CRYSTALLIZATION.* 



By G. D. LiVEiNG, F. E. S. 



There is something very fascinating about crystals. It is not merely 

 the intrinsic beauty of their forms, their picturesque grouping, and 

 the phiy of light upon their faces, but there is a feeling of wonder at 

 the power of nature, which causes substances, in ])assing from the 

 fluid to the solid state, to assume regular shapes bounded by plane 

 faces, each substance with its own set of forms, and its faces arranged 

 with characteristic symmetry; some, like alum, in perfect octahedra; 

 others, like blue vitriol, in shapes winch are regularly oblique. It is 

 this power of nature which is the subject of this discourse. I hoi^e to 

 show that crystalline forms, with all their regularity and synmietry, 

 are the outcome of the accepted princii)les of mechanics. I shall 

 invoke no peculiar force, but only such as we are already familiar with 

 in other facts of nature. I shall (;all in only the same force that ])ro- 

 duces the rise of a liquid in a capillary tube and the surface-tension at 

 the boundary of two substances which do not mix. Whether this 

 force bo different from gravity I need not stop to inquire, for any 

 attractive force which for small masses, such as we sup])ose the mole- 

 cules of matter to be, is only sensible at insensible distances is suffi- 

 cient for my purpose. 



We know that the external forms of crystals are intimately connected 

 with their internal structure. This is betrayed by the cleavages with 

 which ill mica and seleuite everybody is familiar, and which extend to 

 the minutest parts, as is seen in the tiny rhombs which form the dust 

 of cruslicd calcite. It is better marked by the optical properties, single 

 and double refraction, and the effects of crystals on polarized light. 

 These familiar facts lead up to the thought that it is really the internal 

 structure which determines the external form. As a starting-])oint for 

 considering that structure, I assume that crystalline matter is made 

 up of molexudes, ami tliat, whereas in the fluid state the molecules 

 move about amongst themselves, in the solid state they have little 

 freedom. They are always Avithin the range of each other's influence- 

 and do not change their relative places. Nevertheless, these mole, 

 cules are in constant and very rapid motion. Xot only will they com- 

 municate heat to colder bodies in contact with them, l)ut they are 



*A discourse delivered at the Roynl Institition of Great Britain on Friday, May 

 15, 1891,— From Xature, June 18, 1891; vol. xi,iv, pp. 156-160. 



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