1044 Transactioks of the American Institute. 



tlie age of Tiberius, and is now manufactured in the greatest perfect 

 tion. It is one of the most useful arts to mankind ; for by it, in 

 conjunction with tlie grinder's help, we obviate the natural infirmities 

 of tlie eye. Without it, old people, and those whose optic nerves are 

 affected, would be debarred the knowledge of reading letters or books, 

 and would be unable to sit within doors, or in a coach or ship and 

 see all things clearly around them, yet without being exposed either 

 to the scourging heat or freezing cold, or being annoyed with the east 

 wind, or the ingress of extraneous filth. Pure glass will scarcely 

 Teceive any stain, and is easily cleansed again. Although the essential 

 constituents of glass are silex and alkali, it generally contains other 

 substances, such as metallic oxydes, which are designed to modify its 

 external character of hardness, fusibility, brilliancy, color and trans- 

 parency. Many kinds of glass contain either potash or soda ; the 

 first is not much employed by the manufacturers of common glass. 

 Some kinds contain lime and oxyd of lead and alumina and oxyd of 

 iron ; the two latter are however mere accidental impurities. The 

 following constitute the principal materials of glass : 



1. Silex, or sand, which is, as already stated, very abundant on the 

 globe ; the sand mostly employed is the white sand, either obtained 

 from the disintegrated sandstone rocks, which are numerous in the 

 tJnited States, as in Missouri, near St. Genevieve, and Berkshire 

 county, Mass. ; or from the river sand which is found in large beds of 

 wliite sand at Maurice river, in New Jersey and Florida. Drift 

 sand is brought by the winds from the sea coasts or deserts, but mostly 

 from the lower sands of sea shores, as we find them for 100 miles on 

 the Long Island shore ; this sand when washed forms a good sand 

 for glass. 



The infusorial deposits of the silicious shells, called the diatoms, 

 which form immense deposits both inland and on the coasts, yield a 

 good material for the manufacture of glass. 



,2. Alkali. If potash is used, the purified pearlash is employed, 

 particularly for plate glass and the fewer kinds of crown glass, as also 

 soda ash, which is the carbonate of soda, is also used for the better 

 qualities of glass ; while sulphate of potash, glauber salt, salt-cake, or 

 common salt are employed for common glass. 



3. Lime, either as air-slaked, quick-lime, or as carbonate of lime, 

 such as marble or chalk, is used for the manufacture of green glass. 



4. Oxyd of lead,' or litharge, or red lead, are usefully employed in 

 the manufacture. 



