+-----------------------------------------------------------------------+ | | | | | | |Oxide of|Oxide of| | |Silica. |Potash. |Soda. |Lime. |Alumina.| Lead. | Iron. | |--------+--------+--------+--------+--------+--------+--------+--------| |Crown. | 67 | 21 | -- | 10 | 2 | -- | -- | |--------+--------+--------+--------+--------+--------+--------+--------| |Flint. | 44 | 12 | -- | -- | 1 | 43 | -- | |--------+--------+--------+--------+--------+--------+--------+--------| |Plate. | 78 | 2 | 13 | 5 | 2 | -- | -- | |--------+--------+--------+--------+--------+--------+--------+--------| |Bottle. | 59 | 3 | -- | 25 | 6 | -- | 7 | |--------+--------+--------+--------+--------+--------+--------+--------| |Crystal.| 56 | 9 | -- | 3 | -- | 32 | -- | +-----------------------------------------------------------------------+ Cheapest of all glass is bottle glass, where the base is mainly lime. The metal used for medicine bottles contains more potash and is purer and clearer. The use of potash and soda makes the glass more easily fusible; alumina has the opposite effect; lime makes a harder glass; lead gives lustre, increases fusibility, and heightens the refractive power. Hence in glass which is to be cut and polished the employment of lead in sufficient quantity is a factor of the highest importance. This is a point to be specially noted in connection with English glass. Lead--chiefly the oxides known as litharge and minium--in small quantities has long been employed, the introduction of the metal serving as a flux, but lead glass was generally avoided as being too brittle. Merret, writing in 1662, remarks that could this glass be made as tough as crystalline, “it would far surpass it in the glory and beauty of its colours.” It will be noted that the two kinds of glass in which lead is used in quantity are flint glass and crystal. The larger the amount of lead the greater the beauty and brilliancy of the product, a result due, as previously intimated, to the increase in refractive power that is brought about by its addition.