298 Dr. Schafhaeutl on 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 w'ash 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 phsenome- 
non consisting in the insensitiveness of iron to the action of ni- 
tric acid under certain circumstances ; a phaenomenon 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 v/hen 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 we shall soon more accurately describe, all action of 
the acid and all evolution of hydrogen apparently cease; but a 
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 
