326 Prof. Johnston on the received 



Now in certain compound metallic siilphurets the sulphuret 

 of silver is replaced by the disulphuret of copper, and not by 

 the native sulphuret consisting of 7*9 copper + 2*0116 sul- 

 phur. 



Thus in Fahlerz (gray copper), for which the general for- 

 mula is 



R4 ft + 2 Cu4 R^or R^ ]S' + 2 Ag^ R, 



the first representing the copper and the second the silver 

 Fahlerz, we have identity of crystalline form and identity of 



chemical formula, if it be admitted that the compound Ag is 



capable of replacing the disulphuret Cu, and that these two 

 minerals are varieties of the same species. It is so far fa- 

 vourable to this view, that though native crystals of disulphuret 



of copper (Cu) occur in rhomboids of 71° 30' nearly, yet that 

 by fusion of the artificial as was first observed by Mitscherlich, 

 or of the native sulphuret as observed by G. Rose, this com- 

 pound can be obtained in octohedrons like native sulphuret 



of silver (Ag). The occurrence of two substances in any 

 form belonging to the regular system does not, it is true, prove 

 them to be isomorphous, yet in the present case, the several 

 circumstances connected with the forms and apparent mutual 

 replacement of these two compounds are such as to have in- 

 duced some distinguished chemists to consider their isomor- 

 phism as certain. It has however been placed almost beyond 

 doubt by the discovery at Riidelstadt of a sulphuret of cop- 

 per and silver in the rhomboidal form of the sulphuret of 

 copper (Rose, Pogg. xxviii. p. 427), and consisting according 

 to Sander (Ibid. xl. p. 313) of equal atoms of either sul- 

 phuret (Cu + Ag).* 



But the isomorphism of two compounds generally implies 

 an analogy in their atomic constitution, — that they are both 

 sulphurets of the same order. If the compound of copper be 



» 

 a disulphuret Cu, that of silver is most probably a disul- 

 phuret Ag, and if so the atomic weight of silver must be re- 

 diuced one half, or to 6*75. We have here therefore an argu- 

 ment in favour of the old result of Dulong and Petit. 



* In stating that it is placed almost beyond doubt, I take for granted 

 that the crystals f'rona Riidelstadt, of which the analysis is published by 

 Sander in Poggendorff's Annalen for 1837, part ii. p. 313, are different 

 from the imperfect crystals measured by G. Rose, and of which an account 

 is given in his crystallography published in 1833, p. 158. 



