﻿the Electron Theory of Solids. 661 



Hence the electrostatic potential energy for the atom and 



e 2 

 its four associated electrons is 21*15 -r. 



Since there are eight atoms in the cube whose edge is 2d. 

 if A is the density of the diamond and M the mass of a 

 carbon atom, 



8M A 



1 /A\ 1/3 



d = \m) ■ 



Thus the electrostatic potential energy per one atom and 

 four electrons is 



/A\ 13 



21 ' 15 <m) • 



and the energy per unit volume is 



/A\ 4 / 3 

 21-15<*( M ) • 



Hence, by page 736 of the former paper, k the bulk 

 modulus of the diamond is given by the equation 



KM.) 



for the diamond A = 3-52, M=12 X 1'64 x 10" 24 ; hence 

 & = 5-6 x 10 12 , l/jfe=-178 x 10~ 12 . 



This value for 1//- is much less than that, '5 X 10 " l2 , found 

 by Richards. It is, however, in close agreement with 

 *16 x 10~ 12 , the value recently found by Adams (Washington 

 Acad. Sc. xi. p. 45, 1921). 



The properties of solids formed by elements whose atoms 

 have more than four disposable electrons are quite different 

 from those of solids formed by the elements with one, two, 

 or three disposable electrons. The latter are, with the 

 exception of boron, metallic and good conductors of 

 electricity and heat. The former, for instance sulphur and 

 phosphorus, are insulators. Not only do they insulate in the 

 solid state, but they do so after they are fused. They differ 

 in this respect from solid salts which, though they may 

 insulate when in the solid state, generally conduct when 

 molted. This suggests that in the salts there are positively 

 and negatively electrified systems which are fixed when the 

 substance is in the solid state, but can move about when it 

 is liquefied. In such elements as sulphur or phosphorus 

 there does not seem to be any evidence of the existence of 



7 21-15 9 /A\ 4/3 



