EARTHS. 279 



remote in their properties from those usually assigned to metals.- 

 Berzelius. 



GLASS.* 



1. HISTORY. Known to the ancients. Glass beads were found 

 among the ornaments of mummies in the catacombs, near Memphis, 

 supposed to be 1600 years older than the Christian era ; glass was 

 known to the Romans, and glass vessels were discovered in the hous- 

 es of Herculaneum, and a coarse glass in the windows of the houses 

 in Pompeii, which were destroyed by an eruption of Vesuvius, A. D. 

 79 ; glass lachrymatories are found in the tombs of the ancient 

 Greeks. f Glass was however, with the ancients, merely an article 

 of luxury and curiosity, and it is only in modern times that it has 

 come into general use. 



In Europe, it was first made at Venice, and its use, in windows 

 of private houses, was introduced into England in the tenth century, 

 nor was it common until the 13th or 14th century. 



2. COMPOSITION. Essentially a compound of silica, and fixed 

 alkali, with however, various adventitious ingredients; sometimes 

 glass is made of lime, or of the coarsest refuse ashes, and sand. 



3. Principal kinds.' Flint glass ; crown, or window glass ; broad r 

 or coarse window glass ; 'plate glass ; green bottle glass. 



(a.) Flint Glass.^ 120 parts clean white sand, 40 purified pearl 

 ashes, 35 litharge, or minium, 13 nitre, and a little oxide of manga- 

 nese ; or 100 white sand, 80 to 85 red oxide of lead,. 35 to 40 of pearl 

 ashes, 2 or 3 of nitre ; or, (in England,) purified Lynn sand tOO* parts? 

 litharge, or red lead, 60, purified pearl ashes 30. To remove the- 

 color, derived from combustible matter, or oxide of iron, a little nitre,., 

 or black oxide of manganese, or arsenic is added ; the oxigen con~ 

 tained in these substances, either burns the combustible matter, or 

 brings the metallic oxides that may be present, to such a state that 

 they do not color the glass. The fusion takes about thirty hoursv 

 The lead gives to this species of glass greater toughness and softness,* 

 so that it can be cut, ground, and highly polished, and greater densi- 



* Glass is an example of what is called a vitrification. Many earthy and saline 

 substances, and metallic oxides, either alone, or mixed, become by fusion, dense, hard, 

 brittle, shining bodies, usually breaking with aconchoidal fracture, and having more 

 or less of transparency. The slag and scoriae of furnaces are imperfect vitrifications. 



t Specimens were brought out by Mr. Jones, author of " Naval Sketches," anrf 

 are now in the Cabinet of Yale College; they are supposed to be 2200 years oldV 

 Some of them are beautifully irised ; the glass is perfect, and is a little greerrin its 

 shade of color. 



| Called flint glass, because it was formerly made from flints ; and it has beei* 

 called crystal glass, being sometimes made from rock crystals ; both are ignited and 

 thrown into water to crack them, and they are then pulverized. 



