GLASS 



647 



No. 5. Glass of Sevres. 

 No, 6. Guinand's flint-glass. 



Some specimens of ancient glass, analysed by Kichard Phillips, have tho following 

 composition : 



Thus we see that these ancient glasses were all soda-glasses. 



Devitrification. The glasses which contain several bases are liable to suffer different 

 changes when they are melted or cooled slowly. The silica is divided among these 

 bases, forming new compounds in definite proportions, which, by crystallising, separate 

 from each other, so that the general mixture of the ingredients which constituto_ the 

 glass is destroyed. It becomes then very hard, fibrous, opaque, much less fusible, 

 a better conductor of electricity and of heat, forming what Reaumur styled denitrified 

 glass ; and what is called after him ' Reaumur's porcelain.' 



This altered glass can always be produced in a more or less perfect state, by 

 melting the glass and allowing it to cool very slowly ; or merely by heating it to the 

 softening pitch, and keeping it at that heat for some time. The process succeeds 

 best with the most complex vitreous compounds, such as bottle-glass; next with 

 ordinary window-glass ; and, lastly, with glass of potash and lead. 



This property ought to be kept constantly in view in manufacturing glass. It 

 shows why in making bottles we should fashion them as quickly as possible with the 

 aid of a mould, and re-heat them as seldom as may be absolutely necessary. If glass 

 is often heated and cooled, it loses its ductility, becomes refractory, and exhibits a 

 multitude of stony granulations throughout its substance. When coarse glass is 

 worked at the enameller's lamp, it is apt to change its nature in the same way, if the 

 workman be not quick and expert at his business. 



Fusibility, Cooling, Annealing. All glass is more or less fusible ; when it is 

 softened by the action of heat, it may be worked with the greatest ease, and may bo 

 drawn out into threads as fine as those of the cocoon of the silkworm. Glass, when 

 it is submitted to rapid cooling, becomes very fragile, and presents several very 

 remarkable phenomena, among which, as an example, Prince Rupert's drops may 

 be instanced, Glass supports variations of temperature better in proportion as it 

 has been more slowly cooled ; thus, when it has been slightly annealed, or not at all, 

 its fragility may be considerably diminished by annealing it in water, or bettor, in 

 boiling oil. 



Action of Atmospheric and Chemical Agents. The harder and more infusible a glass 

 is, the less it is alterable by the action of atmospheric and chemical agents, with the 

 exception of hydrofluoric acid. Glass which is too alkaline attracts gradually the 

 moisture of the air, and, loses its lustre and polish. Many glasses are perceptibly 

 attacked by a prolonged boiling with water, and a fortiori by acid and alkaline solu- 

 tions ; thus, the bottle-glass is frequently attacked by the tartar which is found in the 

 wine. According to Guyton-Morveau, all glass which is attacked by prolonged 

 boiling with concentrated solutions of alum, common salt, sulphuric acid, or potassa, 

 is of bad quality. 



From these facts we perceive the importance of making a careful choice of the glass 

 intended to be worked in considerable masses, such as the large object-glasses of tele- 

 scopes ; as their annealing requires a very slow process of refrigeration, which is apt 

 to cause devitrified specks and clouds. For such purposes, therefore, no other species 

 of glass is well adapted, except that with bases of potash and lead ; or that with bases 

 of potash and lime. These two form the best flint-glass and crown-glass ; and they 

 should be exclusively employed for the construction of the object-glasses of achromatic 

 telescopes. 



_ Glass, it will be apparent from the analyses given(pp. 645,646), maybe defined, in tech- 

 nical phraseology, to be a transparent homogeneous compound formed by the fusion of 

 silica with oxides of the alkaline, earthy, or common metals. It is usually colourless, 

 and then resembles rock-crystal, but is occasionally stained by accident or design with 

 coloured metallic oxides. At common temperatures it is hard and brittle, in thick 

 pieces ; in thin plates or threads, flexible and elastic ; sonorous when struck ; fracture 

 conchoidal, and of that peculiar lustre called vitreous : at a red heat, becoming soft, 

 ductile and plastic. Other bodies are capable of entering into vitreous fusion, as 



