IKDUSTBIAL MINERALS DIAGRAM 19 225 



struck with iron, sparks appear, which are particles of iron 

 chipped off and ignited by the concussion. 



Mill-stone grit is another kind of silex which is generally of a 

 white or reddish colour, and is hollowed with a number of large 

 and small cavities. In spite of the presence of all these holes, 

 millstone grit is an exceedingly durable stone, with which foun- 

 dations and buildings which are required to possess unusual 

 solidity are constructed. Millstone grit is so called because it is 

 also used to make millstones. For this purpose stones must al- 

 ways be chosen which are both hard and full of holes, so as to 

 crush and bruise the corn better. Millstone grit answers these 

 conditions very well, but it is very rare to find quarries in which 

 millstones can be cut in one block, so they are made of several 

 pieces fitted together, and joined by cement. The millstone is 

 then strongly bound with iron, and allowed to dry for a very 

 long time before being used. These millstones are as solid as if 

 they were made out of a single piece. 



Rock-crystal, agate, glass. Eock-crystal and agate are also 

 formed of the same substance as sand, sandstone, and millstone. 

 This substance is silex, flint, or quartz. When it is perfectly 

 pure, it is also perfectly transparent, and forms rock-crystal. In 

 agates, the quartz is slightly coloured, or traversed by veins of 

 different colours. Eock-crystal and agates are extremely hard, 

 and are frequently used to make ornamental articles. 



Glass is made of sand and soda, which is melted together at 

 an intense heat. Fine glass is often called crystal. It is more 

 sonorous, and is cut easier than glass. It is obtained by adding 

 a definite quantity of litharge to melted glass. 



Clays. Clays are earths of exceedingly fine grain, which form, 

 when mixed with water, an adhesive paste, which can be worked 

 in different ways. They are composed of more or less impure 

 alumina. Clays are sometimes bluish, yellowish, or red. Though 

 they can be mixed with water, water cannot easily penetrate 

 them, and we therefore see the rain remain on the surface of the 

 ground for a long time in clayey districts. 



Bricks, and the different kinds of [crockery and earthenware, 



