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SCIENCE PROGRESS 



which has been in contact with the glass, is seen to be mainly 

 green. This, however, now rapidly oxidises to brown rust on 

 exposure to air, and therefore consists of iron essentially in the 

 ferrous condition. Although the corroded under-side of the 

 metal may be unequally attacked, there is no pitting observable. 

 A very similar green appearance may be obtained by immers- 

 ing iron foil in a saturated solution of a nitrate, such as sodium 

 or potassium nitrate. In this case the iron may be entirely free 

 from the containing vessel, save of course at the four corners of 

 support as in the first experiment; also AB (fig. i) should not 

 be less than about half an inch. If, after a few days, the iron is 

 removed and gently washed with distilled water the green rust 

 steadily oxidises to a brown colour. There is no pitting. This 

 reaction is interesting as being fairly characteristic of nitrates, 

 for in most other aqueous solutions, such as those of the chlorides 



NaNO, 



Fig. 2. 



and sulphates of the alkali metals, the colour of the rust produced 

 varies from a ruddy brown to a much darker shade with varying 

 amounts of green, according to circumstances. A pretty experi- 

 ment is as follows : Prepare two saturated solutions at the 

 temperature of the room, one of sodium nitrate and one of 

 potassium chloride. Pour the former into a gas jar and then 

 add the other very carefully, either pouring on to a piece of cork 

 floating on the nitrate, or else allowing it to flow gently down 

 the side of the jar held in an inclined position. The chloride 

 solution being the less dense floats on the nitrate solution. Now 

 insert a polished strip of iron as in fig. 2. In the course of a few 

 hours a coating of green rust is formed on B C, whilst C D remains 

 perfectly bright. This is well illustrated by the photograph 

 (fig. 3), where the dark portion represents the corroded metal, 

 and the light the uncorroded. This is particularly interesting 

 because we might have expected broivn rust from B to C, and 



