GLASS 



645 



cellence in the art of working glass ; but tho French became eventually so zealous of 

 rivalling them, particularly in the construction of mirrors, that a decree was issued by 

 the court of France, declaring not only that tho manufacture of glass should not dero- 

 gate from the dignity of a nobleman, but that nobles alone should be masters of glass- 

 works. Within the last fifty years, Great Britain has made rapid advances in this 

 important art, and at the present day her pre-eminence in some departments hardly 

 admits of dispute. 



The window -glass manufacture was first begun in England in 1557, in Crutched 

 Friars, London : and fine articles of flint-glass were soon afterwards made in the 

 Savoy House, Strand. In 1635 the art received a great improvement from SirKobert 

 Mansell, by the use of coal-fuel instead of wood. The first sheets of blown glass for 

 looking-glasses and coach-windows were made in 1673 at Lambeth, by Venetian 

 artisans employed under the patronage of the Duke of Buckingham. 



The casting of mirror-plates was commenced in France about the year 1688, 

 by Abraham TheVart : an invention which gave rise soon afterwards to the establish- 

 ment of the celebrated works of St. Gobain, which continued for nearly a century 

 the sole place where this highly-prized object of luxury was well made. In cheap- 

 ness, if not in excellence, the French mirror-plate has been for some time rivalled by 

 the English. 



The analyses of modern chemists, which will be detailed in the course of this 

 article, and the light thrown upon the manufacture of glass in general by tho accu- 

 rate means now possessed of purifying its several ingredients, would have brought 

 the art long since to the highest state of perfection in this country, but for the long- 

 continued vexatious interference and obstructions of our Excise laws now happily at an 

 end. 



The researches of Berzelius having removed all doubts concerning the acid character 

 of silica, the general composition of glass presents now no difficulty of conception. 

 This substance consists of one or more salts, which are silicates with bases of potash, 

 soda, lime, oxide of iron, alumina, or oxide of lead; in any of which compounds we 

 can substitute one of these bases for another, provided that one alkaline base be left. 

 Silica in its turn may be replaced by boracic acid, without causing the glass to lose 

 its principal characters. 



Under the title ' glass ' are therefore comprehended various substances fusible at a 

 high temperature, solid at ordinary temperatures, brilliant, generally more or less 

 transparent, and always brittle. The following chemical distribution of glasses has 

 been proposed : 



1. Soluble glass ; a simple silicate of potash or soda ; or of both these alkalis. 



2. Crown-glass ; silicate of potash and lime. 



3. Bottle-glass ; silicate of soda, lime, alumina, and iron. 



4. Common window-glass ; silicate of soda and lime ; sometimes also of potash. 



5. Plate-glass ; silica, soda or potash, lime, and alumina. 



6. Ordinary crystal-glass ; silicate of potash and lead. 



7. Flint-glass ; silicate of potash and lead ; richer in lead than the preceding. 



8. Strass ; silicate of potash and lead ; still richer in lead. 



9. Enamel ; silicate and stannate or antimonate of potash or soda, and lead. 



The following analyses of these varieties of glass will place their composition more 

 completely before the reader : 



