308 Dr. T. Andrews on the Action of Nitric Acid 



of nitric acid from the sp. gr. of 1'47 to that of 1*5, yet such 

 acids have no action whatever upon iron alone; so that the 

 passage of an electrical current of sufficient intensity is ca- 

 pable of becoming the cause of the solution of iron when 

 acting as the positive pole. The manner of closing the cir- 

 cuit produced no difference in the result. 



It appears therefore, from these observations, that the pass- 

 age of an electrical current of a certain intensity renders iron 

 and bismuth inactive in acids capable of dissolving them, 

 while the passage of a current of a higher intensity causes 

 their solution in acids which otherwise have scarcely any ac- 

 tion upon them. It is true that the required intensities of the 

 currents for these objects are different for each metal, but 

 this necessarily follows from the difference in their chemical 

 relations to nitric acid. But although the peculiar state of 

 the two metals thus appears to be developed by the same 

 cause, it must be carefully observed, that while the chemical 

 action of the acid upon iron is entirely destroyed, its action 

 upon bismuth, and all the other metals which I have examined, 

 except perhaps tin, is only greatly retarded. This distinc- 

 tion, important as it is, does not appear to me to be sufficient 

 to prevent us from referring all the phaenomena to the same 

 general principle. As to the circumstance of the peroxide of 

 lead not protecting bismuth while it protects iron, I have 

 only to observe, that this substance has so little tendency to 

 attach itself to the surface of bismuth, that I have never been 

 able to succeed in coating properly that metal with it; and 

 when I endeavoured to employ iron coated with the peroxide 

 to protect bismuth in nitric acid of sp. gr. 1*4, the peroxide 

 generally separated leaving the surface of the iron exposed. 



Concentrated nitric acid immediately develops the peculiar 

 state of bismuth, as well as of iron, and when a small portion 

 of bismuth is left in nitric acid of sp. gr. 1*5, its solution will 

 occupy some weeks, just as happens when, in the peculiar 

 state, it is kept in nitric acid of sp. gr. 1'4. But even in .the 

 same acid and at the same temperature, it is remarkable what 

 apparently trivial circumstances are capable of determining 

 these two states in bismuth, and the facts which I have now 

 to describe are certainly among the most singular to which 

 this inquiry has led. 



If a small fragment of bismuth (half a grain, for example,) 

 is introduced into nitric acid of the sp. gr, 1-4, at the tem- 

 perature of 40° or 50° Fahr., and allowed to remain at rest, 

 it will usually dissolve in a few seconds, with the disengage- 

 ment of orange fumes; but sometimes, after the solution has 

 proceeded to a certain extent, it will suddenly cease, and the 



