GLASS. 



321 



brought to the form of a cylinder, such as is 

 represented in the figure. The cylinder is 

 then cracked longitudinally, by letting a drop 

 of water run down its length, and following it 

 by a hot iron. It is subsequently reheated, 

 opened, and flattened out into a sheet, which 

 is then cut into paries of smaller size, if required. 

 How are glass 815. GLASS TUBES. To make a glass 

 tubes made? tube, a bulb is first blown, such as is repre- 

 sented on the previous page. An assistant then at- 

 taches his tube to the hot bulb at the opposite side, 

 and moves backward. The glass is thus drawn out, 

 as if it were wax, and the cavity within it is elongated 

 to a smooth and perfect bore. 



816. GLASS BOTTLES Bottles and a 



Glass bottles? . f . 



great variety of other objects of glass, are 

 made by the enlargement of similar bulbs within a 

 mould of the required shape. Bottle glass is usually 

 made of cheaper and less pure materials than window 

 glass, and contains, in addition to the materials before 

 mentioned, alumina and oxides of iron and manganese. 

 It owes its green color to the protoxide of iron. 

 Glass mir- 817. GLASS MIRRORS. Plate glass, such 



rors? as is used for mirrors, instead of being 



blown, is cast in metallic tables of the required shape, 

 and then rolled out and polished. 

 Wha.n*crys~ 818. CRYSTAL GLASS. This name is 

 tal glass ? given to a highly brilliant glass, contain- 

 ing potassa and litharge as bases. It is used for prisms, 

 lenses, lustres, and the finer qualities of cut glass ware. 



14* 



