S06 Dr. T. Andrews on the Action of Nitric Acid 



which soon afterwards dissolved away and the metallic sur- 

 face again became visible. But the action of the acid on the 

 bismuth was now no longer perceptible, and, although not 

 altogether arrested, yet it had become so feeble that a piece 

 of bismuth weighing scarcely half a grain was not completely 

 dissolved by a large excess of acid at the end of two days. 

 Yet during this period the acid was removed, and replaced by 

 a fresh portion. Indeed, the more frequently the solvent was 

 changed, the more slowly did the action proceed,— a result 

 apparently paradoxical, but arising from the circumstance, 

 that the metal in this peculiar state is less able to resist the 

 action of an acid having a metallic salt in solution than that 

 of a pure acid. 



" If the bismuth in this peculiar state was touched by a 

 platina wire, the only effect apparently produced was that of 

 increasing, perhaps, the brilliancy of its lustre ; but when 

 the contact with the platina was broken, the surface of the 

 bismuth became at first covered with a dark film, then reco- 

 vered its metallic aspect as before ; and this series of phaeno- 

 mena always occurred, when connection with the platina was 

 made and broken. 



" Copper gave very similar results to bismuth. The con- 

 tact of platina checked its solution in the same acid and 

 maintained its surface bright. When the platina was re- 

 moved its surface became covered with a black coat of oxide, 

 which was afterwards very slowly dissolved by the acid, leaving 

 the copper in the peculiar or slowly soluble state. But if the 

 copper, while covered with the oxide, was raised from the 

 liquid, the acid adhering to its surface instantly dissolved 

 away the crust : the copper was now however left in its or- 

 dinary state." 



It is obvious that the bismuth and copper, in the preceding 

 experiments, are brought into the peculiar or slowly sokible 

 state, by being made the positive surfaces in a simple voltaic 

 arrangement. I was therefore much surprised to observe 

 that M. Schoenbein should have failed in producing the same 

 effect by making the bismuth act as the positive pole of a pile, 

 while it is well known that iron can be rendered inactive in 

 both ways ; and that from this difference in the bearing of 

 the two metals, he should have concluded, that the peculiar 

 condition of iron is not brought about by the same cause which 

 occasions the inactivity of bismuth. The following experi- 

 ments will, however, show that in this respect there is the 

 most perfect similarity in the behaviour of iron and the other 

 metals. 



When a small bar of bismuth connected, as the positive 



