CHAPTER XII 



Soil (continued) 



Chalk is the rock from which lime is obtained. 

 It is composed of carbonic gas and lime. In 

 order to obtain the lime the chalk is exposed 

 to great heat in furnaces by lime burners. 

 The carbonic gas is disengaged and dispersed 

 in the air and the lime is left. Common build- 

 ing stone and ashlar are chalk. In arable 

 ground chalk is often present in larger or 

 smaller lumps, but more often it is very fine 

 dust which cannot be distinguished from other 

 substances, especially clay. River and spring 

 water almost always contains a small pro- 

 portion of chalk in solution. This provides 

 the stony layer which gradually accumulates 

 inside water bottles and dims the transparency 

 of the glass. Some water contains enough to 

 deposit a mineral crust on the objects over 

 which it flows, such as mosses and aquatic 

 plants and to fill up their arteries. The 

 clearest water, in which absolutely nothing 



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