3 74 Count de Bournon’s Description of 
in which these two substances are combined. I have been na- 
turally led to follow the same order, and to divide the arseniates 
of copper into four different species : and the very interesting 
analysis of this substance, made by Mr. Chenevix, has afforded 
me the most satisfactory sanction to this division. It is thus 
that the chemist and the naturalist, by freely uniting their 
labours, without jealousy or prejudice, ought in all cases to 
proceed, in order to attain that certainty which is the desirable 
recompense of their endeavours. 
First Species. Arseniate of Copper in the form of an obtuse 
octaedron. 
The most simple form under which this species appears, is a 
very obtuse octaedron, formed by the united bases of two tetrae- 
dral pyramids, with isosceles triangular planes ; and this ap- 
pears to be its original form. This octaedron has, in each of 
its pyramids, two opposite planes more inclined than the other 
two; which gives a parallelogrammic form to their common base. 
(Fig. l. Plate XIII.) The two planes more inclined than the 
others, meet at the apex of each of the pyramids, in an angle 
of igo ° ; and at the common base, in one of 50°. The two 
planes which are less inclined, meet at the apex, in an angle of 
115 0 ; and at the base, in one of 65°. 
These planes are commonly .smooth and bright ; sometimes, 
however, they are striated, in a direction parallel to their edges. 
The four planes seldom terminate in one and the same point : 
more commonly the apex is formed into a ridge, the octaedron 
being lengthened, parallel to its less inclined planes ; the base is 
then a square, or at least approaches very nearly to that form. 
(Fig. 2.) 
