202 THE FORMS OF CELLS [cii. 



what they termed "intussusception." The contrast is true, 

 rather, of solid as compared with jelly-Hke bodies of all kinds, 

 living or dead, the great majority of which as it so happens, but 

 by no means all, are of organic origin. 



A crystal "grows" by deposition of new molecules, one by 

 one and layer by layer, superimposed or aggregated upon the 

 solid substratum already formed. Each particle would seem to 

 be influenced, practically speaking, only by the particles in its 

 immediate neighbourhood, and to be in a state of freedom and 

 independence from the influence, either direct or indirect, of its 

 remoter neighbours. As Lord Kelvin and others have explained 

 the formation and the resulting forms of crystals, so we beheve 

 that each added particle takes up its position in relation to its 

 immediate neighbours already arranged, generally in the holes and 

 corners that their arrangement leaves, and in closest contact with 

 the greatest number*. And hence we may repeat or imitate this 

 process of arrangement, with great or apparently even with 

 precise accuracy (in the case of the simpler crystalline systems), 

 by piling up spherical pills or grains of shot. Li so doing, we must 

 have regard to the fact that each particle must drop into the 

 place where it can go most easily, or where no easier place offers. 

 In more technical language, each particle is free to take up, and 

 does take up, its position of least potential energy relative to those 

 already deposited; in other words, for each particle motion is 

 induced until the energy of the system is so distributed that no 

 tendency or resultant force remains to move it more. The 

 application of this principle has been shewn to lead to the produc- 

 tion of planes -f (in all cases where by the limitation of material, 

 surfaces must occur) ; and where we have planes, straight edges 

 and solid angles must obviously also occur ; and, if equilibrium is 



* Cf. Kelvin, On the Molecular Tactics of a Crystal, The Boyle Lecture, Oxford, 

 1893, Baltimore Lectures, 1904, pp. 612-642. Here Kelvin was mainly following 

 Bravais's (tod Frankenheim's) theory of "space-lattices," but he had been largely 

 anticipated by the crystallographers. For an account of the development of the 

 subject in modern crystallography, by Sohncke, von Fedorow, Schonfiiess, Barlow 

 and others, see Tutton's Crystallography, chap, ix, pp. 118-134, 1911. 



f In a homogeneous crystalline arrangement, symmetry compels a locus of one 

 property to be a plane or set of planes; the locus in this case being that of least 

 surface potential energy. 



