the Arseniates of Copper , and of Iron. 183 
nated, at that extremity, by a ridge. (Fig. 17.) In this variety, 
the crystal is often seen placed upon one of its scalene triangular 
sides, and then presents the other at its upper extremity ; an ap- 
pearance which is apt to puzzle the observer, particularly when 
he perceives, among the triangular sides which most generally 
terminate the crystals, nothing but equilateral triangles. This 
modification, in all its forms, is much less common than the 
preceding one ; it is, however, occasionally met with. 
The modification which we have just seen the primitive crys- 
tal assume at one of its solid angles, and only on one side, 
sometimes takes place also, (only on one side,) at its two other 
angles. Then, if the act of crystallization has continued so 
long, under the same mode of increase, that the new edges, 
as A B, (Fig. 15,) produced by the replacing of the solid 
angles, unite together, and give birth to a new equilateral tri- 
angle, placed in a direction contrary to the primitive one ; and 
if, at the same time, the crystal has such a length that the 
secondary planes terminate at the opposite base, and are very 
acute isosceles triangles, the crystal will present the appear- 
ance of a kind of truncated hexaedral pyramid, the base and 
apex of which will be equilateral triangles. (Fig. 18.) The six 
triangular planes which compose the pyramid of this crystal, are 
always acute isosceles triangles ; but three of them have their 
acute angle much smaller than the three others. The sides 
of the base of this kind of pyramid are opposite to the least 
acute angles ; and its truncated apex is opposite to the most 
acute ones ; the triangular planes being placed alternately in 
an opposite direction. I have seen several instances of this 
form ; but I never saw such intermediate varieties as the 
