256 
THE RUSTING OF IRON AND STEEL. 
conditions necessary for the rusting of iron, as the experimental 
evidence recorded in favour of the current view seemed to me very 
insufficient, and the whole question appeared to be one of interest and 
importance. 
I propose to-night to give you a short account of some of the con¬ 
clusions at which I have arrived and of the principal evidence I have 
accumulated in support of them. The final result of the process we 
call rusting is, undoubtedly, oxidation; the question is as to the 
manner in which it is effected. 
In dealing with this question, pure iron was heated in a current of 
hydrogen to remove any oxide on its surface, and the oxygen or air 
employed was highly purified. There are two possible sources of the 
oxygen which enters into combination, one is the free oxygen of the air, 
the other is the combined oxygen of water always present in air. To 
show that the oxygen may be derived from either of these sources, I 
propose to make two experiments. 
To show that iron combines with oxygen gas, I shall heat the metal 
in oxygen, when it oxidises with great violence. Finely divided iron 
will take fire in air at the ordinary temperature. 
Iron can also, under certain circumstances, abstract oxygen from 
water. When heated with the vapour of water (steam) the oxygen is 
removed and the hydrogen of the water set free. With these two 
possibilities in view, experiments were made by enclosing pieces of 
bright iron in glass tubes under the following conditions:— 
(1) . With dry oxygen. ... ... Result: no rusting. 
(2) . With water vapour only. ... „ „ 
(3) . With liquid water only. ... ,, ,, 
(4) . With oxygen and water vapour. ,, ,, 
(5) . With pure oxygen and liquid water. ,, rusting. 
Rusting is thus shown to occur only when iron is in contact with 
oxygen and liquid water; every care was taken to exclude carbon 
dioxide, which therefore plays no necessary part in the rusting process. 
But this conclusion seems to be contradicted by the well-known fact 
that iron rusts in ordinary air out of contact with liquid water. This, 
however, is really no contradiction, since if the temperature of the air 
is constant, no rusting takes place. Rusting only occurs when, owing 
to a falling temperature, water is condensed as a liquid film or dew on 
the surface of the iron. This I can make clear by the experiment in 
progress before you. Two pieces of bright iron are suspended, one in 
the glass chamber of constant temperature over a dish of water, the 
other in the outer air where the temperature fluctuates. After sixty 
hours exposure, we find that the metal in the chamber is untarnished, 
whilst that outside is covered with little spots of rust—the spots 
corresponding with the globules of deposited dew. 
It is thus clear that liquid water and oxygen being necessary for 
rusting to occur, the oxygen may be taken from one or both these 
sources. It was thought that some further light might be thrown on 
this subject by examining the influence exerted by certain metallic 
