THE RUSTING OF IRON 409 



crumbling material known as rust. The condition most 

 favourable to rusting appears from every-day experience to 

 be alternate or simultaneous exposure to air and water, as in 

 the case of the surface of a railway line on which the rain is 

 drying after a shower. 



During the greater part of the nineteenth century rusting 

 was regarded as a simple process of oxidation. At first no 

 attempt was made to explain the part played by water in the 

 change, although the presence of water was recognised as 

 essential. The experiments of Crace Calvert in 1871 indicated 

 that the presence of carbon dioxide was an important factor in 

 rusting. Professor Crum Brown in 1888 definitely formulated 

 the theory that iron changes into rust under the combined action 

 of the oxygen and carbonic acid of the air in presence of liquid 

 water. The carbonate (or bicarbonate) of iron formed as the 

 initial product was supposed to be converted gradually by the 

 action of atmospheric oxygen into a ferric hydroxide or rust. 

 The successive changes were summarised by Crum Brown in 

 the following equations : 



4 (Fe + H 2 + C0 2 ) = 4 FeC0 3 + 4H, (1) 



4 FeC0 3 + 6 H 2 + 6 2 = 2 Fe 2 (OH) 6 + 4 CO, .... (2) 



Liquid water is essential in the process and acts by producing 

 a solution of carbonic acid. Once the action is initiated it becomes 

 continuous, because as much carbon dioxide is regenerated by 

 the process represented in the second equation as is required 

 in the first. This theory of rusting, which may be called the 

 " carbonic acid theory" was generally accepted and its correctness 

 not called into question for several years. 



In the year 1898, however, in a lecture delivered at the Royal 

 Artillery Institution, Woolwich, Prof. Wyndham R. Dunstan 

 brought forward reasons for considering that the change was 

 essentially of a different character; the full account of Prof. 

 Dunstan's experiments, made in conjunction with Drs. Jowett 

 and Goulding, on which this view was based, was not, however, 

 published until 1905. Prof. Dunstan considered that the earlier 

 experiments of Calvert were in some respects erroneous. It 

 was contended that when the utmost care is taken to exclude 

 carbon dioxide, iron will still rust in pure oxygen in contact 

 with water. The older theory that rusting was due to carbon 

 dioxide was considered " quite untenable since it has been 



