ARSENIATED COPPER. 249 



cific charaders, fome are too delicate to be defcribed ; but cuf- 



torn enables the naturalift to feize them ; their adtion on his 



fight is fudden ; the mofl rapid glance embraces the whole of 



them, and the naturalift has frequently formed his opinion long 



before he has thought of accounting to himfelf for it. He is 



not, however, fecured by them from the errors which other 



bulkier and more comparable charaders may afterwards rec- 



tify; but the iirft impreffion received from thefe firft traces, 



very often ferves him as a guide in the method of employing 



the fecond. Among thefe latter characters, fome are of eafy 



application and almoft always poffibie, others require attention 



and particular circumftances to be capable of being employed. 



Thofe which are in moft common ufe, and eafieft, are the form, 



the fracSure, the hardnefs, the fpecific gravity, and the colour. 



Perhaps in a fliilful hand, direded by the habit acquired from 



their ufe, thefe characters are almoft always fufficient for the 



knowledge and claffitication of mineral fubftances. In ftones, 



the colour is the moft variable of all : neverthelefs, it is certain, 



though the true caufe cannot yet be affigned, that each of thofe 



which have been examined hitherto, affedts one only of the 



known colours more readily than it does any of the others. 



But in the metals this character becomes more conftant and 



more effential, and it very feldom varies without the caufe of 



its variation being a change in the nature of the metallic fub- 



ftance itfelf. 



This fa6l granted, when the naturalift employs the exterior The confidera- 



fpecific charaders, to afcertain the fubjed which determines tionoffomeof 



'_ . . -' tlie characters 



his enquiry, from the moment at which the agreement of thefe may be omitted. 



characters, or their differences with thofe fliown by known ^^^'^ ^^'^y ^^ 

 lubftances, puts him in a fituation to pronounce on the identity n^ral inferences, 

 or the difference of their nature, do not you believe that he has 

 then the liberty of retrenching, on the one hand, thofe which 

 do not agree with the opinion which he had previoufly thought 

 it right to embrace ; and, in the fecond place, to fubjeCt the 

 others to fuppofitions which may occafion a change in their 

 afpect to connect them with that which he wiflies, when na- 

 ture itfelf has not offered traces, free from doubi, of the pro- 

 bability of the modification which he admits in thefe charac- 

 ters ? 



Permit me to obferve to you, Sir, that this is precifely what This has been 

 appears to me to be the fubftance of your obfervalions on the ^"^ ''y M. 



arfeuiated 



