THE RUSTING OF IRON 417 



known to be determinative of chemical action in general : namely, 

 the possibility of the existence of an electric circuit. The inter- 

 action of iron with water and oxygen appears to be impossible 

 in the absence of an electrolyte, just as the union of hydrogen 

 and oxygen has been shown by recent experiments to be also 

 impossible in absence of impurities. In the case of iron the 

 presence of a trace of acid, by rendering the water an electrolyte, 

 fulfils the conditions requisite for action to occur. In the case 

 of ordinary atmospheric corrosion the acid is usually carbonic 

 acid. 



The misapprehension or misconception of this position has 

 given rise to some discussion on the subject in the columns of 

 Nature. Thus, it has been suggested that whilst carbon dioxide, 

 oxygen and water are essential for the rusting of pure iron, the 

 last two alone may be sufficient to cause the rusting of impure 

 forms of the metal. But rusting in such cases appears to be due 

 to the production of acids owing to the oxidation of impurities 

 in the iron, these acids playing the same part as carbonic acid 

 in the rusting of pure iron. 



The remarkable sensitiveness of iron to attack by water 

 containing even small amounts of carbonic acid may be illus- 

 trated by a simple experiment. Distilled water which has 

 been shaken with or left in contact with ordinary air is 

 poured on a perfectly clean, polished plate of iron. After forty 

 seconds, when the metal is seen to be perfectly bright, the 

 water is run into a porcelain basin containing a drop of 

 a dilute solution of potassium ferricyanide ; a marked blue 

 coloration shows the presence of dissolved iron. Using rain 

 water, the amount of iron dissolved in the ferrous state (as 

 ferrous bicarbonate) is still greater and may be detected after 

 thirty seconds' contact. Distilled water which has been well 

 boiled and thus freed from carbonic acid, does not, however, 

 dissolve a trace of iron. On the other hand, when iron borings 

 are left in contact with water saturated with carbonic acid, 

 hydrogen is steadily evolved from the surface of the metal and 

 the latter passes into solution. The influence of carbonic acid 

 in promoting rusting can be well realised from the following 

 comparative data, which show the rate of absorption by iron of 

 oxygen from ordinary air and from air almost entirely freed 

 from carbonic acid. 



