GLASS-CUTTING. 



385 



The best are first blown, and afterwards cut and polished. Of whatever 

 kind of glass the article may be, it is so brittle that the slightest blow would 

 break it, a bad quality which is got rid of by a pro- 

 cess called "annealing," that is, placing it while 

 quite hot on the floor of an oven, which is allowed to 

 cool very gradually. This slow cooling takes off the 

 brittleness, consequently articles of glass well annealed 

 are very much tougher than others, and will scarcely 

 break in boiling water. 



The kind generally used for ornamental cutting 

 is flint-glass. Decanters and wine-glasses are therefore 

 made of it ; it is very bright, white, and easily cut. 

 The cutting is performed by means of wheels of 

 different sizes and materials, turned by a treadle, as in 

 a common lathe, or by steam power ; some wheels 



* ' Fig. 390. -Plate-glass casting- 



are made of fine sandstone, some of iron, others of bringing out the P ot. 



tin or copper ; the edges of some are square, or round, or sharp. They arc 



Fig. 391. Glass furnace. (See also fig. 390 

 for detail.) 



Fig. 392. Glass-cutting. 



used with sand and water, or emery and water, stone wheels with water 

 only. 



In a soluble form silicic acid is found in springs, and thus enters into 

 the composition of most plants and grasses, while the shells and scales of 

 " infusoria " consist of silica. As silicate of alumina, z>., clay, it plays a 

 very important role in our porcelain and pottery works. 



BORON is found in volcanic districts, in lakes as boracic acid, in com- 

 bination with oxygen. It is a brownish-green, insoluble powder, in a free 

 state, but as boracic acid it is white. It is used to colour fireworks with the 

 beautiful green tints we see. Soda and boracic acid combine to make 



25 



