133 On the Lazo of Symmetry^ 



form than the octohedron. And, in like manner, other species 

 of minerals, of which there are about a dozen having the same 

 cleavage form, are never found crystallised as rhomboids or prisms, 

 or as any forms whatever, which are more dissimilar to spheres 

 than the octohedron ; but, on the other hand, their angles and 

 edges, (those parts the presence of which make them to differ 

 from spheres), are often thrown off. So exquisitely, indeed, is 

 this change effected in some instances, that Mr Phillips informs 

 us that he possesses a crystal of fluate of lime, whose cleavage 

 form is of course bounded only by eight planes ; but whose ac- 

 tual form is so bevelled, and bevelled again, that were the crys- 

 tal complete, it would be bounded by no fewer than 322 planes, 

 (Phillips"* Mineralogy, p. 170). In like manner, when the 

 cleavage form is the cube, as is the case in about another dozen 

 of mineral species, analogous truncations and bevelments of an- 

 gles and edges may be observed. And even where the cleavage 

 form is the dodecahaedron, as is the case with nine or ten species, 

 though that form itself possesses much of the contour of the 

 sphere, still replacements of the salient parts by planes, are often 

 to be observed, all of them consisting in so many approxima- 

 tions to a spherical superficies. 



Now, while it thus appears that the actual forms of such crys- 

 tals as have tessular cleavages, do never depart farther from the 

 sphere than their cleavage forms, but, on the other hand, usu- 

 ally approximate more nearly to it ; with regard to those whose 

 cleavage forms are not truly tessular, it may be remarked, that 

 xheir actual forms do not unfrequently simulate the tessular as- 

 pect, thus shewing, even in still more adverse circumstances, a 

 conatus towards a spherical contour. Such a phenomenon may, 

 for instance, be often observed in vesuvian, oxide of tin, tung- 

 ■state of lime, phosphate of titanium, meUite, and others, nature 

 being sometimes able to produce this effect only having recourse 

 to the process of hemitrope, as in carbonate of lead, chabasie, 

 &C. In other minerals, also, whose cleavage forms are most 

 diverse from tessular, such as bipyramids, rhomboids and 

 prisms, the same phenomena may constantly be observed^ name- 

 ly, edges and angles subjected to replacement by planes, so 

 that the most salient parts of the figure may be cut off, and the 

 actual forms reduced to a nearer coincidence with that which 



