1891.] Professor G. D. Liveing on Crystallisation. 375 



WEEKLY EVENING MEETING, 

 Friday, May 15, 1891. 



Sir James Criohton Browne, M.D. LL.D. F.R.S. Treasurer and 

 Vice-President, in the Chair. 



Professor G. D. Liveing, M.A. F.R.S. 



Crystallisation. 



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

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

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

 at the power of nature which causes substances in passing from the 

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

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

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

 and others, like blue vitriol, in shapes which are regularly oblique. 



It is this power of nature which will be the subject of this discourse. 

 I hope to show that crystalline forms with all their regularity and 

 symmetry are the outcome of accepted mechanical principles. I shall 

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

 with in other natural phenomena. In fact, I shall call in only the 

 same force that produces 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 is different from gravity I shall not stop to 

 inquire. Any attractive force which for small masses, such as we 

 suppose the molecules of matter to be, is only sensible at insensible 

 distances, is sufficient for my purpose. 



We know that the external form of a crystal is intimately con- 

 nected with its internal structure, with the chemical nature, the 

 arrangement and the motions, of the molecules. This internal struc- 

 ture betrays itself in the cleavages with which every one is familiar 

 in mica and selenite, which extend to the minutest parts, so that when 

 calc-spar is crushed, even the dust consists of tiny rhombs. It is 

 still better seen in the optical characters. The regular crystals, like 

 common salt, give no double refraction, while those less regular refract 

 doubly, and indicate different degrees of symmetry by their action on 

 polarised light. These familiar facts suggest that it is the internal 

 structure which determines the external form. 



As a starting-point for considering that structure I assume that 

 crystals are made up of molecules, and that in the solid state the 

 molecules have little freedom ; that they are always within the range 

 of each other's influence, and cannot change their relative places. 

 Nevertheless, these molecules must be in constant and very rapid 

 motion. Not only will they communicate heat to colder bodies which 

 touch them, but they are always radiating, which means that they are 



2 c 2 



