GLASS AND GLASS-MAKING 



obtained by combustion of certain marine plants; 

 in the present day it is produced very easily by artifi- 

 cial means. Potash may be employed instead of soda; 

 it is not less common and widely known; it exists in 

 all ashes. 



"Here then we have the key to all those profound 

 mysteries of Murano, Bohemia, and St. Gobain. A 

 mirror is a valuable object produced from the commonest 

 materials. To assist the memory, let me thus sum up 

 the preceding remarks. When warming your feet, 

 if you look at yourself in the mirror, remember that 

 the mirror which adorns your mantelpiece can be 

 manufactured by the help of that same mantelpiece 

 and fireplace beneath: the stones furnish the silica, 

 the ashes the potash, the marble the lime, and the fire 

 is the only mysterious agent required for the trans- 

 formation. 'Glass,' according to the old saying, 'is 

 the offspring of fire.' ' 



The predominating silicate used frequently deter- 

 mines the name of the product. The terms "soda 

 glass," "lime glass," etc., indicate that the soda or 

 lime silicates predominate over the other silicates 

 present in a particular glass. Most of the ancient 

 glass was soda glass, but the later Venetian glass 

 contained potassium and calcium in considerable 

 quantities. Bohemian glass contains the silicates 

 of potassium and calcium. Flint glass is a mixture 

 of the silicates of potassium and lead. Bottle glass 

 is usually a mixture of the silicates of calcium, alumi- 

 num, and sodium. 



The different silicates impart certain definite qualities 



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