MANUFACTURE OF GLASS. 237 



for bottles, and other utensils, intended to be cut 

 and polished, and for various ornamental purposes. 



Crown-glass differs from the last, in containing 

 no lead. It is made of soda and fine sand. It is 

 used for panes of windows, &c. 



Bottle-glass is the coarsest sort of all. It is 

 made from kelp and common sand. Its green 

 colour is owing to iron. It is the least fusible. 



Glass is sometimes coloured by mixing with it 

 while in a fluid state, various metallic oxyds. It 

 is coloured blue, by the oxyd of cobalt ; red, by 

 the oxyd of gold ; green, by the oxyd of copper 

 or iron ; yellow, by the oxyd of silver or anti- 

 mony, and violet, by the oxyd of manganese. 



The hardness of glass is very considerable ; its 

 specific gravity varies from 2. 3 to 4, according to 

 the quantity of metallic oxyd which enters into 

 its composition. Though glass, when cold, is 

 brittle, it is one of the most ductile bodies known. 

 When liquid, if a thread of melted glass be drawn 

 out, and fastened to a reel, the whole of the glass 

 can be spun off; and by cutting the threads of a 

 certain length, there is obtained a sort of feather of 

 glass. A thread of glass may be thus drawn or 

 spun so fine, as to be scarcely visible to the naked 

 eye. Glass is almost perfectly elastic, and is one 

 of the most sonorous bodies. Fluoric acid dis- 

 solves it at common temperatures, and alcalis in a 

 great degree of heat. These are the only sub- 

 stances known which act upon it. 



Glass utensils require to be gradually cooled 

 in an oven : this operation is called annealing, and 

 is necessary to prevent their breaking by change 

 of temperature, wiping, or slight accidental 

 scratches. 



