﻿260 



Dr. J. A. Ewing on the 

 Fisr. 2. 



Along each row the polarity preserves the same direction,, 

 but the polarity of each row is opposite to that o£ each conti- 

 guous parallel row. This description applies equally to all 

 three axes. The whole group (fig. 3) consists of the quartettes 

 of fig. 2 piled alongside of and also on top of one another. In 

 this way we arrive at what I take to be 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 

 liquid state. As a rule the actual process of crystal-building 

 goes on dendritically ; branches shoot out, and from them 

 other branches proceed at right angles, leaving interstices to 

 be filled in later. We have, therefore, to conceive of the 

 molecules as piling themselves preferably in rows rather than 

 in blocks, though ultimately the block form is arrived at. In 

 this position of maximum stability each molecule has its six 

 poles touching poles of contrary name. 



Now comes a point of particular importance. Imagine two 

 neighbouring molecules in the same block to be turned round, 

 each through one right angle, in opposite senses. They will 

 now each have five poles touching five poles of contrary name, 

 but the sixth pole will touch a pole of the same name as itself. 

 They are still stably situated, bat much less stably than in the 

 original configuration, and they will revert to that confi- 

 guration if set swinging through an angle sufficient to exceed 

 the limited range within which they are stable in the new 

 position. 



Similarly we may imagine a group of three, four, or more 

 molecules, each to be turned through a right angle, thereby 

 constituting a small group with more or less stability, but 

 always with less than would be found if the normal configu- 

 ration had been preserved. The little group in question may 

 be made up of molecules in a row, or it may be a quartette or 



