25S ARSENIATED COTVtKi 



to indicafe a difference in the fpecies into which I have fepa^ 

 rated them, while to bring them to a iingle one, you have beert 

 obliged to confider as nothing; all the exterior fpecific charac- 

 ters, with the exception of the form alone, and you have ufed 

 the latter only in eftablidiing an hypothefis refpefling it, to 

 %vhich, neither artificial means, fuch as fplittingj nor natural 

 indications, fuch as fecondary planes on a primitive cryf}al,nor 

 the retaining of primitive planes on the others have led you* 

 If in the explanation of embaraffing hth, it were permitted 

 thus to make nature fpeak when (he is filent, I have no hefi- 

 tation in alTerling that a naturaiift fo well informed, and fo 

 praclifed in the art of calculation as you are, would find very 

 few obflacles in refolving all the fpecies into each other when- 

 ever he chofe. 



Of the four fpecies of arfenlated copper which I have de- 

 fcribed, there ftill remains one> in refpecl of which you have 

 not made any calculation of approximation; it is the fourth^ 

 which, as I have flated, has for a primitive cryftal a tetrahe- 

 dral prifm with an equilateral triangle for a bafe* Neverthelefs 

 you do not exclude it when you draw your conclufions on the 

 doubt which you believe to exift on the divifion of the arfeni- 

 ated coppers into four fpecies, and you direct this doubt equally 

 to the fourth. Since according to your fuppofition this can 

 only be as a primitive to the firft cryftal, that it may alfo be in 

 a ftate to be brought to it, I have thought it right to add like- 

 wife the comparifon of this fourth fpecies with the firft* 



C'lwpurifon of the Fourth Species ivith the Firjl. 



Compa vifon of The colour of the firft fpecies is a deep ii<y-blue, which fome- 

 tine to.uth times changes to grafs-green. That of the fourth is a brilliant 



IrSiT"^^ ^ " deep verdigris, but its furface is very readily difcoloured, doubt- 

 jefs by oxidating, and it then becomes black: this renders the 

 cryftals opaque, whicli, when they have not experienced this 

 alteration, are beautifully tranfparent; this is very unufual 

 among thofe which have been Jiaturally expofed to the free air 

 during a certain time. This change however exifts only at the 

 furface; by fcratching the cryftals lightly, their fine colour is 

 readily reftored to them. I have neVer perceived any thing 

 refembling this fa6l, which unqueflionably depends on the na-i 

 ture of the fubftance of this fpecies, either among the cryllais 

 of the firft, or among thofe of the fecond and third. 



The 



