654 SCIENCE PROGRESS 



on joining them at the other extremity an equal but oppositely 

 directed difference of potential would be developed : con- 

 sequently, a circuit could not be established in which an 

 electric current would flow. To secure a flow of current, the 

 introduction of a third component into the system is required 

 to afford the necessary slope of potential. 



For the various reasons given it may be contended therefore 

 that the answer to Dr. Cushman"s " one crucial question " — 

 " Does iron pass into solution, even to the slightest extent, in 

 pure water?" — must be, on the practical side at least: "That 

 it is not proven that it does"; on the theoretical: "That it is 

 highly improbable that it does, not to say impossible that it 

 should." Or perhaps it should be said, as it cannot be denied 

 that, strictly speaking, no substance is insoluble in water, that 

 if iron dissolved it would dissolve mechanically in the water 

 and in the absence of acids it would not rust, even if oxygen 

 were present. But such an ideal condition of purity is only 

 conceivable ; it could not be realised. 



Taking Moody and Friend's observations into account, it 

 may be said to be proved that whenever iron is dissolved acid 

 is present and active. Objection has been taken to Moody's 

 conclusion on the ground that the inactivity of the iron he used 

 towards water was due to the fact that by cleaning the iron in 

 chromic acid before exposing it to oxygen and water he rendered 

 it passive — but the account he gives is full of proof to the 

 contrary. Thus he states that rusting began when ordinary 

 air was allowed to enter into the tubes in which iron had been 

 exposed during long periods in contact with purified air and 

 water without any rusting being apparent ; also that the iron 

 at once began to rust when and where it came into contact 

 with the glass tube in which it was enclosed. I still have in 

 my possession tubes prepared by Moody several years ago in 

 which iron is sealed up in contact with water and oxygen and 

 not the least rusting is apparent, except at one spot where, 

 owing to careless handling by students, the V-shaped tube 

 having been turned upside down, the iron rod was allowed to 

 come into contact with the glass and to remain in contact with 

 it during a short time. Friend has also observed that contact 

 with glass promotes rusting. This observation is in itself most 

 significant as showing the influence that acids have in pro- 

 moting rusting — the effect cannot well be ascribed to any 



