330 SULPHURET OF LEAD, COPPER, AND ANTIMONY. 



5*643*. This species, when it is perfectly pure, and without 

 any mixture of iron, may be cut nearly with as much ease as 

 lead, of which it has almost the colour. It cuts perfectly 

 smooth, and with a metallic lustre. This sulphuret alters 

 spontaneously to a deep black by the oxidation of its surface. 

 Variegated cop- 2d, The double combination of copper and iron with 

 sulphur, known by the name of bunt kupferertz, which was 

 given it by the Germans, and the composition of which ap- 

 pears to be from 0*60 to 0*65 copper, 0*18 to 0*1 5 iron, and 

 22 to 0-25 sulphur; proportions deduced from the analyses 

 of ten different specimens, made by Mr. Chenevix at my re- 

 quest. Its primitive form is a cube + ; and its specific 



gravity 



terminating in a point, or very nearly so. Frequently these pyra- 

 mids touch each other at their bases, when, the prism disappear- 

 ing, they give rise to three different dodecaedra with isosceles 

 triangular faces. In one of these dodecaedra, the planes meet at 

 the summit at an angle of 1 12? 38' ; in another at one of 96° 52' ; 

 and in the third at one of 53° 8'. These crystals are almost pecu- 

 liar to Cornwall; every where else the crystals of sulphuret of 

 copper are very rare. 



* This specific gravity was given by two very perfect crystals 

 united together, weighing about 258 grains, and perfectly pure. 

 Authors that have mentioned this substance give its specific gravity 

 from 4"810 to 5'338. Beside that the specific gravity of no sub- 

 stance can vary to such a degree, the greatest is certainly below 

 the truth ; it was probably taken from some of its amorphous 

 varieties ; and a great number of trials with these has taught me, 

 that their specific gravity varies considerably ; and never equals 

 that of the crystals. No doubt this is owing to cavities in their in- 

 terior, which in fact may be frequently seen on breaking pieces of 

 this sulphuret. In weighing this sulphuret of copper too, we should 

 not take crystals that are grown black or oxided on their surface'; 

 the black oxide of copper not being easily permeable to water, there 

 always remains in this case a great deal of air between the surface 

 of the piece of sulphuret and the water in which it is weighed ; 

 and as we cannot entirely free it from this, the specific gravity ob- 

 tained is always much less than it ought to be. 

 Crystal* rare. t additional note. The crystals of bunt kupferertz are very rare. 

 Cornwall, which has furnished mineralogists with so many scarce 

 species of this metal, has produced some very fine groupes, though but 

 few. The cube, which is the primitive crystal of this substance, is the 



form. 



