648 GLASS 



phosphoric acid, boracic acid, arsenic acid, as also certain metallic oxides, as of 

 lead and antimony, and several chlorides ; some of which are denominated glasses. 



Silica, formerly styled the earth of llints, which constitutes the basis of all com- 

 mercial glass, is infusible by itself in the strongest fire of our furnaces ; but its 

 vitreous fusion is easily effected by a competent addition of potash or soda, either 

 alone or mixed with lime or litharge. The silica, which may bo regarded as be- 

 longing to the class of acids, combines at the heat of fusion with those bases, to form 

 saline compounds ; and hence glass may be viewed as a silicate of certain oxides, 

 in which the acid and the bases exist in equivalent proportions. Were these pro- 

 portions, or the quantities of the bases which silica requires for its saturation at 

 the melting point, exactly ascertained, we might readily determine beforehand the 

 best proportions of materials for the glass-manufacture. But as this is far from 

 being the case, and as it is, moreover, not improbable that the capacity of satura- 

 tion of the silica varies with the temperature, and that the properties of glass also 

 vary with the bases, we must in the present state of our knowledge, regulate the 

 proportions rather by practice than by theory, though the latter may throw an in- 

 direct light upon the subject. For example, a good colourless glass has boon found 

 by analysis to consist of 72 parts of silica, 13 parts of potash, and 10 parts of lime, in 

 95 parts. If we reduce these numbers to the equivalent ratios, we shall have the 

 following results, taking the atomic weights as given by Berzelius : 



This glass would therefore have been properly better compounded with the just 

 atomic proportions, to which it nearly approaches, viz. 71 '49 silica, 14' 67 potash, and 

 8 '84 lime, instead of those given above as its actual constituents. 



The proportions in which silica unites with the alkaline and other oxides are 

 modified by the temperature as above stated ; the lower the heat, the less silica will 

 enter into the glass, and the more of the base will in general be required. If a glass 

 which contains an excess of alkali be exposed to a much higher temperature than that 

 of its formation, a portion of the base will be set free to act upon the materials of the 

 earthen pot, or to be dissipated in fumes, until such a silicate remains as to constitute 

 a permanent glass corresponding to that temperature. Hence the same mixture of 

 verifiable materials will yield very different results, according to the heats in which 

 it is fused and worked in the glass-house; and therefore the composition should 

 always be referrible to ' the going ' of the furnace. When a species of glass, which at 

 a high temperature formed a transparent combination with a considerable quantity of 

 limo, is kept for some time in fusion at a lower temperature, a portion of the lime 

 unites with the silica into another combination of a semi-vitreous or even of a stony 

 aspect, so as to spoil the transparency of the glass altogether. There is probably a 

 supersilicato and a subsilicate formed in such cases ; the latter being much the more 

 fusible of the two compounds. The Eeaumur's porcelain already mentioned, is an 

 example of this species of vitreous change in which now affinities are exorcised at a 

 lower temperature. An excess of silica, caused by the volatilisation of alkaline 

 matter with too strong firing, will bring on similar appearances. 



The specific gravity of glass varies from 2-3 to 3'6. That of least specific gravity 

 consists of merely silica and potash fused together ; that with lime is somewhat denser, 

 and with oxide of lead denser still. Plate-glass made from silica, soda, and limo, has 

 a specific variety which varies from 2'3 to 8 - 6 ; crystal or flint-glass containing load 

 from 3-0 to 3'6. 



The density of several glasses without lead is as follows : 



Old Bohemian glass (Dumas) 2-396 



Bohemian bottle-glass 3782 



Do. window-glass 2-642 



Fine glass, called Bohemian crystal 

 Mirror-glass of Cherbourg (Dumas) 



Do. St. Gobain 



Do. Newhaus, 1812 (Scholz) 



2-892 

 2-506 

 2-488 

 2-551 



Do. do. 1830 2-653 



The power of glass to resist the action of water, alkalis, acids, air, and light is in 

 general the greater the higher the temperature employed in its manufacturo, the smaller 



