226 INDUSTRIAL MINERALS DIAGRAM 20. 



Chalk is a very soft and white limestone, which becomes a paste 

 in water. When this paste is sifted, it is dried, and called whitening. 



There is a very finely-grained grey or yellowish limestone 

 which is used for taking impressions. A drawing is made on 

 these stones with a particular kind of ink, and by pressing the 

 design against a sheet of white paper, we can take as many im- 

 pressions as we wish. This process is called lithography, from two 

 Greek words which mean writing on stone. 



We have mentioned that all limestones yield quick-lime when 

 heated to redness, and this operation is performed in lime kilns. 

 When quick-lime is thrown into water, it grows hot, and forms a 

 paste with it which is mixed with sand to make mortar. There 

 are three kinds of lime ; fat lime, poor lime, and hydraulic lime. 



Fat lime is produced by the hardest limestones. A large pro- 

 portion of sand can be mixed with it to make mortar, and it is 

 therefore economical, but the mortar is not very firm. 



Poor lime is generally not so white, and much more of it must 

 be employed to make mortar, but it holds much better. 



Lastly, hydraulic lime, or Roman cement, is made of limestones 

 which contain a large proportion of clay. It is then enough to 

 mix it with water to produce a paste which immediately becomes 

 very hard, so it is used to construct works which are to be sub- 

 merged. When hydraulic lime is placed in water it becomes 

 harder and harder. 



Sands and Sandstones. There are entire layers of sand in the 

 earth similar to that on the sea-shore. They are sometimes white, 

 sometimes bluish, and at other times red in consequence of 

 containing iron. 



Tripoli is an exceedingly fine and hard sand which is used to 

 polish metals. 



Sandstone is entirely formed of conglomerated grains of sand. 

 Sandstone is sometimes friable, and easily disintegrated, and at 

 other times it forms a very durable stone which is used for paving 

 the streets of towns. 



Silex, or Flint, is formed of the same substance as grains of 



