,58 Count de Bournon's Description of a 
infinitely more consequence in metals than in stones) of the first- 
mentioned sulphuret is always a bright deep yellow, whereas 
that of the other is a blackish gray; this circumstance alone 
would be sufficient to create some doubts respecting the identity 
of these two substances, but their other characters also present 
very striking differences. The gray sulphuret is harder: its 
powder, instead of being of a greenish-brown colour, like that 
of the yellow sulphuret, is black. This powder, when thrown 
upon an iron heated to redness, emits neither the smell of sul- 
phureous acid, nor the beautiful phosphorescent light of the other. 
The specific gravity of the gray sulphuret, taken from crystals 
of a perfectly determinate form, was always found to be between 
4460 and 4560, the mean being 4512 ; while that of the yellow 
double sulphuret is, as I have already stated, 4058. The crystal 
that had the highest specific gravity came from Cornwall, and 
belongs to the kind I have already described as containing only 
copper and iron combined with the sulphur; which kind, I 
think, ought to be considered as a standard of comparison in 
this species of double sulphuret. The specific gravity of the 
above crystal was 4558. 
A question here naturally presents itself, to which, if we 
consider the present state of our knowledge, it appears not very 
easy to furnish an answer. As the true sulphuret of copper is 
of a blackish-gray colour, and the tetraedral gray double sul- 
phuret ( Fahlerz ) is also of that colour, how happens it, that 
the yellow double sulphuret ( Kupferkies ) has always that 
brilliant yellow colour which characterises it, and which is at 
the same --time the principal cause that leads many minera- 
logists to consider it as being nothing more than a martial 
pyrites mixed with copper? To answer this question, as I 
