1050 Transactions of the American Institute. 



Bottle glass is composed of the coarsest materials of silex, soda, lime 

 oxyd of iron, and clay. It is generally of less specific gravity than 

 any other variety : it is tougher and resists chemical action. In New 

 Jersey, green sand is added to spread glass for beer bottles, etc., etc. 



Lead glass comprises three varieties, crystal, fluid glass, and strass, 

 differing in the proportions of litharge and red lead they contain ; it 

 may be shown that crystal glass contains but little oxyd of lead, in 

 comparison to the famous paste called strass, which contains more 

 oxyd of lead than silica. The crystal glass is composed of 100 parts 

 of silica, ten parts oxyd of lead, thirty -ive parts purified potash, and 

 thirteen parts carbonate of lime. The common flint glass contains 

 100 parts silica, sixty-six parts oxyd of lead, twenty-six parts purified 

 potash, and seven parts saltpeter. Optical glass contains 100 parts 

 silica, 100 parts oxyd of lead, twenty-three parts purified potash, and 

 a very small proportion of saltpeter and borax. Strass contains 100 

 parts silica, 133 parts oxyd of lead, and thirteen parts purified pot- 

 ash. The dried and mingled materials are then thrown into the 

 white-hot melting pots, and when full of melted glass, the mouths of 

 the oven are closed. Some heavy combinations of lead sink to the 

 bottom, while the salts, whicli will not incorporate with the glass, 

 rise to the top as a scum, called glass gall and sandiness. The 

 greater part of this is skimmed off. Strass is the basis of a beautiful 

 glass, and was invented in the seventeenth century by a man named 

 Strass, of Strasburgh, who first conceived the importance of imita- 

 ting the real gems as respects their hardness, specific gravity, and 

 refraction of light, and the white mass obtained by his receipt has 

 produced a beautiful base for imitating the diamond, the rock 

 crystal, and the white topaz. It is now manufactured in large quan- 

 tities in France, as a base also for the production of all other colored 

 gems, such as ruby, emerald, sapphire amethyst, aquamarine, garnet 

 chrysoprose, opal, hyacinth, robellite, indigolite, or blue turmaline, 

 chrysolite, turquoise, lazulite, and agate. Although the properties 

 which are usually considered as constituting excellence in glass 

 for ordinary purposes may be easily obtained, yet in glasses 

 for optical instruments, and to be employed in the examination 

 of objects so remote and so minute as to require the most 

 undeviating accuracy, the difficulty of obtaining the metal (or 

 the mass) sufficiently free from the defects to which glass is inci- 

 dent, has until a late period baffled every attempt to produce a lens, 

 except of comparatively small dimensions; although purity, uuchange- 



