering the water soft. Such water is called temporarily 

 hard water. Boiler and teakettle scale are deposits from 

 temporarily hard water. Permanently fyard water cannot 

 be softened by boiling. 



Emulsions. Experiment 66. Put a few drops of kerosene or 

 other oil into a test tube half full of water. Since the oil is lighter 

 than the water it rises to the surface. Shake the test tube vig- 

 orously. Does the oil mix with the water? Set the test tube 



A LIMESTONE CAVE 



A cavern dissolved out by water. Hard water 

 trickling in and evaporating has formed the 

 columns. 



