on the Causes of the Neutrality of Iron in Nitric Acid, 173 



acting as the positive pole, shows the most absolute chemical 

 indifference to nitric acid of all strengths. This fact appears 

 to me to speak strongly against the truth of the English phi- 

 losopher's hypothesis ; for if we assume for an instant that at 

 the moment of immersion of the positive iron wire in the di- 

 luted nitric acid, (in consequence of the decomposition of 

 water thus caused,) a thin layer of this questionable oxide is 

 formed upon it, and that in this circumstance the evolution 

 of the oxj'gen gas originates, we cannot conceive how the 

 oxide so formed can remain for one instant in dilute acid 

 without being dissolved in it, viz. in an acid so diluted that, 

 according to Faraday, the oxide cannot remain any longer 

 indifferent. In other words, if the chemical indifference of 

 the iron to the nitric acid were materially influenced by the 

 degree of dilution of the latter, the iron under the above- 

 mentioned circumstances would remain active, a nitrate of iron 

 would be formed, and no evolution of oxygen from the metal 

 would take place. Experience, however, shows that exactly 

 the reverse of what one might have expected, according to 

 the above-mentioned hypothesis, takes place. Faraday, to be 

 sure, mentions that iron in nitric acid (the strength oi which, 

 however, is not given,) is dissolved even when it acts as the 

 positive pole in it. According to my experiments, of which 

 I may assert that they were conducted with the greatest pos- 

 sible care and accuracy, no trace of this metal is dissolved 

 under the circumstances stated in nitric acid which is several 

 times diluted with water. 1 kept an iron wire, which was 

 united to the positive pole of a cmironne des tasses of 15 jars, 

 for many hours in such an acid without being afterwards able 

 to distinguish the smallest trace of iron in it. If however 

 to such an experiment acid of common strength, as, for in- 

 stance, of 1*35, is added, the result is somewhat different; in 

 this case it contains, after some time, always a little oxide of 

 iron. According to my conviction this is not formed in the 

 acid, but a nitrate of iron is produced on that part of the wire 

 which reaches above the acid, by the acid vapours continually 

 rising, which nitrate is then conducted down into the acid 

 by capillary action. A more important fact, to which I must 

 draw attention, is the circumstance that the iron wire which 

 dips into the diluted acid and is indifferent to it, is acted upon as 

 soon as the electric current ceases to pass through it. U, for 

 example, we allow the iron wire to remain in the acid to be 

 examined, and interrupt the current anywhere, the wire im- 

 mediately appears surrounded by heavy yellowish brown 

 striae, which are a nitrate of iron. From these facts it ap- 



