﻿Molecular Structure of Metals. 



259 



Iron and most of the useful metals crystallize in the cubic 

 system ; for simplicity we may limit what has to be said to 

 them. Imagine a molecule possessing polarity equally in 

 three directions, defined by rectangular axe-. TVe need not 

 for the present purpose inquire to what the polarity along the 

 axes is due ; it will suffice to assume that the molecule has 

 six poles, three positive and three negative, and that the>e 

 repel the like and attract the unlike poles of other molecules. 

 We may make a model by using three magnetized rods fixed 

 at right angles to one another at their middle points. I imagine, 

 further, that the molecule has an envelope 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 a\<>< 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 of force exerted by the poles of A. 

 In other words, we have this formation : — 



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: — 



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

 of polavitv are free to turn as a rigid system. 



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