THE RUSTING OF IRON 419 



containing traces of acid on iron in presence of atmospheric 

 oxygen. Certain kinds of iron are more susceptible than others 

 to attack, in virtue of their different degrees of purity. To 

 prevent rusting it is necessary primarily to exclude every 

 trace of acid ; and, as this is generally impracticable, the alter- 

 native is to prevent contact of the iron with water and the 

 atmosphere by means of some protective coating such as paint. 

 Whether, in the case of steel, the internal structure can be 

 so modified by a suitable and inexpensive treatment that the 

 metal shall be nearly rustless, is a problem that still remains 

 open and urgently needs investigation. 



