LESSONS IN CHEMISTRY. 139 



though less powerful than chlorine it is preferred in some cases, 

 because it is less liable to injure the fabric. The articles to be 

 bleached are moistened and hung in a chamber in which sulphur 

 dioxide is produced by burning sulphur. The coloring matters 

 seem to form colorless compounds with the acid and in some cases 

 the colors maybe restored by neutralizing the acid. Sulphur diox- 

 ide, like carbon dioxide, will not support combustion and is used 

 to stop combustion. This acid is an antiseptic promptly arrest- 

 ing fermentation. It is used as a disinfectant and as an insecti- 

 cide. The dioxide becomes an acid so readily by taking water 

 from the air that it is often difficult to decide whether a given 

 result is due to the dioxide or the acid. 



While sulphurous acid is useful in many other ways its principal 

 use is in the manufacture of sulphuric acid, formula H 2 S0 4 . It 

 is a powerful acid which has very many practical uses. It is said 

 that more than 100,000 tons of it are used each year in England 

 alone. The steps involved in the manufacture of sulphuric acid are 

 the formation of the sulphur dioxide, and sulphurous acid H 2 S0 3 , 

 which combined with one atom oxygen forms sulphuric acid. The 

 last step is the most difficult and most interesting. The sulphur- 

 ous acid cannot readily get the necessary oxygen from the air, 

 but nitric oxide, formula NO, takes oxygen from the air forming 

 nitrogen peroxide N0 2 . The peroxide gives off one atom of oxy- 

 gen to the sulphurous acid and sulphuric acid is formed, the nitric 

 oxide simply carrying oxygen from the air to the sulphurous acid. 

 Sulphurous acid is supplied from burning sulphur or decomposing 

 iron pyrites. Nitric acid, from which the nitric oxide is formed, 

 is supplied by the decomposition of niter by sulphuric acid. 

 A mixture of sulphurous and nitric acids, air and steam are 

 conveyed into a leaden chamber containing a layer of water. 

 The nitric acid by the action of the sulphurous acid is reduced to 

 nitric oxide, which takes oxygen from the air and gives it up to 

 the sulphurous acid, converting it into sulphuric acid. This is 

 absorbed by the water forming a weak acid which is concentrated 

 by evaporation, at first in leaden pans and then in glass retorts 

 or platinum stills. 



