GLASS. 483 



in glass is at once discovered by its surface acquiring a metallic 

 lustre when heated to redness in the reducing flame. 



Silicates of alumina, of the oxides of iron, magnesia, and 

 potash or soda. Green or bottle glass, of which wine-bottles, 

 carboys, and glass articles of low price consist, is a mixture of 

 these silicates. It is formed of the cheapest materials, such as 

 sand, with soap-makers' waste, lime that has been used to render 

 alkali caustic, &c. In the bottle-glass of this country the small 

 quantity of alkali is chiefly soda. The alkaline sulphates when 

 fused with silica and carbonaceous matter, lose their sulphuric 

 acid, and become silicates ; even common salt is decomposed by 

 the united action of silica and the aqueous vapour in flame, but 

 much of it is lost from its own volatility. The proportion of 

 silica to the bases is much l^ss in this than in the other kinds of 

 glass, the oxygen of the former being to the latter as 2 to 1 ; and 

 the oxygen of the alumina and peroxide of iron equal to that of 

 the potash and lime. This glass is in fact a mixture of neutral 

 and subsilicates, and is more apt than any of the preceding spe- 

 cies to assume a crystalline structure when maintained long in 

 a soft condition by heat. A bottle of green glass may be devi- 

 trified, or converted into what is called Reaumur's porcelain, by 

 enveloping it in sand, and placing it where its temperature is 

 kept high for several weeks, as in a brick kiln or porcelain fur- 

 nace. It has been supposed that the glass loses its alkali in 

 these circumstances, and is thus more easily crystallized, but the 

 proportion of alkali is found undiminished after the change. 

 Glass of all kinds, however, when strongly and repeatedly 

 heated loses alkali, from its volatility ; the glass then becomes 

 harder and less fusible, and is not so easily wrought, a circum- 

 stance which may sometimes be remarked in blowing a bulb 

 upon a tube which has been too long exposed to the blow-pipe 

 flame. 



SECTION III. 



LITHIUM. 

 Eg. 80.33 or 6.41; L. 



Lithium is the metallic basis of a rare alkaline oxide lithia 

 discovered in 1818 by Arfwedson*. The name lithia (from 



* An. dc Ch. et dc Ph. t. 10. 



