EARTHS. 281 



and 1 oz. of borax after fusion, 5 drachms of nitre are added ; or, 

 rock crystal pulverized, 3 oz., white lead, 8 oz., and borax, 2 oz., 

 and half a grain of manganese. This is a paste in which the lead 

 and borax answer the purpose of a flux. 



Some principal colors are given by the following oxides of metals. 

 Antimony gives yellow, and the same is produced by muriate of sil- 

 ver, and by oxide of zinc, white clay, and yellow oxide of iron ; 

 manganese produces violet ; gold, many shades of violet, red and 

 purple ; cobalt, blue ; chrome, green, or red ; iron, red, and a great 

 many other colors and shades ; and many varieties are imparted by 

 mixtures of different oxides. Fluxes for the colors are made of 

 borax, pearl ashes, lead, &c. These imitations of the gems, except 

 in lustre, are often equal in beauty to the originals, but they are soft, 

 and easily defaced. 



(g.) Stained glass. The art of staining glass was introduced into 

 England, in the 13th century, in the reign of king John. Many of 

 the ancient Gothic churches in Europe, are ornamented by stained 

 glass, the panes of the windows having pictures painted upon them. 

 The glass used for this purpose, is made without oxide of lead, be- 

 cause that addition would make it too fusible, so that it would lose 

 its shape during the second heating. The colors, ground in water, 

 are laid on the glass, which is heated under a muffle, until the colors 

 are melted, and united to the glass ; and the pieces, to prevent their 

 bending, are supported upon the biscuit of unglazed porcelain, or 

 some other suitable substance.* 



(h.) Medallions encased in glass. They appear to be something 

 like the biscuit of porcelain introduced into the glass, while in fusion ; 

 they are called crystallo ceramie, and are very beautiful. f 



(i.) Enamels are glasses, more or less opake, stained with various 

 colors ; one of the most common is stained by oxide of tin or oxides 

 of tin, arsenic and lead more or less mixed, as in watch faces. 



Dr. Bigelow informs us,J that the beautiful imitation of porcelain, 

 made in Boston, and now seen in the shops, is flint glass, containing 

 a portion of white arsenic, upon which its opacity depends. 



Remarks. Green glass is much harder and less fusible than white 

 flint, and as it contains no lead, it is also much fitter to contain cor- 

 rosive chemical agents. Glass is very ductile, as is proved by its 

 being spun into the most delicate threads ; it is highly elastic, form- 

 ing the finest toned bells and musical instruments ; it expands and 

 contracts less than any other substance by variation of temperature, 



* I have seen modern stained glass in the windows in the University, Cambridge 

 Eng. and in Hartford, Conn, (the latter of Boston manufacture,) less beautiful, 

 however, than the ancient. 



t Heads of Washington, Franklin, Napoleon, and other distinguished persons, 

 have been executed in this way. t Technology, 460. 



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