6 Defcrtpthn of the Arfeniales 



blue, and green colours, as the kind already deferibed ; but, 

 allhouffh thefe colours are frequently very deep, they never 

 arc lb brilliant as in that kind. 



It is very uncommon to find pieces of this ore that are not 

 mixed, and frequently even penetrated, with gray vitreous 

 copper ore. That which accompanies the arfeniate ot Huel 

 Gorland mine, oilers, in this refpeft, an uncommon and 

 very particular appearance. The yellow ore is mechanically 

 mixed with ihe vitreous ore, fo as to form a compound, in 

 which, by the affillance of a lens, the fmall particles belong- 

 ing to each of thofe two ores may be very clearly diftinguifhed. 

 The fpecific gravity, as well as the quantity of copper, in this 

 ore, varies confiderahlv, according to the proportions in which 

 the yellow copper ore and gray vitreous copper ore are mixed 

 together; fometimes they feeni to be mixed in equal propor- 

 tions, or nearly fo. 



Nature has eftabliflied ver\^ remarkable differences between 

 the arfeniates of copper; and thefe take place not only in 

 their forms, but likewife in their hardnefs and fpecific gra- 

 vity. Thefe diflerences arife, either from the manner in 

 which the arfenic acid is combined vtith the copper, or from 

 the difierent proportions in which thefe two fubftances are 

 combined. I have been naturally led to follow the fame 

 order, and to divide the arfeniates of copper into four different 

 fpecies: and the very intereding analyfis of this fubftance, 

 made by Mr. Chenevix, has afforded me the moft fatisfac- 

 tory fanftion to this divifion. It is thus that the chemift 

 and the naturalift, by freely uniting their labours, without 

 jealoufy or prejudice, ought in all cafes to proceed, in order 

 to attain that certainty which is the dofirable recompenfe of 

 their endeavours, 



Firft Species. — Arfemute of Copper in the Form of an ohtufe 

 O£l(iedron, 



The moft fimple form under which this fpecies appears, i» 

 a very obtnfe odaedron, formed by the united bafes of two 

 tetraedval pyramids, with ifofceles triangular planes ; and 

 this appears to be its original form. This oftaedron has, in 

 each of its pyramids, two oppofite planes more inclined than 

 the other two; which gives a parallelogrammic form to their 

 common bafe. '^Fig. i. Plate I.) The two planes more in 

 dined than the others, meet at the apex of each of the py- 

 ramids, in an angle of 13 /; and at the common bafe, in one 

 of 50^. The two planes which are lefs inclined, meet at the 

 apex, in an angle of j 15°; and at the bafe, in one of 65°. 



4 Thefe 



