480 



SODIUM. 



boiling water. The solution may be applied to objects of wood, 

 and when dried by a gentle heat forms a varnish, which imbibes 

 a little moisture from the air, but is not decomposed by car- 

 bonic acid, nor otherwise alterable by exposure. Stuffs im- 

 pregnated with the solution lose much of their combustibility, 

 and wood is also defended by it, to a certain degree, from com- 

 bustion. 



GLASS. 



The alkaline silicates, cooled quickly or slowly, never 

 exhibit a crystalline structure, but are uniformly vitreous. 

 They are the bases of the ordinary varieties of glass, which 

 contain earthy silicates besides, but appear to owe the 

 vitreous character to the silicates of potash and soda. The 

 silicate of lime and the silicate of the protoxide of iron 

 crystallize on cooling, so does the silicate of lead, unless it 

 contains a large excess of oxide of lead. The addition of the 

 silicate of potash or soda deprives them entirely of this pro- 

 perty ; the silicate of alumina considerably diminishes it. But 

 if silicates of potash or soda are heated for a long time, the 

 alkali may in part escape in vapour, and if other bases exist in the 

 compound, it then often assumes a crystalline structure on cool- 

 ing. The alkaline silicates by themselves are soluble in water, and 

 decomposed by acids ; the silicate of lime is also dissolved by 

 acids, but the double silicates, on the contrary, resist the 

 action of acids, particularly when they contain an excess of 

 silica. The following table exhibits the composition of the best 

 known kinds of glass, from the analyses of Dumas and of 

 Faraday : 



COMPOSITION OF VARIETIES OF GLASS. 



Silicate of soda and lime. To form window-glass 100 parts 

 of a quartzy sand are taken, with 35 to 40 parts of chalk, 30 to 



