THE CORROSION OF IRON AND OTHER METALS 651 



cases, when substances are mixed which by themselves have 

 no electrical conducting power, solutions are obtained which 

 conduct readily is also in no way accounted for by the 

 hypothesis. 



In 1883, without considering any of the difficulties inherent 

 in the Clausius hypothesis, Arrhenius put forward the pro- 

 position that not only was the conductivity of solutions to be 

 ascribed to the presence in them of the dissociated, electrically 

 charged ions of the dissolved substance but that it was pro- 

 portional to the degree of dissociation and he assumed that 

 the dissociation was practically complete in dilute solutions 

 of strong acids. He recognised, moreover, that there was a 

 close correspondence between chemical activity and electrical 

 conductivity in the case of acids ; in fact, his conclusions were 

 summarised in the pregnant sentence : " L'activite electrolytique 

 se confonde avec l'activite chimique." 



The conclusions at which Arrhenius arrived were so 

 fascinating and the correlation of chemical with physical 

 activity was so welcome, that the hearts of chemists were 

 captured forthwith ; it was a case of unreflecting love at first 

 sight. The fact was overlooked that the solvent was in no 

 way taken into account ; the violence of the assumption made 

 in supposing that practically all of the molecules of the dis- 

 solved substance " went smash " while those of the solvent 

 remained unaffected was not remarked. To the present day, 

 conductivity is very generally ascribed to the presence of free 

 dissociated ions and to these alone. 



After this digression, consideration of Cushman and Gardner's 

 argument maybe resumed. Having stated that three "theories" 

 of rusting are to be considered : (i) The carbonic acid theory; 

 (2) the peroxide theory ; (3) the electrolytic theory, they proceed 

 to dismiss (i) and (2) on various grounds, and then state the 

 third in the following terms : 



" The Electrolytic Theory. — From the standpoint of the modern 

 theory of solutions, all reactions which take place in the wet 

 way are attended with certain readjustments of the electrical 

 states of the reacting ions. The electrolytic theory of rusting 

 assumes that before iron can oxidise in the wet way it must 

 first pass into solution as a ferrous iron. The subject has been 

 interestingly treated by Whitney, who discussed it from the 

 standpoint of Nernst's conception of the source of electro- 

 motive force between a metal and a solution. When a strip 



