VII.] CRYSTALLIZATION. 81 



The hexagonal theory has the advantage of making the 

 axis of symmetry, to which the optic axis also is parallel, 

 to be a crystallographic axis.^ But I believe the rhombo- 

 hedral theory to be the sound one : partly because it 

 makes no exception to the law that every crystal has three 

 axes, and partly because, by referring the cube and the 

 rhombohedron to homologous axes, it recognises the homo- 

 logy between the cubic and the rhombohedral systems, 

 and the possibiKty of gradational forms between the two ; 

 while the hexagonal theory obscures these truths, by re- 

 ferring the cube and the rhombohedron to axes which are 

 not homologous. 



Among organisms the true affinities and homologies Affinities, 

 are not always the apparent ones : one organism may often aOT^rent 

 be said to be disguised in the likeness of another. The 

 whales, for instance, are mammals in the likeness of fishes: 

 the chitons are molluscs in the likeness of annelids. The 

 same occurs in at least one known instance among crystals. 

 The crystals of Herschellite have the form of hexagonal Heisdiei- 

 prisms, which were formerly referred without hesitation 

 to the hexagonal or rhombohedral system. But it has 

 been found by examination with polarised light, by which 

 the optical properties of crystals are tested, that the 

 hexagonal crystal of Herschellite really consists of six 

 triangular crystals, belonging to the right prismatic system ; 

 though they are not recognisable as such by their form, 

 which is altered in consequence of their being thus 

 united.2 



This brings us to the curious and beautiful subject 

 of compound crystals. These have been compared to Compound 

 animal monstrosities; but this is an error, or rather a^^'^^ ■ 

 stupidity, which has arisen from comparing crystals with 

 the higher organisms instead of with the lower ones. The 

 true analogy of such crystals as Herschellite is to flowers. 



I prefer, however, to call it cubic, and so to keep before the mind's eye 

 the homology betwen the axes of the cubic, rhombohedral, and prismatic 

 systems. 



1 See p. 71 for the meaning of this expression. 



^ Dr. Von Lang in Philosophical Magazine, December 1864. 



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