420 TENTH ANNXFAL REPORT, ETC. 



In respect to the electro-motive power, tlie film rendering iron 

 passive stands very near platinum. 



We shall now consider briejfiy the explanations ' given by different 

 physicists of the phenomena of passivity. 



Faraday (Phil. Mag., 1836, p. 53) supposes iron to become coated 

 with an insoluble film of oxide in concentrated nitric acid. This 

 view was attacked on many sides, but all the facts being properly 

 weighed in their relations, it is not possible to avoid considering this 

 as the basis of the correct theory of passivity. 



3I0USS011 and De la Rive supposed that the iron was protected by a 

 film of nitrous acid, (Pog. Ann., XXXIX, 330,) an hypothesis which 

 Schonbein has conclusively proved to be untenable, (Pog. Ann. 

 XXXIX, 342.) In fact, a nitrous acid film cannot be maintained as 

 a ground of explanation of the passivity of iron, because, as we have 

 seen, these phenomena are not limited to nitric acid. 



Martens presents the view (Pog. Ann. XXXVII, 393 ; LIX, 121) 

 that the passivity which iron assumes by heat is independent of its ox- 

 idation, the incorrectness of which Schonbein (P. A. LIX, 149,) as 

 well as Beetz (P. A. LXII, 234), have amply shown experimentally. 



Schonhein himself, who gathered most of the material for establish- 

 ing a theory of passivity, and has interwoven his memoirs on this 

 subject with various theoretical considerations, is unable to express 

 himself decidedly in favor of any one of the explanations given above. 

 He believes the explanation of the phenomena to be still an open 

 question. 



The views developed at the beginning of this section harmonize on 

 essential points with those which Beetz (P. A. LXVII, 186) and 

 Kollman (P. A. LXXIII, 406) have given. The latter has presented 

 a new proof of the existence of an oxide film on passive iron. He has 

 shown that rendering an iron wire passive is always attended with a 

 diminution of its conductive capacity, which evidently can be ascribed 

 only to a badly conducting envelope. [?] 



I have finally to mention a new series of experiments which Wetzlar 

 instituted twenty years after he had first made known to the chemical 

 public the remarkable indifference which such an oxidable metal as 

 iron exhibited in a liquid, giving up its oxygen so readily. 



Wetzlar has investigated the electro- motive relation of iron treated 

 in various ways, not with a galvanometer, but with a condensing 

 Bohnenherger electo'oscojje. 



In his experiments he used plates of wrought iron and steel having 

 a thickness of a few lines, and 2| or 2f inches in diameter, and fitted 

 to each other perfectly by well planed surfaces. The side opposite 

 the surface of contact had in its middle a hole for receiving an insu- 

 lating handle. He obtained the following results : 



1. If one of two clean and bright iron or steel plates^ of homoge- 

 neous character, as previously ascertained by a condenser, be rtibbed 

 with rust or polishing paper, it acts positively towards the unrubbed 

 plate. 



In this case from eight to ten contacts with the collector suffice to 

 impart a complete charge. 



2. If the contact surface of a clean steel plate be moistened with 



