464 THE CHEMISTRY OF GLASS. 



acicntitic footing, and conducted with the precision of other chemical processes. 

 The many varieties of glass that had been made acccording to arbitrary 

 rules deduced from experience were found to be mixtures of silicate in vary- 

 ing proportions; or, to be more specific, mixtures of the silicate of potash 

 or soda, or of both, with one or more silicates of the earths or metals, as those 

 of lime, alumina, baryta, iron, lead, etc. Thus common window glass is a 

 silicate of soda or potash, or both, with that of lime, and usually of alumina ; 

 flint glass of potash and lead, and so on. It would take whole columns of 

 the Journal to enumerate the varieties of glass with their composition ; but 

 their properties depend upon a few simple chemical facts. The alkaline 

 silicates, or those of potash and soda, are soluble in water ; the others used 

 in glass-making are insoluble. The mixtures of the two, in different num- 

 bers and proportions, vary in the power of resisting the action of acids and 

 alkalies, fusibility, hardness, brilliancy and other respects. Lead, for 

 instance, tends to make glass softer, more fusible, and more lustrous, which 

 fits it for optical and ornamental purposes, but spoils it for the jDurposes of 

 the chemist, who wants a hard and infusible glass, not readily acted upon 

 by chemical agents. Lime, on the other hand, renders glass refractory and 

 less susceptible to the action of acids and alkalies. The Bohemian glass, so 

 much esteemed for chemical apparatus, is essentially a silicate of potash and 

 lime. 



The coloring of glass is effected by the admixture of small quantities of 

 metallic oxides when it is in a state of fusion. Common bottle glass owes its 

 •dark-green color to the presence of oxide of iron in the sand used in its 

 manufacture. Suboxide of copper produces a red glass, and gold a magnif- 

 icent ruby tint. The oxides of antimony and uranium are used for difii^erent 

 shades of yellow, oxide of cobalt for blue, binoxide of manganese for an am- 

 •cthyst purple, a mixture of the oxides of cobalt and manganese for black, 

 and various other oxides or mixtures of oxides for the multijjlicity of hues and 

 tints required for artistic and ornamental purposes. Natural gems owe their 

 .beauty of coloring to the blending of metallic oxides with the colorless silica, 

 -and all their exquisite tints can be reproduced b}^ the skill of the glass-maker. 



The value of glass for most of its uses depends largely on the factthat 

 it is an amorphous, that is, a non-crystalline substance. When certain va- 

 aneties of glass are heated nearly to the melting point and then slowly 

 cooled, the silicates partially separate and crystallize, and the mass becomes 

 ■opaque, like porcelain. This devitrified glass, as it is called, may be re- 

 stored to its original transparent state by again fusing it. 



While for ordinary purposes glass is one of the most joermancnt of com- 

 pounds, it is not so indifferent to chemical action as it appears. Water acts 

 more or less on all kinds of glass. Faraday found that powdered i)late- 

 .glass reddens moist turmeric paper, showing that a portion of its alkali is 

 readily dissolved out. Pure water boiled for a long time in glass vessels 

 likewise becomes alkaline. Mere exposure to moist air gradually causes a 

 •decomposition of the surface of the glass, especially when there is ammonia 



