660 



TRANSACTIONS OF SECTION G. 



molecule lias an euvelope in the shape of a sphere, which touches the spherical 

 envelopes of its neighbours, and assume that these spheres may turn on one 

 another without friction.^ 



Think now of the process of crystal-building with a supply of such spherical 

 molecules for brickbats. Starting with one molecule, let a second be brought up 

 to it and allowed to take up its place under the action of the polar forces. It will 

 have a position of stability when a positive pole in molecule A touches (or lies 

 in juxtaposition to) a negative pole in molecule B, with the corresponding axes 

 in line, and when the further condition is satisfied that the axes in molecule B 

 whose poles are not touched by A are stably situated with respect to the field oi 

 force exerted by the poles of A. 



In other words, we have this formation : — 



Fig. 1. 



B 



A B 



For convenience of representation in the diagram the poles are distinguished by 

 the letters N. and S., but it must not be assumed that the polarities with which 

 we are here concerned have anything to do with magnetism. 



Suppose, now, that the crystal is built up by the arrival of other molecules, 

 each of which in its turn assumes the position of maximum stability consistent 

 with formation in cubic or normal piling. The group in that case takes an 

 arrangement which is essentially a repetition of this quartette : — 



Fig. 2. 



Alono- each row the polarity pi-eserves the same direction, but the polarity oi 

 each row is opposite to that of each contiguous parallel row. This description 

 applies equally to all three axes. The whole group (fig. 3) consists of the 

 quartettes of tig. 2 piled alongside of and also on top of one another. In this way 

 we arrive at what I take to ba the simplest possible type of cubic crystal. 



In this grouping each molecule has the alignment giving maximum stability, 

 and it seems fair to assume that it will take that alignment when the crystal 

 grain is formed under conditions of complete freedom, as in solidifying from the 



' Or, let the envelope be a shell of any form, inside of which the axes of polarity 

 are free to turn as a rigid system, 



