SULPHUROUS ACID. 89 



173. When sulphur is burnt in the air, it forms sul- 

 phurous acid, a transparent, invisible gas, possessing 

 a very strong and suffocating smell ; it is easily dis- 

 solved by water, and the solution then obtained has 

 a strongly sour taste. 



174. A portion of sulphurous acid is formed when- 

 ever we light a common brimstone match ; and the pecu- 

 liar suffocating smell then perceived is occasioned by 

 this acid, and not by the sulphur alone, which has no 

 smell, but which is used to tip the matches because it 

 has a strong affinity for oxygen, and requires less 

 heat to enable it to commence combination than the 

 wood itself; but when it has once fairly got alight, 

 it then produces heat enough to fire the wood. 



175. Sulphurous acid is a transparent gas, rather 

 heavier than common air, readily soluble in water, 

 very poisonous when breathed, and extinguishing the 

 flame of combustible matters ; it does not form any 

 important compounds when combined with bases, and 

 its principal interest consists in its bleaching powers. 

 It has long been used to destroy color in things 

 desired to be bleached, which are merely hung up 

 over some burning sulphur, and exposed to the fumes 

 of sulphurous acid thus formed. Articles thus bleached, 

 however, frequently retain an unpleasant smell of 

 sulphurous acid. 



176. It is likewise used in fumigation, being con- 

 sidered destructive of contagion ; and, therefore, sul- 

 phur is burnt in houses where persons have died of 

 an infectious disorder, and in other circumstances 



8* 



