386 SELENIUM. 



acid solution is filtered, and evaporated in vacuo over sulphuric 

 acid, till it attains a density of 1.347, which must not be ex- 

 ceeded, as the acid solution begins then to decompose sponta- 

 neously into sulphurous acid, which escapes, and sulphuric acid 

 which remains in the liquor. 



Properties. This acid has not been obtained in the anhy- 

 drous condition. Its aqueous solution has no great stability, 

 being decomposed at its temperature of ebullition. The same 

 solution exposed to air in the cold slowly absorbs oxygen, ac- 

 cording to Heeren, and becomes sulphuric acid. But neither 

 nitric acid, nor chlorine, nor peroxide of manganese oxidize this 

 acid, unless they are boiled in its solution. Its salts are per- 

 fectly stable, either when in solution, or when dry, and are in 

 general very soluble, having some analogy to the nitrates. A 

 hyposulphite, when heated to redness, leaves a neutral sulphate, 

 and allows a quantity of sulphurous acid to escape, which would 

 be sufficient to form a neutral sulphite with the base of the sul- 

 phate. This class of salts was particularly examined by Heeren.* 

 Hyposulphuric acid is imagined to exist in acid compounds pro- 

 duced by the action of sulphuric acid on some principles of 

 organic chemistry, in the sulpho-napthalic acid, for instance. 



SECTION VIII. 

 SELENIUM. 



Eg. 494.58 or 39.63 ; Se; density of vapour unknown. 



This element was discovered by Berzelius in 1817, in the 

 sulphur of Fahlun, employed in a sulphuric acid manufactory in 

 Sweden, and was named by him selenium, from SeX^v^, the moon, 

 on account of its strong analogy to another element tellurium, 

 which derives its name from tellus, the earth. It is one of the 

 least abundant of the elements, but is found in minute quantity 

 in several ores of copper, silver, lead, bismuth, tellurium and 

 gold in Sweden and Norway ; and in combination with lead, 

 silver, copper and mercury in the Hartz. It is extracted from a 

 seleniferous ore of silver of a mine in .the latter district, and 

 supplied for sale in little cylinders of the thickness of a goose- 

 quill, and three inches in length ; or in the form of small medal- 



* Poggendorff's Annalen, v. vii, p 77. 



