206 SCIENCE PROGRESS 



effect of which, however, is so slight that in neutral solutions no 

 pitting is observed. 



The masses of rust are mostly of the green variety, and 

 rapidly oxidise on removal and exposure to air. The metal 

 really looks much prettier than the photograph indicates owing 

 to the colours ranging from light moss-green through dark green 

 to dark brown, the edges being relieved with the ruddy tinge of 

 ordinary brown rust. 



By increasing the quantity of alkali to about twice normal, 

 that is, 112 grams of caustic potash per litre, no corrosion of 

 any kind will take place, whatever the concentration of the 

 chloride. 



Particularly pretty results are obtainable by suspending 

 pieces of iron foil in weakly alkaline solutions of potassium 

 chloride by means of glass hooks. The rust accumulates in the 

 form of threads and hangs down from the metal like skeins of 

 brown silk. This is illustrated by fig. 6, where A B is the 

 corroded metal, the lower portion being rust. 



OXYGtN 



OXYC.EN 



Partial immersion of iron in weakly alkaline salt solutions 

 also yields interesting results, the corrosion occurring locally, but 

 being particularly severe at the surface of the liquid where thick 

 masses of green and brown rust accumulate. 



4. The Corrosion Zone 



If a sphere of iron is suspended in a tank of still water it 

 tends to combine with the dissolved oxygen in its immediate 

 vicinity. Fresh supplies of oxygen gradually diffuse towards 

 the iron from surrounding layers of water until equilibrium sets 

 in. When this has been attained a more or less spherical shell 

 might be sketched out in the water as represented in section by 

 the circle ABC in fig. 7 through which oxygen is constantly 



