3*70 ROCKS. 



of a pure refractory clay, until fusion has taken place and the 

 material has settled. The glass is afterwards worked by 

 blowing, or moulded, into the various forms it has in market ; 

 and it is finally annealed — or in other words, is very slowly 

 cooled— to render it tough. A little oxyd of manganese is 

 usually employed to correct the green color which glass is 

 apt to derive from any oxyd of iron present. But if the man- 

 ganese is in excess, it gives a violet tinge to it. 



The following chemical distribution of glasses has been 

 proposed : 



Soluble glass. A simple silicate of potash or soda, or of 

 both of these alkalies. 



Bohemian or crown glass. Silicate of potash and lime. 

 . Common window and mirror glass. Silicate of soda and 

 lime ; sometimes also of potash. 



Bottle glass. Silicate of soda, lime,, alumina, and iron. 



Ordinary crystal glass. Silicate of potash and lead. 



Flint glass. Silicate of potash and lead ; more lead than 

 in the preceding. 



Strass. Silicate of potash and lead — still more lead. 



Enamel. Silicate and stannate, or antimonate of potash 

 or soda and lead. 



Glass was manufactured by the Phoenicians, and the later 

 Egyptians. According to Pliny and Strabo, the glass works 

 of Sidon and Alexandria were famous in their times, and 

 produced beautiful articles. The Romans employed glass 

 to some extent in their windows, and remains of this glass 

 are found in Herculaneum. Window glass manufacture was 

 first commenced in England in 1557. 



Sand for casting is a fine siliceous sand, containing a little 

 clay to make it adhere somewhat and retain the forms into 

 which it may be moulded. It must be quite free from lime. 



Tripoli is a fine grained earthy deposit, having a dry, 

 harsh feel and a white or grayish color. It contains 80 per 

 cent, of silica, mostly derived from the casts of animalcules. 

 It is valuable as a polishing material. 



Marl. Marl is a clay containing carbonate of lime. The 

 material is valuable as manure. The term is also improper- 

 y applied to any clayey earth used in fertilizing land. The 

 green sand in New Jersey is sometimes called marl. 



Fuller's earth is a white, grayish, or greenish-white earth, 

 having a soapy feel, which was formerly used for removing 

 oil or grease from woolen cloth. It falls to pieces in water, 



