SELENIUM. 513 



(b.) Insipid, insoluble in water, but soluble in an excess of acid. 



(c.) The compound of alumina, fluoric acid, and soda, may be 

 made to crystallize, and is even found native in the cryolite; and 

 in the topaz, fluoric acid is combined with fluoric acid. 



The properties of the t fluate of silica have been incidentally detail- 

 ed, perhaps to a sufficient extent. The following facts may how- 

 ever be advantageously recapitulated. 



FLUATE OF SILICA. 



1. This compound is always formed when fluoric acid is obtained 

 in glass vessels ; more perfectly if a little powdered flint or sand be 

 mixed with the materials. 



2. Silica is thus suspended in the gaseous form, and is permanent 

 over quicksilver. 



3. Water throws down a part of it. 



4. Glass vessels are corroded' both by liquid and gaseous fluoric 

 acid ; Bergman obtained crystals from a fluoric solution, which Four- 

 croy regards as fluate of silica. 



5. Alkalies decompose the fluate of silica, and triple compounds 

 are often thus formed. 



6. A similar compound is formed when fluoric acid attacks glass ; 

 softer siliceous stones that contain no alkali, are attacked by fluoric 

 acid, with more difficulty. 



7. Fluate of silica is decomposed by heat. 



Remarks. It is very probable that the progress of chemical 

 analysis will bring to light more native combinations ,of earths, with 

 the fluoric acid ; a number have been added within a few years. 



Gay-Lussac and Thenard remark, that they had a quantity of 

 fluor of the purest and most beautiful appearance, in which the eye, 

 aided by a magnifier, did not enable them to discover any silex, 

 which nevertheless yielded silicated fluoric acid gas. 



The fluates of zirconia, glucina, and yttria are formed upon the 

 same principles as the other earthy fluates, but are of no impor- 

 tance.* 



SEC. VIII. SELENIUM. 



1. DISCOVERY. By Berzelius, in 1818. The iron pyrites of 

 Fahlun, in Sweden, afford by sublimation, sulphur, which being em- 

 ployed in the manufacture of sulphuric acid, a reddish substance f 

 was constantly deposited in the bottom of the leaden chambers. It 

 was principally sulphur, but on burning it, an odor like that of decay- 



* See Recher. Phys.-Chim. Tom. II, p. 27. 



t In this substance, besides the selenium, Berzelius found mercury, tin, cop- 

 per, zinc, iron, lead, and arsenic. 



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