SILICON; BORON 361 



dioxide and carbon. The gaseous products enter a condenser in 

 which the tetrachloride assumes the liquid form: 



2C1 2 + SiO 2 + 2C -* SiCl 4 + 2CO. 



Silicon tetrachloride is a colorless liquid (b.-p. 59) which fumes 

 strongly in moist air, giving silicic acid and HCL Mixed vapors 

 of SiCU, NH 3 and H 2 O produce a very dense white smoke, con- 

 sisting of minute particles of NH 4 C1 and silicic acid. This smoke 

 was utilized during the war for screening vessels from submarines. 



Glass. Calcium carbonate (limestone) interacts at a high 

 temperature with sand in the same way as does sodium car- 

 bonate : 



Si0 2 + CaC0 3 -> C0 2 1 + CaSi0 3 



giving calcium silicate. Now sodium silicate, when alone, is 

 soluble in water. Calcium silicate is insoluble, but forms a 

 brittle, crystalline mass. By using both sodium and calcium 

 carbonates, and employing a larger proportion of sand than 

 that shown in the equation, a material is obtained which has 

 the qualities required in glass. When cooled, the molten mass 

 becomes viscous and finally, for all practical purposes, solid. 

 Yet it does not crystallize it is amorphous. It is also prac- 

 tically insoluble in water. 



By pouring the viscous material into moulds, or stamping 

 it with dies, articles of pressed glass are obtained. Bottles are 

 blown, by taking up a sufficient mass of the hot, thick liquid on 

 the end of an iron tube, inserting it in a mould, and blowing until 

 the outline of the mould is filled. Window glass is made by 

 blowing an immense, elongated bubble (6 by l\ ft.), ripping it 

 while still hot and soft, and flattening it out. Plate glass for 

 windows and mirrors is manufactured by pouring out the material 

 upon a cast-iron table, with a raised rim, and passing a large, 

 heated iron roller over it. The plate is subsequently ground flat 

 on both sides and polished with rouge (Fe 2 3 ). 



