THE CORROSION OF IRON 81 



copper nitrate of any strength, without causing copper to be 

 deposited on the iron, whilst pieces from a batch of iron which 

 rusts always cause the deposition of copper from the same 

 solutions of copper salts. Sometimes the copper is deposited 

 quickly, whilst at other times several hours may elapse before 

 the deposition of copper takes place. The metal is always 

 attacked at one or more points and the deposition of copper 

 spreads from these points over the whole surface in a very 

 short time. 



" It is clear from these experiments that physical differences 

 in iron of the same high state of chemical purity can cause most 

 profound differences in its behaviour. 



" If the theory is true, we should expect such results. In the 

 case of the iron which does not rust and is unaffected by solu- 

 tions of copper sulphate or copper nitrate, the metal is probably 

 physically homogeneous, at any rate on the surface. There 

 would, therefore, be no differences of solution pressure — no 

 electrically different parts — on the surface of the metal, and so 

 no tendency for the metal to pass into solution by electrolytic 

 action when the metal was put into an electrolyte. Since iron 

 could not therefore pass into solution, we should not expect 

 rusting to take place, nor should we expect the deposition of 

 copper on the iron from solutions of copper salts." 



Mr. Lambert states further that when pieces of metal which 

 had been exposed during several months to the action of air 

 and water without corroding were pressed upon by an agate 

 pestle in an agate mortar and again exposed, they rusted in the 

 course of a few hours, rust forming first at the edges which had 

 not been pressed, while the pressed portions remained bright. 

 In the same way, copper was precipitated immediately when the 

 pressed pieces were placed in a solution of copper sulphate, 

 deposition commencing at the unpressed edges. He assumes 

 that the difference in physical state was the cause of the 

 difference in the behaviour of the iron before and after it was 

 subjected to pressure. But this explanation is not good enough. 



In the first place, it is open to question whether two parts of 

 a plate composed of a homogeneous material would be at 

 different potentials after the one had been subjected to pressure 

 if pressure had no effect in altering the chemical nature of the 

 material and merely changed the electrical resistance of the one 

 relatively to the other. 



In the second place, it is improbable that different specimens 

 of iron prepared by reducing oxidised iron in hydrogen — the 

 6 



