164 MANUFACTURE OF GLASS. 



milky like the dial-plate of a watch. When any combustible body 

 is present, it is usual in some manufactures to add a little white 

 oxyd of arsenic. This supplying oxygen, the combustible is burnt) 

 and Hies off; while the revived arsenic is at the same time vola. 

 tilized. 



There are several kinds of glass adapted to different uses. The 

 best and most beautiful are the flint and the plate glass. These, 

 when well made, are perfectly transparent and colourless, heavy 

 and brilliant. They are composed of fixed alkali, pure siliceous 

 sand, calcined Hints, and litharge, in different proportions. The 

 Hint-glass contains a large quantity of oxyd of lead, which by cer- 

 tain processes is easily separated. The plate-glass is poured in the 

 melted state upon a table covered with copper. The plate is cast 

 half an inch thick, or more, and is ground down to a proper degree 

 of thinness, and then polished. 



Crown.glass, that used for windows, is made without lead, 

 chiefly of fixed alkali fused with silicious sand, to which is added 

 some black oxyd of manganese, which is apt to give the glass a tinge 

 of purple. 



Bottle. glass is the coarsest and cheapest kind : into this little or 

 no fixed alkali enters the composition. It consists of an alkaline 

 earth combined with alumina and silica. In this country it is 

 composed of sand and the refuse of the soap boiler, which consists 

 of the lime employed in rendering his alkali caustic, and of the 

 earthy matters with which the alkali was contaminated. The 

 most fusible is flint glass, and the least fusible is bottle glass. 



Flint-glass melts at the temperature of 10 Wedgewood ; crown- 

 glass at 30 ; and bottle.glass at 47. The specific gravity varies 

 between 2'4S and 3 '3 8. 



Glass is often tinged of various colours by mixing with it, while 

 in fusion, some one or other of the metallic oxyds; and on this 

 process, well conducted depends the formation of pastes or facti- 

 tious gems. 



Blue glass is formed by means of oxyd of cobalt. 



Green, by the oxyd of iron or of copper. 



Violet, by oxyd of manganese. 



Red, by a mixture of the oxyds of copper and iron. 



Purple, by the purple oxyd of gold. 



"White, by the oxyd of arsenic and of zinc. 



Yellow, by the oxyd of silver and by combustible bodie?, 



