418 



SCIENCE PROGRESS 



Percentage of total oxygen in 100 c.c. of air 

 absorbed by 10 grams of iron. 



These experiments show clearly that when the amount of 

 carbonic acid is very small the absorption of oxygen takes 

 place with extreme slowness. In ordinary air absorption takes 

 place fairly rapidly, beginning almost immediately and con- 

 tinuing until nearly the whole of the oxygen is removed. 



The rusting of steel as distinguished from iron requires 

 consideration as presenting certain peculiar features. It was 

 pointed out in the Report of the Committee on Steel Rails 

 already referred to, that the condition and mode of distribution 

 of the constituents of steel, not merely their nature and quantity, 

 should be taken into account in judging of the suitability of the 

 metal for the manufacture of rails. The character of a steel 

 depends not only on its chemical composition but much more 

 on the thermal treatment to which it has been subjected. 

 Steel may appear, when examined microscopically, to be nearly 

 homogenous or it may appear more or less heterogeneous ; 

 the iron may be present in two forms, a- and /3-ferrite, and 

 also in the "carbide" forms — "pearlite" or " cementite." The 

 micro-structure appears largely to determine the behaviour 

 on rusting. In steel, the iron (ferrite) is attacked during 

 atmospheric corrosion much more rapidly than the carbide of 

 iron. In certain cases, particularly when manganese is present, 

 the iron (ferrite) is found to form large distinct veins and laminae ; 

 these appear to be more readily attacked than the rest of the 

 mass during atmospheric corrosion, forming superficial furrows. 

 It is plainly of importance that a micro-structure as resistant 

 as possible to rusting should be assured in cases where resist- 

 ance to rusting is one of the principal requirements to be met. 

 The influence of the addition of certain elements such as nickel, 

 in diminishing the liability of steel to rust, has recently been 

 recognised ; and certain varieties of steel containing nickel are 

 now known which are almost entirely resistant to atmospheric 

 corrosion. Much remains to be learned in this direction. 



To sum up : the cause of rusting is the action of liquid water 



