ARTIFICIAL STONE 271 



in the manufacture of lime for cement and fertilizer. Sand- 

 stone is easily quarried, and some kinds are very durable. 

 The brownstone houses in many cities show early signs 

 of decay ; this is because of a poor grade of stone having 

 been used. The " life " of a building stone is the length 

 of time before it shows signs of crumbling or chipping. 

 Compare the life of building stones as given in the fol- 

 lowing table. Granite, although extremely hard and du- 

 rable, is not very desirable for small houses. Slate is used 

 for roofing many houses. While it makes a hot roof, it 

 also makes a safe and durable one. 



Artificial stone. We are coming more and more to use 

 cement, hollow tile, and concrete in the construction of 

 our houses. Some of the most beautiful, as well as econom- 

 ical, small houses are now made of concrete or hollow tile, 

 with a coating of stucco, a kind of plaster which is placed 

 over the outside of the house. It has the advantage when 

 used, over wooden houses, of giving protection against fire. 

 Lime and sand are important building materials also. 

 Ordinary mortar made by mixing lime with sand and 

 water hardens more slowly than cement, the hardening 

 being caused by the taking up of carbon dioxide, which 

 changes the lime to calcium carbonate, or as the mason 



