THE CORROSION OF IRON 7; 



and observe a still lower degree of conductivity without ever 

 arriving at water pure and simple. 



It is worth while, however, to analyse Mr. Lambert's state- 

 ments with regard to the case of a piece of ordinary commercial 

 iron in contact with the electrolyte water ; they are as follows : 



" Whenever we have two metals (or two modifications of 

 the same metal) which are electrically different, that is, have 

 different solution pressures and they are placed in metallic 

 contact in an electrolyte, then the relatively electropositive 

 part will dissolve with the production of an electric current 

 flowing through the electrolyte from the electropositive to the 

 electronegative pole and in the opposite direction through 

 the metal. 



"The rate of such a reaction depends (a) on the magnitude 

 of the difference of electric potential — that is, the difference of 

 solution pressure between the two metals, and (b) on the 

 resistance offered by the electrolyte to the passage of the electric 

 current. 



" No part of the metal can dissolve unless an electric current 

 actually passes through the electrolyte. The rate of the re- 

 action may, however, be so infinitesimal that the amount of 

 metal passing into solution will not be sufficient to respond to 

 chemical tests even after long periods. 



" Let us consider, in this light, the case of a piece of 

 ordinary commercial iron in contact with the electrolyte water. 



" Such a piece of iron is impure and not homogeneous — 

 there are some parts of it which have a different solution 

 pressure from other parts and so when it is placed in contact 

 with the electrolyte water we have all the conditions for the 

 production of an electric current. 



" If the conditions are such that an appreciable electric 

 current can pass between the two electrically different parts 

 of the iron, the metal will dissolve at the relatively electro- 

 positive parts. The fact that iron is practically insoluble in 

 pure water (in the absence of oxygen), shows that the current 

 which actually does pass is so infinitesimal that the amount 

 of iron dissolved cannot be detected, even after long periods, 

 by chemical means. 



" This may be due to two causes, namely (a) the small magni- 

 tude of the electromotive force, owing to the small differences 

 of potential between the electrically different parts of the metal 

 and (b) the great resistance offered to the passage of an electric 

 current by the electrolyte. 



"The writer's experiments seem to be generally accepted as 

 proving beyond any doubt that commercial forms of iron will 

 undergo corrosion quite readily in contact with pure water and 

 pure oxygen in the complete absence of carbonic acid or any 



