172 Prof. Schoenbeiii*s Uemarks on Faraday's Hypothesis 



water, the expelled ferruginous matter being indicated by. 

 the ^^ gossan." This usually consists of quartz, as well as iron- 

 ochre, 8cc., and it abounds in copper veins, but not in those 

 of tin. 



XXXVI. Remarks on Faraday's Hypothesis with regard to 

 the Causes of the Neutrality of Iron in Nitric Acid. By Pro- 

 fessor SCHCENBEIN.* 



¥N the July Number of the Philosophical Magazine for 

 ■■- 1836, Mr. P^araday has set forth an equally simple and in- 

 genious hypothesis on the cause of the passive condition as- 

 sumed by iron in common nitric acid under certain circum- 

 stances. This distinguished philosopher supposes that the 

 peculiar action of this metal is caused, first, by a thin layer 

 of oxide which is formed round the iron wire under these cir- 

 cumstances ; and, secondly, by the property of this oxide to 

 be insoluble in nitric acid of a certain degree of concentration. 

 According to Faraday, therefore, the real cause of the inac- 

 tivity of the iron would be purely mechanical ; that is, the in- 

 activity of the iron would originate in the fact, that the metallic 

 iron and nitric acid do not come into actual contact. In the 

 same way Faraday seems also to explain the fact which I have 

 observed, that when a complete circuit of the electric current 

 is formed, oxygen gas is evolved at the positive iron wire. As 

 probably his paper on the subject in question will find a place 

 in this [Poggendorff's] journal, I do not consider it as necessary 

 to enter into a more detailed account of the hypothesis of which 

 we are speaking, for the purpose of subsequently referring to 

 it, and I proceed, therefore, at once to bring forward the facts 

 which appear to stand in opposition to those of Faraday. I 

 must first remark, that the surface of an iron wire which has 

 been made passive by repeated immersions in nitric acid of 

 1*35 (see my last paperf ) exhibits a much cleaner and brighter 

 metallic surface than a wire which has been cleaned in any 

 other manner, and therefore not the slightest trace of a coat- 

 ing of oxide can be observed by the eye. I will, however, 

 not lay much stress upon this circumstance, although I think 

 it deserves some consideration. In one of my former treatises 

 I have mentioned the fact, that iron wire, in whatever way it 

 has been made passive towards common nitric acid, acts like 

 common iron in considerably diluted acid, whilst an iron wire 



• From Poggendorff's Annalen der Thysik und Chemie^ vol. xxxix. p. 137- 

 t Prof. Schoenbein's paper here referred to appears to correspond with 



his first paper on the subject in Lond. and Edinb. PhiK Mag. vol. ix. p. 53. 



—Edit. 



