EXPERIMENTS CONCERNING CORROSION 203 



effect in later experiments. It must be remembered, too, that 

 fluctuations in the intensity of the light and temperature, the 

 composition of the air and the nature of the containing vessel all 

 play an important part in determining the final results. 



If these points are carefully borne in mind the reader will be 

 saved from many disappointments and failures in carrying out 

 the experiments detailed below. 



1. Different Types of Iron Rust 



Let us place a rectangular piece of iron foil in a beaker in 

 such a manner that its four corners rest in contact with the sides 

 and bottom of the glass, as indicated diagrammatically in fig. 1. 

 Now cover with distilled water to such a depth that the level of 

 the liquid A shall not fall by evaporation as low as the top of 

 the metal B, otherwise disturbing effects will ensue. 



What do we observe ? In the course of eight or nine minutes 



Fig. i. 



a faint yellow skin begins to make its appearance on the surface 

 of the metal and after a short time the iron becomes covered 

 with a thin film of brown rust. In the course of two or three 

 days the rust thickens but remains fairly evenly distributed over 

 the surface of the metal. The colour likewise remains fairly 

 constant and practically no green rust appears. 



If we remove the iron and gently rub it, the rust will easily 

 wipe off and a localised thin green stain may or may not be left 

 behind on the metal, according to circumstances. There should 

 be no pitting. 



This is the simplest or " normal " form of rusting, the brown 

 layers consisting of a very pure hydrated ferric oxide, which will 

 be referred to in the sequel as brown rust. 



The experiment may be varied by laying the foil flat on the 

 bottom of the beaker, and covering with water as before. After 

 a few hours the surface of the iron becomes covered with an even 

 layer of brown rust, but upon lifting up the foil the under-side, 



