THE CORROSION OF IRON AND OTHER METALS 447 



A simpler method is to pour off the solution and rinse out 

 the vessel (a glass jar is the most convenient) several times with 

 distilled water; the testing liquid is then quickly poured over 

 the iron so as to cover it. The whole process should be done 

 as rapidly as possible and without unduly shaking the iron. 



With regard to the discovery of passivity, whereas ordinary 

 active iron is at once attacked by the diluted nitric acid, iron 

 which has been rendered passive by treatment with a particular 

 agent remains bright and unaffected by the acid during times 

 varying from a few seconds to several hours ; if it be touched 

 with a piece of active iron or scratched with a sharp glass rod, 

 action at once sets in, spreading from the point touched over 

 the whole surface. 



When active iron is introduced into a dilute solution of 

 sulphate of copper, a film of copper is at once deposited ; 

 passive iron may remain hours in the solution unaffected and 

 bright but if touched or scratched in the manner before 

 described copper is at once deposited, the film spreading 

 gradually from the point touched over the whole surface. 



In distilled water exposed to the atmosphere, active iron 

 usually begins to rust visibly within eight to ten minutes. 

 Passive iron will sometimes remain bright during one to two 

 hours before rusting begins and then the action is very local 

 during a considerable time. The most uniform results are given 

 with iron which has been etched with diluted nitric acid {d = 1*2). 



The results obtained by applying such tests to iron after ex- 

 posure in various solutions are collected in the table on next page. 



And not only iron but other metals — aluminium, copper, 

 magnesium and zinc — are rendered passive, so that they resist 

 aerial oxidation, by treatment with alkalies or with chromates and 

 a number of other salts. The results are regarded by Dunstan 

 and Hill as clear proof that in all cases the passivity is due to 

 the formation of a film of oxide on the surface of each metal As- 

 suming this to be true, there is no novelty in the conclusion. It 

 has long been recognised that the use of iron coated with zinc 

 as a roofing material and that of aluminium is only possible by 

 reason of the protection afforded by the coating of oxide on the 

 surface of the metal. 1 The only rational explanation hitherto 



' In the case of iron, the apparently special passivity caused by nitric acid 

 may be due, if not to a particular mechanical or crystalline condition of the 



