1890.] The Passive State of Iron and Steel 117 



a research of this intricate nature. He feels, however, rewarded by 

 the measure of success which has ensued. The general tentative 

 conclusion he has arrived at, as a result of careful experimentation, 

 is that the passive state of iron and steel ought not to be regarded as 

 fixed or static, the electro-chemical observations tending to show that 

 the passivity is a property influenced more or less by various condi- 

 tions, such as variation of the molecular structure and chemical 

 composition of the iron and steel, different strengths of nitric acid, 

 modification of attendant physical conditions, magnetism, tempera- 

 ture, &c. It is known that when bright iron is immersed in nitric 

 acid of 1*4 sp. gr., the iron is not acted upon, but remains passive in 

 the acid, which appears to exert no perceptible effect upon the metal. 

 " Under certain circumstances iron is not acted upon at all by nitric 

 acid. Iron in this state is termed passive, and this condition is 

 brought about by dipping the metal into concentrated nitric acid and 

 then washing it " (' Treatise on Chemistry,' by Sir Henry E. Roscoe, 

 F.R.S., and Dr. C. Schorlemmer, F.R.S.). The late Dr. Jno. Percy, 

 F.R.S., the eminent metallurgist, referring to the passive state of iron, 

 remarked that " this is a very curious and interesting subject, which 

 may possibly be one day found to admit of valuable practical applica- 

 tion." The passive state of iron appears first to have been observed 

 just a century ago by Keir, and brought before the notice of the 

 Royal Society in 1790 (' Phil. Trans.,' 1790, p. 379) ; he observed that 

 strong nitric acid had no action on iron when the metal was placed 

 therein. Bergman, Scheurer-Kestner, Schonbein, and Buff also 

 made some observations on the passivity of iron previous to the year 

 1848. Faraday and Beetz were disposed to attribute the passive state 

 of iron to the immediate formation of a very fine envelope or film of 

 oxide on immersion of the metal in concentrated nitric acid, whereas 

 Sir John Herschel (' Annales de Chimie,' vol. 54, 1833, p. 87) con- 

 sidered the phenomenon was due to a certain permanent electric state 

 of the surface of the metal. 



Westlar (' Annales des Mines,' vol. 2, 1832, p. 322) observed that 

 when iron or steel had been immersed in nitrate of silver solution it 

 failed to precipitate copper from its solutions. 



Braconnot (' Annales de Chimie,' vol. 52, 1833, p. 288) noticed that 

 filings or plates of iron were not at all affected in strong nitric acid at 

 ordinary temperatures. 



In the present memoir are presented the results of a study of cer- 

 tain magnetic, temperature, and other conditions which the author 

 found to affect the passive state of iron and steel, and he is extending 

 the inquiry into the relative passivity of the various modern steels 

 and other aspects of the subject. The author observed, in course of 

 experimentation some years ago, that when bright iron or steel was 

 magnetised whilst immersed in concentrated nitric acid, its passive 



