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XXVIII. Letter from Mr. Fauaday to Mr. Brayley on some 

 former Researches relative to the peculiar Voltaic Condition 

 of Iron reobserved In/ Professor ScHOENBUiN, supplejnentary 

 to a Letter to Mr. Phillips, in the last Number. 



My dear Sir, Royal Institution, July 8, 1836. 



1 AM greatly your debtor for having pointed out to me Sir 

 John F. W. Herschel's paper on the action of nitric acid 

 on iron in the Annalcs de Chimie et de Physique ; I read it at 

 the time of its publication, but it had totally escaped my me- 

 mory, which is indeed a vei'y bad one now. It renders one 

 half of my letter (supplementary to Professor Schoenbein's) in 

 the last Number of the Philosophical Magazine, p. 57, super- 

 fluous; and I regret only that it did not happen to be recalled 

 to my attention in time for me to rearrange my remarks, or at 

 all events to add to them an account of Sir John Herschel's 

 results. However, I hope the Editors of the Phil. Mag. will 

 allow my present letter a place in the next Number; and en- 

 tertaining that hope I shall include in it a few references to 

 former results bearing upon the extraordinary character of 

 iron to which M. Schoenbein has revived the attention of 

 men of science. 



" Bergman relates that upon adding iron to a solution of 

 silver in the nitrous acid no precipitation ensued*." 



Keir, who examined this action in the year 1790tj made 

 many excellent experiments upon it. He observed that the 

 iron acquired a peculiar or altered state in the solution of sil- 

 ver; that this state was only superficial; that when so altered 

 it was inactive in nitric acid ; and that when ordinary iron was 

 put into strong nitric acid there was no action, but the metal 

 assumed the altered state. 



Westlar, whose results I know only from the Annates des 

 Miiies for 1832 J, observed that iron or steel which had been 

 plunged into a solution of nitrate of silver lost the power of 

 precipitating copper from its solutions; and he attributes the 

 effect to the assumption of a negative electric state by the part 

 immersed, the other part of the iron having assumed the po- 

 sitive state. 



Braconnot in 1833^ observed, that filings or even plates of 

 iron in strong nitric acid are not at all affected at common 

 temperatures, and scarcely even at the boiling-point. 



Sir John Herschel's observations are in reality the first 

 which refer these phasnomena to electric forces; but Westlar's, 



* Phil. Trans. 17S0, p. .374. f Hid., pp. 374, 37!). 



I Annalcs dc.i Mines, 1832, vol. ii. p. 322; or Mag. de Pharm. 1830. 



§ Annalcs de Chimie et de Physiqite, vol. iii. p. 288. 



