triple Sulphur et, of Lead, Antimony, and Copper, &c. 55 
appears to me to be a double sulphuret, of copper and iron ; but 
constituting a species distinct from the gray sulphuret of the 
same form. Chemistry has not yet ascertained, in a certain and 
satisfactory manner, the proportions of the constituent parts of 
this yellow double sulphuret. Its primitive form is a regular 
tetraedron, of which the octaedral form it sometimes exhibits, 
is only a modification, produced by each of its solid angles 
having been replaced by a plane, which is perpendicular to the 
axis passing through these angles. That this is really the case, 
is also proved, by my having seen specimens of this double sul- 
phuret, (and chiefly among those that came from Cornwall,) 
which exhibited several of the well known modifications of the 
regular tetraedron ; a circumstance that never takes place in the 
octaedral sulphuret of iron, even when it happens accidentally 
to contain a portion of copper.* 
* Though I do not admit that the above-mentioned yellow copper ore, is merely a 
sulphuret of iron, with copper interposed within its substance, I am far from assert- 
ing, that the last mentioned sulphuret does not sometimes contain a portion of copper 
interposed within it; but, when that happens, the 6opper is generally in much smaller 
quantity, and its proportions are very irregular, insomuch that, in a hundred weight 
of sulphuret of iron containing copper, the quantity of copper varies, from a few 
ounces to several pounds. Besides, this sulphuret of iron, in the above-mentioned 
circumstances, constantly preserves the external characters that are peculiar to it ; 
which, as we shall hereafter see, are totally different from those of the yellow double 
sulphuret. 
But, what appears to me worthy of remark is, that when this sulphuret of iron, thus 
mixed with copper, assumes a determinate form, that form is always one of those 
belonging to the octaedral sulphuret of iron ; whereas, when gold happens to be, in 
the above manner, interposed within the substance of this sulphuret, it is always in 
that kind which assumes the form of striated cubes, or in that dodecaedral modification 
of the above form which has pentagonal planes. 
The constancy of the above facts, sufficiently shows that they are not owing to 
