192 CHEMISTRY. 



away, a particle of silica takes the place of every removed 

 particle of wood, so that at length all the wood is gone, and 

 is wholly replaced by silica. Because the shape and all the 

 lines of the wood are preserved, the common idea is that the 

 wood is turned to stone ; but, as you see, stone has merely 

 taken the place of wood. Silicified wood is found in great 

 quantity in certain parts of California and Oregon. Large 

 trunks of trees are found completely silicified. Such speci- 

 mens are erroneously called petrifactions, as if they were 

 the result of turning into stone. 



262. Glass. In making glass, the silica or silicic anhydride 

 is made, by an intense and long-coatinued heat, to unite 

 with various bases, according to the kind of glass required. 

 Window-glass is made by uniting silica with soda and lime ; 

 plate-glass, crown-glass, and the beautiful Bohemian glass, 

 by uniting silica with lime and potash ; and green-bottle 

 glass is commonly a silicate of lime and alumina, combined 

 with oxides of iron and manganese, and sodium and potas- 

 sium, its green color being produced by the oxide of iron. 

 Glass is in reality a very complex mixture of true salts. 

 What is called enamel is an opaque glass, made so by some 

 substance which, though it be thoroughly mixed with the 

 glass in melting, does not melt with it. Stannic oxide is 

 commonly used for this purpose. The silica used in mak- 

 ing glass is in the forms of sand, quartz, flint, and old broken 

 glass. The materials are subjected to intense heat in clay 

 pots for about forty-eight hours, in order to bring the mass 

 into a proper state to be worked. The manner in which 

 the melted glass is made into various articles we will not 

 detail. The common mode of making window-glass is de- 

 scribed in Part I, 216. 



263. Coloring Glass. The various colors are given to glass 

 mostly by metals. We have already mentioned the bottle- 

 green color imparted by the ferrous oxide. Ferric oxide gives 



