on certain stony and metalline Substances , See. 183 
The black crust with which the surface of the stone is coated, 
although it is of no great thickness, emits bright sparks, when 
struck with steel : it may be broken by a stroke with a hammer ; 
and seems to possess the same properties as the very attractable 
black oxide of iron. This crust is, however, like the substance 
of the stone, here and there mixed with small particles of iron 
in the metallic state : they may easily be made visible, by passing 
a file over the crust, as they then become evident, on account 
of their metallic lustre. This is more particularly the case with 
respect to the crust of those stones which remain to be men- 
tioned, they being much more rich in iron than that I have just 
described; a circumstance I think it needless to repeat, in the 
following descriptions of them. The stone now treated of, does 
not, when breathed upon, emit an argillaceous smell : the same 
remark may be applied to all the others. 
The specific gravity of this stone is 3332. 
STONE FROM YORKSHIRE. 
This stone, the constituent parts of which are exactly the 
same as those of the stones from Benares, differs from them, 
however. 
First. In having a finer grain. 
Secondly. That the substance described as being in the form 
of small globular or elliptical bodies, is not so constantly in those 
forms, but is also found in particles of an irregular shape ; a 
circumstance that is not met with in the other stones : these 
bodies are likewise, in general, of a smaller size. 
Thirdly. The proportion of martial pyrites, which has pre- 
cisely the same characters as that in the stones from Benares, is 
less; on the contrary, that of the iron in a metallic state, is 
much greater. The quantity I was able to separate by means 
