298 Dr. Schafhaeutl 07i the Different Species of 



surrounded by a ring of black sediment, about a quarter of an 

 inch distant from it; this ring, which was about three-quarters 

 of an inch broad, was formed of rays tending towards the 

 centre of the cup and having scarcely any connexion be- 

 tween them ; the opposite end of the fragment, which lay in 

 the magnetic meridian, was covered with fibrous bundles of 

 the black sediment like the pole of a magnetic needle dipped 

 into iron filings. 1 decanted the liquid in order to wash the 

 remaining fragment and to free it from its black covering ; 

 but on touching it, I found it converted into a soft black pla- 

 stic substance. Exposed to the air, it soon changed its black 

 appearance to that of brown, and attracted very rapidly 

 moisture from the atmosphere, until it became nearly liquid. 

 Hydrochloric acid heated did not appear to act upon it; ni- 

 tric acid first evolved some nitric oxide gas, converting it 

 into a brown pulpy mass of a greater volume ; nitro-muriatic 

 acid at a boiling heat dissolved it almost completely ; it melted 

 before the blowpipe, after disengaging copious fumes, into a 

 globule of silicate of iron, strongly acting upon the magnet. 

 Twenty grains of it burnt with chromate of lead and chlorate 

 of potassa, developed carbonic acid gas, azote, and water. 

 The greatest part consisted of iron, but only a trace of silica 

 was separated, too small to be weighed. 



This leads me to the consideration of a paradoxical phaenome- 

 non consisting in the insensitiveness of iron to the action of ni- 

 tric acid under certain circumstances ; a phsenomenon which 

 has hitherto excited so much attention, and has been attributed 

 to an electrical inactivity of the iron itself, or to a film of hy-- 

 drogen or binoxide of hydrogen protecting the surface of the 

 iron. We have seen that when a considerable mass of iron 

 is attacked by acids a black skeleton always remains, which 

 very quickly changes colour when exposed to the air. We 

 have further seen, that cast iron, which is a mixture of carbu- 

 ret of iron and silicon, is even attacked by hydrochloric acid 

 on the surface only ; that is to say, that as soon as the surface 

 is coated with a pellicle of that black or brown residuum 

 which w^e shall soon more accurately describe, all action of 

 the acid and all evolution of hydrogen apparently cease; but 

 we learn from the latter experiment that in time an invisible 

 action always takes place, by which the mass of iron is slowly 

 decomposed, and the remainder assumes a different shape ac- 

 cording to the concentration or the different chemical proper- 

 ties of the acid itself 



When the liquid in which the iron is immersed contains 

 free acid, the decomposition of water stops as soon as the iron 

 is covered with a pellicle, which prevents the close contact of 



