g^O ARSENIATED COPPER, 



direded to determining this bafe, or the primitive generating 

 cryftal of all thofe of this fubftance. I was not long in dif- 

 covering that among the cryftals which I had fubjeded to this 

 examination, there exited two forms which could not, in ^py 

 way, be conneded with the others : analyfis has fince fliown, 

 that one of thefe belonged to an arfeniated iron, which had 

 been improperly cited as belonging to copper, and the other 

 to a combination of copper and iron with the arfenical acid, 

 which had not been known before. With refped to the other 

 crjftals, as the appearance offered by each of them contradided 

 the opinion which conneded them, it became neceflary to de- 

 pend in the baft poffible rnanner on all the other exterior cha- 

 raders which this fubftance could offer to the mineralogift, to 

 attain to fome refult refpeding it. This is precifely what I 

 have done, and when the aggregate of thefe charaders forced 

 me to recognize four very diftind fpecies in the mafs of fub- 

 flances, which I fufpeded might belong to the combination of 

 the arfenical acid and copper, 1 confefs I experienced fome 

 fatisfadion in obferving that theanal)'fis of a chemifi^, fojuftly 

 efleemed as Mr. Chenevix is, fandioned, in fome meafure, 

 the divifion to which obfervation had led me. You remark. 

 Sir, that thefe analylis, on being repealed by M. Vauquelin, 

 varied in their refult : it follows neceffarily, that this fupport 

 fails, or at leafl becomes uncertain for me : I abandon it tljere- 

 fore, and leave to chemiftry the difcuflion of a fad wiiich be- 

 longs to it, and was to me only a powerful auxiliary, to con- 

 fine myfelf within the ftrid limits of mineralogy properly fo 

 called. 

 Preliminary ob- ^^^ allow me. Sir, firft to make fome previous obfervalions 

 fervations on the on the method which, it appears tome, fliould be followed to 

 dualn*" mjnera- determine the union or feparation of fubRances, and after- 

 logical enquiries, wards, on the poffibility of finding feveral fpecies placed under 

 the combination of the fame- acid with the fame bafe, but with- 

 out doubt, having eflential differences in the manner of com- 

 bination. 



The methods to be employed by the mineralogifl in the ftudy 

 of mineral fubllances, are comprized in the examination of the 

 peculiar marks which nature has impreffed on each of the in- 

 dividuals which decorate and enrich its bofom, and which his 

 great habit of obfcrving has taught him to recognize. Of thefe 

 marks, which vye dcfignate by the expreffion pf exterior fpe- 



cifiq 



