162 On the Crystalline Forms of Artificial Sails. [March, 



There is a bright cleavage parallel to the edge between M and 

 M' and perpendicular to A, and consequently parallel to the great 

 diagonals of the primary terminal planes. The crystals may be 

 also cleaved parallel to A, and to M and M'. The figure, it has 

 been observed, is symmetrically drawn, but the crystals are 

 frequently much distorted by the disproportionate extension of 

 some planes and the disappearance of others ; so that it is not 

 always easy to compare them with any general type of the 

 whole. We may, however, be much assisted in this comparison 

 by the character of the plane //, which, in all the crystals I have 

 seen, is striated, as shown in the figure. By the assistance of 

 this plane, and the bright cleavage plane perpendicular to it, and 

 by measurement, we may be enabled to compare the crystals 

 with the enoraved figure, however irregularly formed they may 

 be. On very many of them the planes b and b'" are so much 

 enlarged as nearly, if not entirely, to exclude the other four 

 plane> which appear in front of the figure; while at the back, 

 the planes parallel to b' and b" are similarly enlarged. If these 

 four planes were so much extended as to exclude all the others 

 which appear on the figure, an irregular tetrahedron would 

 result. 



On some crystals there is one and sometimes two planes 

 replacing the edge between A and M, one of them measuring 

 with A about 156°, and the other about 145^°. 



titrate of Silver. 

 Primary form a right rhombic prism. 



Pond 116° 36' 



Mond 148 



MonM' 129 31 



d on d' 126 48 



On some crystals received from Mr. 

 Tesch^macher the planes d were barely 

 visible ; while on others from Mr. Cooper, 

 those planes encroached so much on M 

 and M' as to leave only minute portions 

 of these visible. 



