The Story-Book of the Fields 



substances combine. Before absorbing the 

 water the lime is called quick-lime ; after- 

 wards, when reduced to powder, it has the 

 name of slaked lime. This slaked lime is 

 made into a paste with water, well mixed and 

 kneaded with sand. It is now mortar. This 

 is the mortar that is inserted between layers 

 of stone to bind them together and to 

 strengthen the building. 



There is another observation which will 

 explain the part played by the mortar. If we 

 examine the water that has covered the 

 slaked lime for some daj r s we shall see a thin 

 transparent skin, like ice, floating on the 

 surface. This small solid crust is a substance 

 similar to that from which the lime was ex- 

 tracted ; it is chalk, or carbonate of lime. 

 You have been told that two things are 

 required for the formation of such a sub- 

 stance, viz., lime and carbonic acid gas. 

 The lime is provided by the water, which must 

 hold it in solution, as it covers a thick layer 

 of the material, while the carbonic acid gas 

 comes from the air, where it is always present 

 in a small proportion. So the lime is able to 

 absorb slowly the small amount of carbonic 

 acid gas in the atmosphere and to resume its 

 former condition as chalk. 



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