554 IRON. 



acid. If passive iron be made a negative pole (chlorous) in ni- 

 tric acid, it also ceases to resist solution. The indifference to 

 chemical action exhibited by iron when passive, is not confined 

 to nitric acid of the density mentioned, but extends to various 

 saline solutions which are usually acted upon by iron. An in- 

 difference to nitric acid of the same kind can also be acquired by 

 other metals as well as iron, particularly bismuth (Dr. An- 

 drews), but in a much less degree. That the peculiar condition 

 of the iron, which enables it to resist solution in the nitric acid, 

 is of a voltaic nature cannot be doubted, but its exact character 

 is still very imperfectly defined. In the consideration of the 

 subject, the circumstance is not to be overlooked, that iron 

 may, and does dissolve in nitric acid in two different ways : 

 (1) When directly oxidated by the decomposition of the acid, 

 and (2) by substitution for the hydrogen of the nitrate of 

 water, as the same metal dissolves in oil of vitriol. The first 

 mode of solution is not known to be connected with voltaic ac- 

 tion, but the second is so, and should be promoted by rendering 

 the iron positive or zincous ; the condition which actually pre- 

 vents all solution, and makes iron passive in nitric acid of 1.3 

 density. But if the predominating tendency of iron is to dis- 

 solve in acid of that strength by direct oxidation, which is very 

 probable, it comes to be a question whether increasing the dis- 

 position of the metal to dissolve in the other mode, or by substi- 

 tution, may not counteract the former tendency, and thereby im- 

 pede the solution of the metal. The passive condition would then' 

 be represented as the result of an antagonism in the two forces 

 which act simultaneously upon the metal.* Schoenbein has ob- 

 served, however, an action of thin films or pellicles of foreign 

 matter adhering to metallic surfaces, which he thinks, with 

 reason, may be concerned in the phenomenon, and which is in- 

 teresting independently of that relation. Platinum wire, after 

 being plunged for a few seconds in hydrogen gas, acts as a po- 

 sitive metal, or as if it were zinc, when associated with clean 

 platinum in dilute sulphuric acid. This can be explained only 



* Dr. Andrews has indeed drawn the conclusion, from observation, that the 

 ordinary chemical action of a hydrated acid upon the metals which dissolve 

 in it, is in general diminished when the acid is concentrated, by the voltaic 

 association of these metals with such metals as gold, platinum, &c ; while on 

 the contrary, it is increased when the acid is diluted. Trans, of the Royal 

 Irish Academy, 1838 ; or Becquerel, vol. v, pt. 2, p. 187- 



