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SCIENCE PROGRESS 



given of the passivity of iron is that the surface has been 

 covered with a protecting layer of oxide ; other explanations 

 have been advanced but they all lack probability. In the case 

 of aluminium, no other explanation seems possible. 



The conclusions arrived at by Dunstan and Hill from the 

 whole series of experiments under consideration are of special 

 value perhaps as showing the attitude of mind of workers in 

 this field and the need in which we stand of clear conceptions as 

 to the nature of chemical change. They are verbally as follows : 



" None of the existing theories adequately explains how 

 the various inhibiting agents prevent rusting. A satisfactory 



deposit, to the production of a resistant form of higher oxide. The insolubility 

 in acids of ignited ferric oxide and of the magnetic oxide is well known. 



The impenetrability of the layer of oxide on the surface of aluminium under 

 certain conditions is very remarkable. Magnesium may be ignited in steam 

 without difficulty and burns brilliantly when fired (Moody, Chemical Society 

 Proceedings, 1891, p. 20). After giving the demonstration with magnesium, 

 year after year, the writer has repeated the experiment with aluminium, in proof 

 of the " thickness " of its skin : this year, for the first time, the metal caught fire 

 and burnt brilliantly — as it should do. The reason of the different behaviour was 

 not apparent. 



