The manufacture of optical glass--Raw materials-- Mixing--Furnaces and crucibles--Kilns for heating pots--Transfer of pots from kiln to melting furnace-- Introduction of cullet and raw materials--The fining process, difficulties and limitations--The stirring process--The final cooling of the glass--Rough sorting of the glass fragments--Moulding and final annealing of the moulded glass--Grinding and polishing of plates and discs for examination; smallness of yield obtained-- Difficulty of obtaining large blocks of perfect glass _p._ 223 CHAPTER XIV. MISCELLANEOUS PRODUCTS. Glass tubing--Gathering and drawing of ordinary tubes-- Special varieties of tube--Combustion tubes--Tubes of vitreous silica--Varieties of vitreous silica-- Transparent, glass-like silica ware--Great cost of production--Translucent “milky” silica ware produced electrically--Great thermal endurance of vitreous silica--Sensitiveness to chemical action of all basic substances at high temperatures--Glass rod and fibre-- Glass wool--Quartz fibres--Glass beads--Artificial gems--Use of very dense flint glass coloured to imitate precious stones--Means of distinguishing imitations-- Precious stones produced by artificial means--Chilled glass--Great strength and fragility of chilled glass-- Rupert’s drops--Manufacture of “tempered” glass by Siemens--De La Bastie’s process--Massive glass, used for house construction and paving blocks--Water-glass (silicate of soda or potash), manufacture in tank furnaces--Glass for lighthouse lenses and searchlight mirrors--Production by casting glass in iron moulds-- Sizes and types of lenses and prisms produced _p._ 238 APPENDIX--Bibliography of Glass Manufacture _p._ 253 GLASS MANUFACTURE CHAPTER I. THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS.