THE EARTH'S CRUST 177 



6. 1705:56-57. Hard and Soft Water. 



c. 1706 : 327. Calcium Compounds and Hardness of 



Water. 



d. 1707 : 359-361. Temporary and Permanent Hardness. 



e. 1708:268. Hard Water. 

 /. 1709:247-248. Hard Waters. 



g. 1712 : 65-67. Soft and Hard Waters. 



Experiment 68. Temporary Hardness. 



Apparatus: Kipp generator, test tube, test-tube holder, 

 burner. 



Materials: Marble in lumps, hydrochloric acid, calcium 

 hydrate (limewater). 



a. Fill a test tube half full of limewater, and pass carbon 

 dioxide, from the Kipp generator, into it slowly. Tell what 

 happens. Continue to pass in the carbon dioxide, and tell what 

 happens. 



6. Warm the contents of the test tube, and tell what hap- 

 pens. The heat causes the excess of carbon dioxide to pass 

 off, and the calcium carbonate is no longer soluble. Thus 

 heating water which is temporarily hard causes the dissolved 

 material to be precipitated. 



125. THE EARTH'S CRUST 



We see agents of disintegration acting on .the earth's surface, 

 and by working backward we shall always be led to the reali- 

 zation that once all must have been solid rock, without 

 any covering of soil. By analyzing soil, we find that it is 

 merely changed rock, altered both physically and chemically, 

 but containing the material which goes to make up the rock. 

 Review Section 30, Chemical and Physical Changes. 



Gradually the surface became covered with a thin soil 

 which slowly grew thicker as the agents of erosion continued 



