110 Mr. R. T. Forster on the 



regular succession of the particles. He however failed in account- 

 ing for the formation of the ordinary octahedron^ as such a form 

 could not result unless from a mixture of little tetrahedrons and 

 octahedrons, or unless the tetrahedrons were united by their 

 corners, leaving spaces between them. The necessity for such 

 an arrangement so unaccountable and so totally at variance with 

 the rest of his theory was evidently subversive of it. He more- 

 over does not attempt to explain why the decrements that we 

 have spoken of take place, nor does it seem possible to do so on 

 his hypothesis. He seems to have begun at the wrong end, and 

 instead of investigating how certain particles could unite to form 

 certain crystals, he contented himself with showing that such 

 crystals might be formed by piling up the molecules in a par- 

 ticular manner. 



The last and most successful writer is Mr. Dana, the American 

 mineralogist, who first published his views on the subject in the 

 year 1836. 



He considers the molecules to be spherical^ and possessed of 

 sLx poles, each 90° distant from the nearest four; three adjacent 

 poles being of one denomination and three of another, those of 

 a like sort repelling, and those of an unlike attracting each other. 

 By means of this hypothesis he very satisfactorily explained the 

 cubical formation, and pointed out, like Haiiy, how the many 

 secondaries can be obtained from the cube by certain decrements 

 at the edges or angles. He also accounted for the formation of 

 twin crystals, by showing how two molecules may accidentally 

 unite at a point of equilibrium between two or three poles. He 

 supposed the molecules to be in the first system spheres ; in the 

 second spheroids ; in the third spheroids, the poles being at the 

 extremities of equal conjugate diameters; in the fourth ellipsoids ; 

 in the fifth the same, the poles being at the extremities of axes, 

 two of which are inclined to each other ; and in the sixth the 

 same, all the axes of the molecules being inclined. He, however, 

 does not in the least account for the formation of secondary 

 planes (although he wishes us to suppose so), nor of the occur- 

 rence of hemihedral forms. In this respect all writers on the 

 subject have erred in not entering sufficiently fully on the inves- 

 tigation; either contenting themselves, like Haiiy, with consi- 

 dering how certain solids could be built up to form other solids, 

 or, like Dana, merely observing how molecules, if possessed of 

 certain properties, will unite after certain laws. 



The problem, as it appears to me, is of a much more general 

 nature ; we have to consider a number of molecules to which we 

 have given certain properties, and we have to investigate what 

 forms they will assume, and what modifying influences may come 

 into play dui'ing the process. We cannot expect, that, after 



