180 SEVENTH REPORT—1837. 
the theoretical considerations by which the force of this objec- 
tion is for the present, at least, suspended. 
If, then, replacement in atomic proportions without change 
of form imply an absolute analogy of constitution, the sulphu- 
rets of copper and silver possess this analogy. In grey copper 
(fahlerz), represented by the general formula, RR + 2CutR, the 
Cu in the second member of the formula is often replaced by 
Ag (in the silver fahlerz) without change of form. If we sup- 
pose Ag and Cu to be capable of replacing each other, all the 
varieties of the grey copper may be represented by the same 
formula RAR + RAR. But if they replace each other, the forms 
of these sulphurets as they occur in nature uncombined should 
be identical. This has not hitherto been observed to be the 
case. The sulphuret of silver (Ag) is an octohedron, that of 
copper (Cu) is arhomboid. By fusion, however, that of cop- 
per is obtained in octohedrons, while that of silver is rhomboidal 
in the double sulphuret (Ag +Cu) from Rudelstadt*. There is 
every reason, therefore, for believing that these two compounds 
can replace each other, and that they are not only isomorphousf, 
but that they form an isodimorphous group, as represented in 
the table. 
It appears, then, in the present state of our knowledge, to 
follow that the two sulphurets in question are analogous in con- 
stitution, and must both be represented by the same formula, 
Ror R. It is an interesting coincidence with this result, that 
the atomic weight of silver deduced by Dulong and Petit from 
their researches into the specific heats of the metals, is only one 
half of that which is generally received. From this agreement, 
and because it involves fewer changes, it is probable that the 
compounds in question are both disulphurets and represented by 
the formula R. 
* Rose, Pog. An. xxviii. p.427. Sander, id. xl. p. 313. 
+ If isomorphous, the formula for Polybasite Cu hy +4Ag9 = might be 
As As 
expressed by Re. For the analysis of Polybasite, formerly confounded with 
brittle sulphuret of silver (sprédglaserz), see Pog. An. xxviii. p. 156. 
