Prof. Schoenbein on a peculiai- Voltaic Conditio7i of Iron. 53 



communication ; but I thought that it would be useful to unite 

 undei- one point of view what has been done or proposed in 

 different countries towards the attainment of extensive simul- 

 taneous observations upon the laws of terrestrial magnetism. 



Accept, Sir, the acknowledgement of the profound respect 

 with which I have the honour of being, 



Your Royal Highness's, &c. &c., 

 Berlin, April, 1836. Alexander von Humboldt. 



XII. On a peculiar Voltaic Condition of Iron., hy Professor 

 Schoenbein, of Bale; in a Letter to Mr. Faraday ; mth 

 further Experime^its on the same Subject, hy Mr. Faraday, 

 communicated in a Letter to Mr. Phillips. 



To Michael Faraday, Esq., D.C.L., F.R.S., &c. 

 Sir, 

 A S our Continental and particularly German periodicals are 

 ■^^^ rather slow in publishing scientific papers, and as I am 

 anxious to make you as soon as possible acquainted with 

 some new electro-chemical phenomena lately observed by me, 

 I take the liberty to state them to you by writing. Being 

 tempted to do so only by scientific motives, I entertain the 

 flattering hope that the contents of my letter will be received by 

 you with kindness. The facts I am about laying before you 

 seem to me not only to be new, but at the same time deserving 

 the attention of chemical philosophers. Les void. 



If one of the ends of an iron wire be made red hot, and at 

 ter cooling be immersed in nitric acid, sp. gr. 1'35, neither the 

 end in question nor any other part of the wire will be affected, 

 whilst the acid of the said strength is well known to act rather 

 violently upon common iron. To see how far the influence of 

 the oxidized end of the wire goes, I took an iron wire of 50 

 in length andO"''5 in thickness, heated one of its ends about 3" 

 in length, immersed it in the acid of the strength above men 

 tioned, and afterwards put the other end into the same fluid. 

 No action of the acid upon the iron took place. From a si- 

 milar experiment made upon a cylindrical iron bar of IG' in 

 length and 4<"' diameter the same result was obtained. The 

 limits of this protecting influence of oxide of iron with regard 

 to quantities I have not yet ascertained ; but as to the influ- 

 ence of heat, I found that above the temperature of about 75^^ 

 the acid acts in the common way upon iron, and in the same 

 manner also, at common temperatures, when the said acid con- 

 tains water beyond a certain quantity, for instance, 1, 10, 100, 

 and even 1000 times its volume. By immersing an iron wire 

 in nitric acid of sp. gr. 1-5 it becomes likewise inditterent to 

 the same acid of r35. 



