THE RUSTING OF IRON AND STEEL. 
259 
iron, which holds in solution, or intimate association, carbide of iron, 
the hardness chiefly depending on the quantity of carbide which is 
present. Recently the exact structure of steel has been made evident 
by photographing the surface highly magnified. The steel is then found 
not to possess a uniform structure, and the metallic iron may be seen 
imbedded, often in distinct veins, in the mass of the carbide. This is 
well shown in the photograph which I will throw on the screen of a 
highly polished steel surface magnified 140 diameters. The metallic 
iron appears white and the carbide black. It was interesting to 
ascertain how these two components .behave during rusting. The 
next photograph of the same piece of steel tells its own story. The 
pure iron has been attacked and the falling away of the rust has left 
furrows in the surface. The carbide is little if at all “ rusted.” 
I will now show you a photograph of another piece of the same steel, 
highly polished, in which the oxygen and carbide are more evenly 
distributed, and another photograph of the same piece of metal after 
it has rusted. The corrosion is in consequence more general and the 
furrowing is not noticeable. Photographs of steel which have been 
immersed in hydrogen peroxide show that the attack has been on 
the iron (ferrite), scarcely at all on the carbide, there being an exact 
parallelism between aerial oxidation or rusting of steel and the action 
of hydrogen peroxide on the same metal. This mode of rusting of 
steel has an important bearing on the corrosion, and consequent loss 
of mechanical strength, in railway lines and other steel structures. 
From an examination which I have recently made, of the mode of 
rusting of a large number of sections of steel rails, it appears, as you 
have seen, that the rusting may occur uniformly over the entire 
surface, and this happens when the two constituents of steel are 
distributed uniformly throughout the mass of metal, or furrows may 
be formed in those cases where large veins of pure iron occur in the 
mass leading, in the case of railway rails, to a rapid deterioration 
and wearing away of the surface. In the metallurgy of steel regard 
therefore ought to be paid to those conditions which lead to a uniform 
distribution of the two constituents. In future, it seems probable that 
the examination of the micro-structure of steel will be considered to be 
a necessary adjunct to its chemical analysis. 
In this lecture I have endeavoured to give you some account of the 
new view about the rusting of iron and steel, and I think you will 
agree with me that the results have practical importance as well as 
scientific interest, as throwing some light on what has hitherto been 
regarded as one of the simplest and most important of natural 
chemical processes. 
DISCUSSION. 
Professor McLeod : I shall be very glad to make a few remarks upon this 
interesting subject which Professor Dunstan has brought before us and in which 
I think he has proved his point to a large extent, that the cause of rusting is the 
formation of peroxide of hydrogen. But I think he has left out one or two facts 
that will support his theory. 
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