the Arseniates of Copper , and of Iron. 185 
they are inclined. When these new planes have acquired an 
increase of sufficient extent to make the primitive planes of 
the prism totally disappear, and to replace them, the crystal is 
changed to a triedral pyramid with a truncated apex ; the base 
and truncated apex of which are equilateral triangles. (Fig. 21 .) 
When it happens that the crystal has, at the same time, gone 
through this modification and that which replaces the solid 
angles of its other extremity, and these two modifications 
have commenced at the very origin of the formation of the 
crystal, there is a particular period of its progress, in which the 
crystal is lengthened into a hexaedral prism, with acute trian- 
gular isosceles planes, having for their bases two equilateral 
triangular planes, perfectly equal. (Fig. 22.) After this period, 
if the act of crystallization continues, the crystal assumes the 
appearance of an extremely acute rhomboid, the acute solid 
angles of which are replaced, more or less completely, by an 
equilateral triangular plane; (Fig. 23,) and finishes at last by 
taking the form of a perfect rhomboid. (Fig. 24.) 
All these varieties, though less common than those of the 
first modification, are yet frequently to be met with, except- 
ing that of Fig. 22, which is extremely rare, and of which I 
have seen only two or three crystals ; in general, however, 
the crystals of these varieties are very small, and their form 
cannot be well seen without the assistance of a magnifying 
glass. 
It frequently happens, that two of the elongated triedral prisms 
(Fig. 10,) are closely united to each other, by one of the sides 
of the prism; whence results a kind of made, (Fig. 25,) the 
form of which is a rhomboidal tetraedral prism, of 6 o° and 
120 0 ; but there is always discernible, on the terminal sur- 
MDCCCI. B b 
