236 MANUFACTURE OP GLASS. 



which he rolls it again on a smooth surface to 

 polish it, and repeats the blowing, until the glass 

 is brought as near the size and form of the vessel 

 required as he thinks necessary. 



If it be a common bottle, the melted glass at the 

 end of the tube is put into a mould of the exact 

 size and shape of its body, and the neck is formed 

 on the outside, by drawing out the ductile glass. 



If it be a vessel with a wide orifice, the glass in 

 its melted state is opened and widened with an 

 iron tool ; after which being again heated, it is 

 whirled about with a circular motion, and by means 

 of the centrifugal force thus produced, is extended 

 to the size required. Should a handle, foot, or 

 any thing else of the kind, be required, these are 

 made separately, and stuck on in its melted state. 



WindoxiD'glass is made in a similar manner, ex- 

 cept that the liquid mass at the end of the tube is 

 formed into a cylindrical shape, which being cut 

 longitudinally by scissars or sheers, is gradually 

 bent back until it becomes a flat plate. 



Large plate glass, for looking-glasses, S^x, is 

 made by suffering the mass in a state of complete 

 fusion to flow upon a table, with iron ledges to 

 confine the melted matter, and as it cools, a metal- 

 lic roller is passed over it, to reduce it to an uniform 

 thickness. There are various kinds of glass manu- 

 factured for different purposes ; the principal of 

 these are flint-glass, crown-glass, and bottle-glass. 



Flint-glass is the densest, most transparent, 

 colourless, and beautiful. It is sometimes called 

 crystal. The best kind is said to be manufactured 

 in London, from 120 parts of white siliceous sand, 

 40 parts of pearl-ash, 35 of red oxyde of lead, IS 

 of nitrate of potash, and 25 of black oxyde of man- 

 ganese. It is the most fusible glass. It is used 



