322 SULPHUR. 



class of compounds, in which hyposulphurous acid and sulphu- 

 ric acid are included. 



SULPHUROUS ACID SERIES. 



Sulphurous acid SO 2 



Sulphuric acid SO 2 + O 



Hyposulphurous acid. . . . SO 2 + S 

 Chlorosulphuric acid. . . . SO 2 + Cl 

 lodosulphuric acid. . . . SO 2 + 1 



Nitrosulphuric acid. . . . SO 2 -fNO 2 



SULPHURIC ACID. 

 Eg. 501.17, or 40.1 2; SO 3 ; density 2762 ; Q^j. 



Chemists have been in possession of processes for preparing 

 this acid since the end of the fifteenth century. It is of all re- 

 agents the one in most frequent use, being the key to the pre- 

 paration of most of the other acids, which, in consequence of its 

 superior affinities, it separates from their combinations, and 

 being the acid preferred to others from its cheapness, for various 

 useful and important purposes in the arts. 



Preparation. Sulphuric acid was first obtained by the distil- 

 lation of green vitriol or copperas, a native sulphate of iron, 

 and this process is still followed at Nordhausen in Saxony, for 

 the preparation of a highly concentrated acid. The sulphate of 

 iron contains seven equivalents of water, and is first dried, by 

 which its water is reduced considerably below a single equivalent, 

 and then distilled in a retort of stoneware at a red heat. When 

 the experiment is performed on a small scale, the heat of an ar- 

 gand spirit lamp is sufficient ; and in the place of copperas, the 

 sulphate of iron previously peroxidized, the sulphate of bismuth, 

 of antimony, or of mercury may be employed. The first effect 

 of heat upon the dried copperas, is to cause an evolution of sul- 

 phurous acid gas, a portion of sulphuric acid being decomposed 

 in converting the protoxide of iron of that salt into peroxide. 

 Vapours afterwards come over, which condense into a fuming 

 liquid, generally of a black colour, and of a density about 1.9, 

 which is the Nordhausen acid, and contains less than one 

 equivalent of water to two of sulphuric acid. This acid is pre- 

 ferred for dissolving indigo } and for some other purposes in the 

 arts, and is the best source of anhydrous sulphuric acid. 



