482 SODIUM. 



crown-glass, and the beautiful Bohemian glass are of this compo- 

 sition. In the most remarkable varieties the oxygen of the bases 

 is to that of the acid as 1 to 4, and the oxygen of the lime to that 

 of the potash in proportions which vary from 1 and f , to 1 and 

 1. This is the glass of most difficult fusibility, and therefore 

 most suitable for the combustion tubes employed in organic 

 analysis. From its purity, and the absence of oxide of lead, it 

 is also made the basis of most coloured glasses, and of stained 

 glass. To produce coloured glasses certain metallic oxides are 

 mixed with the fused glass in the pot, oxide of cobalt for in- 

 stance, for a blue colour, oxide of copper for green, peroxide of 

 manganese for purple, and peroxide of uranium for a delicate 

 lemon yellow tint. Arsenious acid and peroxide of tin render 

 glass white and opaque, like enamel. In stained glass, on the 

 other hand, the metallic oxides are merely applied with proper 

 fluxes to the surface of the glass, which is then exposed in an 

 oven to a temperature sufficient to fuse the colouring matter. 

 Different shades of yellow and orange are thus produced by 

 means of oxide of silver, and a superb ruby red by a proper, 

 but difficult, application of suboxide of copper. 



Silicates of potash and lead. These substances enter into the 

 composition of the purer and more brilliant species of glass in 

 use in this country, such as that called crystal, of which most 

 drinking vessels are made, flint-glass for optical purposes, and 

 strass, which is employed in imitations of the precious stones. 

 For crystal, the materials are taken in the following proportions, 

 120 parts of fine sand, about 40 of purified potashes, 35 of 

 litharge or minium, and L2 of nitre. In this glass, the oxygen 

 of the bas^es is to that of the silica, as 1 to a number which 

 may vary from 7 to 9, and the oxygen of the potash is to that 

 of the oxide of lead, as 1 to a number varying from 1 to 2.5. 

 In flint-glass and in strass, the oxygen of the bases is to that 

 of the silica as 1 to 4, and the oxygen of the potash is to that of 

 the oxide of lead as 2 to 3 in flint-glass, and as 1 to 3 in strass, 

 (Dumas). The more oxide of lead glass contains, the higher 

 its density ; the density of this kind of glass exceeding 3.6, 

 while that of the Bohemian glass does not rise higher than 2.4. 

 Glass containing oxide of lead is recommended by its greater 

 fusibility and softness, by which it is more easily fashioned into 

 various forms, and by its great brilliancy, which is remarkable 

 in lustres and other objects of cut glass. The presence of lead 



