Red Lead │Pb_{3}O_{4} │ 683 Saltpetre │KNO_{3} │ 101 Sand │SiO_{2} │ 60 Soda Carbonate │Na_{2}CO_{3} │ 106 Sodium Fluoride│NaF_{3} │ 61 Sulphate of │Na_{2}SO_{4} │ 142 Soda │ │ Tin Oxide │SnO_{2} │ 150 Uranium Oxide │UO_{2} │ 272 Zinc Oxide │ZnO │ 81 ───────────────┴───────────────┴───── ------------------------------------------------------------------------ CHAPTER III THE CHEMICAL AND PHYSICAL PROPERTIES OF GLASS The main essential and peculiar property of glass is its transparency. When subjected to a gradually increasing temperature, glass becomes softened, and whilst hot it is plastic, ductile, and malleable, in which state it can be cut, welded, drawn, or pressed. A thread of glass can be drawn so thin and fine that it can be twisted and bent to a remarkable extent, showing that glass is flexible. The above properties shown by glass while softened under heat permit it to be shaped and formed by a variety of methods, so that in the manufacture of the different kinds of glass we find goods pressed, blown, drawn, moulded, rolled and cast from the hot metal. Upon cooling, the form given to them is retained permanently. Another property of glass is its conchoidal fracture and liability to crack under any sudden change of temperature. Advantage is taken of this peculiarity in dividing or cracking apart glass when necessary, during the stages of the manufacture of any glass article. [Illustration: _By permission of_ _Melin & Co._ HORIZONTAL CRACKING-OFF MACHINE ]