424 Introduction to the Study of Science 



nealed the glass is ground down to one half or one fourth of an 

 inch thickness and polished. The grinding is done entirely, 

 and the polishing partly, by machinery. 



Cut-glass articles are made from the lustrous flint glass. 

 After they are molded, they are decorated with designs cut by 

 means of emery wheels, and then polished by machinery and 

 by hand. Imitation diamonds are similarly made from the 

 same kind of glass. Other jewels are imitated by suitably 

 coloring flint glass. 



Sand is the principal material used in making glass. In the 

 study of sedimentary rocks (pages 407, 415) we found that sand 

 is mostly quartz, which is derived from the decomposition of 

 granite, an igneous rock. This leads to an inquiry into the 

 nature of quartz. 



Exercise. Examine a piece of quartz, testing it for the following 

 properties. It is crystalline, forming six-sided crystals, transparent, 

 and very hard, as it scratches glass or is as " hard as flint," flint being 

 a different form of the same substance as quartz or sand. It is fusible 

 at the very high temperature produced in the electric furnace. It is 

 often found in large crystals which may be cut into lenses for spec- 

 tacles or other optical instruments, or to imitate diamonds. Its use 

 in the Cooper-Hewitt lamp may be cited. 



203. Silica. Quartz as considered by chemists is a com- 

 pound made up of two elements, silicon and oxygen. Silicon, 

 a mineral which does not exist separately or free in nature, 

 unites freely in the ratio of one part to two parts of oxygen 

 to form silicon dioxid, or silica. Quartz is thus seen to be 

 silica, and sand of the pure white variety is practically pure 

 silica. Other sands are mixtures of silica and various other 

 minerals. 



Abundance and distribution. Silica is very widely dis- 

 tributed and abundant in the earth's crust. Wherever sand 

 is found, in whatever condition and state, whether loose or com- 

 pact as sandrock or quartzite, there is silica. In the desert, on 

 the shore and bed of stream, lake, and ocean, on mountain and 



