﻿660 Sir J. J. Thomson : Further Studies on 



Compressibility of the Diamond. 



In the diamond we have a quadrivalent element crystal- 

 lizing in the regular system. The arrangement of the carbon 

 atom in the diamond has been shown by Sir W. H. Bragg 

 and Professor W. L. Bragg to be given by the following 

 scheme. They occupy 



a, the corners of a cube ; 



b, the centres of its faces ; 



c, 4 of the centres of the 8 cubes into which the large 



cube is divided by planes bisecting its sides at right 

 angles. 



We shall take this cube as our unit ; it contains eight 

 carbon atoms. Since carbon is quadrivalent, it must contain 

 32 electrons ; these electrons will be situated 



a, at the middle points of the edges of the cubical unit: 



this accounts for 3 ; 



b, at the centres of each of the faces of the 8 small cubes : 



this accounts for 24 ; 



c, at the centres of the four small cubes not occupied by 



the carbon atoms : this accounts for 4 ; 



d, one at the centre of the large cube. 



Making use of this unit, we can calculate the electrostatic 

 potential energy due to the charges on the atoms and 

 electrons. Let E be the charge on a carbon atom, e that on 

 an electron. 



The electrostatic potential energy of a carbon atom 



ly r ) 



I find to be equal to 



^(149-346. £-35-13. E), 

 Z Oj 



where 2d is the side of a unit cube. Since E =4#, this reduces 



to 



e 2 

 17-65. ^. 



d 



The electrostatic potential energy of an electron I find to 

 be 



PU=r 



\ e - i?. 149-346 -147-59 e\ 



2 d L4 J 



1 e 2 



