813 . 



attributes the cause of passivity to the presence of a skin of oxide 

 which separates the metal from the liquid. With this view Habkr 

 and GoLDscHMiDT '), Habkr and Maitland ') and Krassa ') concur. 

 These last investigators assume that the skin of oxide does not 

 entirely cover the metal, but is porous or presents cracks, through 

 which the metal is partly in contact with the liquid. This accounts 

 for the fact that there are different degrees of passivity, which are 

 caused by a more or less perfect separation between metal and liquid. 



Also DuNSTAN and Hill^) consider the formation of a protecting 

 layer as the cause of the passive state, which iron and other metals 

 assume in a solution of potassium bi-chromate. This oxide theory 

 is rejected by Muller and Königsberger *) and Micheli '), who could 

 not optically demonstrate the presence of oxide on a passive metal. 



According to the oxide theory passivity is a mechanical pheno- 

 menon, whereas the other theories ascribe the cause of passivity to 

 a chemical change in the metal or in the liquid. 



Of these theories the oxygen theory comes nearest to the oxide 

 theory. According to this theory, which is defended by Jordis ^), 

 Fredenhagen *), MuTHMANN and Fraunberger '), and Flade ^"), the metals 

 in passive state possess a charge of oxygen, which retards the going 

 into solution of the metal catalytically. The oxygen present can partly 

 combine with the metal to oxide, which dissolves in the solid metal, 

 or is absorbed by the metal ^^). 



In opposition to this oxygen theory the hydrogen theory may be 

 placed, according to which the metals in active state have a hydrogen 

 charge, which accelerates the solution of the metal catalytically. 

 The hydrogen can partially be present as hydrure, which dissolves 

 in the metal. This hydrogen theory is chiefly advocated by Grave ^') 

 and Rathert ^'). One view, however, does not exclude the other, 

 it is possible that fn active state a hydrogen charge furthers the 



I) Zeitschr. f. Elektrochemie 12, 49 (1905). 



5) Ibid. 13, 309, (1907). 

 8) Ibid. 15, 490, (1909). 



*) Journ. Ghem. Soc 99, 1853, (1911). 



6) Zeitschr. f. Elektrochemie 15, 742, (1909). 



6) Arch. Sci phys. Nat. Geneve, 115, 122, (1900). 



7) Zeitschr. f. Elektrochemie 11, 787, (1905). 



8) Zeitschr. f. phys. Chemie 43, 1, (1913); 63, 1, (1908). 



9) Silzungsber Bayr. Akad. 34, 201, (1904). 



10) Zeitschr. f. physik. Chemie 76, 513, (1911). 



II) Bennetts and Burnham, Zeitschr. f. Elektrochemie 23, 377, (1916). 

 12) Zeitschr. f. physik. Chemie 77, 513, (1911). 



1») Ibid. 86, 567, (1914). 



58 

 Proceedings Royal Acad. Amsterdam, Vol. XX. 



