JOHN A. WILLIAMS. 249 



or the double silicate, are soluble in water, the addition 

 of one or more of the insoluble silicates renders the glass 

 practicably insoluble. 



For convenience the several varieties of glass may be 

 classified, as follows : 



1. Flint glass or crystal. Silicate of potassium and 

 lead. Example, lamp chimneys and tableware. 



2. Common window and plate glass. Silicate of sodium 

 and calcium or silicate of potassium and calcium. 



3. Refractory Bohemian glass. Silicate of potassium 

 and calcium. 



4. Coarse green bottle glass. Silicate of sodium, 

 calcium, iron and aluminium. Example, champagne 

 bottles. 



5. Ordinary bottle glass. Silicate of sodium, calcium, 

 iron and aluminium ; the quality of the product depend- 

 ing upon the purity of the materials used. 



(xlass is a compound in which the constituents are 

 chemically united, but the elements may be present in 

 various proportions, and they may be changed at will. 

 This table shows how the proportions may vary in several 

 varieties of calcium soda glass : 

 CaO. Na'O. SiO\ 



11.7 13. 75.3 Na'O. CaO. 6 SiO'. 



13.7 10.8 75.5 5 NaU 7 CaO. 36 SiO'. 



12.8 11.8 75.4 5 Na'O. 6 CaO. 33 SiO\ 

 Under the influence of a cherry red heat and upwards 



all glasses are capable of being first softened and after- 

 wards undergoing complete fusion. Glasses in which 

 silicic acid predominates being fused with much greater 

 difficulty than those in which it is present in normal 

 quantities. Silicic acid is in itself infusible, but in 

 combination with the metallic oxides its fusibility de- 

 pends upon the nature and quantity of the oxides 

 present. All the bases employed in the manufacture of 

 glass, being considered fluxes, since they aid fusion, the 



205 



