AlANLFACTURE OF GLASS. 163 



And jOlbs. of the lava of an iron-furnace: these materials must be 

 well mixed." 



The materials employed in the manufactory of glass are by che- 

 mists reduced to three classes, namely, alkalies, earths, and me. 

 tallic oxides. 



The fixed alkalies maybe employed indifferently ; but soda is 

 preferred in this country. The soda of commerce is usually mixed 

 with common salt, and combined with carbonic acid. It is proper 

 to purify it from both of these foreign bodies before using it. This, 

 however, is seldom done. 



The earths are silicia, (the basis of flints), lime, and sometimes 

 a little alumina, (the basis of clay). Silicia constitutes the basis 

 of glass. It is employed in the state of fine sands or flints; and 

 sometimes, for making very fine glass, rock crystal is employed. 

 When sand is used, it ought if possible to be perfectly white ; for 

 when it is coloured with metallic oxides, the transparency of the 

 glass is injured. Such sand can only be employed for very coarse 

 glasses. It is necessary to free the sand from all the lo:>e earthy 

 particles with which it may be mixed, which is done by washing 

 it well with water. 



Lime renders glass less brittle, and enables it to withstand better 

 the action of the atmosphere. It ought in no case to exceed the 

 twentieth part of the silicia employed, otherwise it corrodes the 

 glass pots. This indeed may be prevented by throwing a little clay 

 into the melted glass ; but in that case a green glass only is ob. 

 tained. 



The metallic oxyds employed are the red oxyd of lead or litharge, 

 and the white oxyd of arsenic. The red oxyd of lead, when added 

 in sufficient quantity, enters into fusion with silicia, and forms a 

 glass without the addition of any other ingredient. Five parts of 

 minium and two of silicia form a glass of an orange. colour and full 

 tf striae. Its specific gravity is five. The red oxyd of lead ren- 

 ders glass less brittle and more fusible ; but, when added beyond a 

 certain proportion, it injures the transparency and the whiteness of 

 the glass. 



The white oxyd of arsenic answers the same purposes with that 

 of lead ; but on account of its poisonous qualities it is seldom used. 

 It is cuftomary to add a little nitre to the white oxyd of arsenic, to 

 prevent the heat from reviving it, and rendering it volatile. When 

 added beyond a certain proportion, it reader* glass opaque and 



M 2 



