168 



METALLOIDS. 



It is composed of sulphur and oxygen, in the proportion 

 of one atom of the former to two of the latter. The ter- 

 mination " ous" indicates, as in other cases, a smaller 

 proportion of oxygen than is contained in some other 

 acid composed of the same elements. 



407. PREPARATION. It has already been 



How is sul- 

 phurous acid shown that this acid may be prepared, by 

 prepared? burning sulphur in oxygen. Another, and 

 better method, is to heat, oil of vitriol, with bits of cop- 

 per. The oil of vitriol is thus 

 deprived of part of its oxygen, 

 and converted into sulphurous 

 acid. The process may be con- 

 ducted in a test-tube. By lead- 

 ing the gas through a smaller 

 tube, into a vial partly filled with 

 water, a solution of sulphurous 

 acid may be obtained, possessed of the same bleaching 

 and other properties as the gas itself. When the evolu- 

 tion of the gas commences, the heat of the lamp is no 

 longer required. 



Explain the 408. EXPLANATION. Copper has a very 



process. strong affinity for oxygen, and takes it 



from the oil of vitriol, which possesses it in large pro- 

 portion. The oil of vitriol, thus deprived of part of 

 its oxygen, is converted into sulphurous acid gas. 



409. USE IN PRESERVING WINES. Slll- 



W ny ^.? sul- 

 phurous acid phurous acid, in small quantities, is some- 



7eTtowlnes d ? times added to wine > to prevent its sour- 

 ing. This change is owing to the absorp- 

 tion of oxygen from the air. Sulphurous acid is a 



