THE CORROSION OF IRON AND OTHER METALS 645 



having the least mercury is first acted upon and, by solution of 

 the zinc, is soon placed in the same condition as the other parts 

 and the whole plate rendered superficially uniform. One part 

 cannot therefore act as a discharger to another ; and hence all 

 the chemical power upon the water at its surface is in that 

 equable condition which, though it tends to produce an electric 

 current through the liquid to another plate of metal which can 

 act as a discharger, presents no irregularities by which any one 

 part, having weaker affinities for oxygen, can act as a discharger 

 to another." 



No better explanation has yet been advanced. We are still 

 content to think of the zinc surface as reduced to "an equable 

 condition " ; whatever the explanation, the effect produced is 

 very remarkable. 



Although the presence of mercury serves to protect the zinc 

 against acid, action sets in immediately the amalgamated surface 

 is touched by any "metallic" conductor relatively negative to 

 zinc. It is not improbable that the zinc is protected because it 

 is covered with a liquid film of a saturated solution of zinc in 

 mercury so constituted that the uppermost layer consists of 

 mercury only, much as an aqueous solution may be thought 

 of as covered with a tenuous film of pure water ; the electro- 

 positive zinc may be thought of as brought to the surface 

 immediately contact is made with the relatively electronegative 

 conductor : then as being at once gripped by the acid radicle 

 and drawn into solution. 



The essential act by which attack is determined is the 

 formation of a conducting circuit of three components, two 

 being metals, the third an acid ; all are conductors of electricity 

 but one of them, the acid solution, is decomposed or electrolysed 

 in conducting — that is to say, it is an electrolyte. These are 

 the conditions apparently that obtain in any and every case of 

 chemical change — in other words, the occurrence of chemical 

 change is dependent on the production of an electric current 

 but this current is only produced as the change is consummated : 

 the two phenomena are interdependent and inseparable. Such 

 may be said to be the electrolytic theory of chemical change — 

 the theory being that change only takes place when, to use 

 Faraday's words, " active voltaic circles " are formed. 



The evidence brought forward in support of this view during 

 the past five-and-twenty years is remarkably complete and 

 cannot well be gainsaid. In case after case it has been shown 



