750 Sir J. J. Thomson on the Application of the 



.atom acts as a separate and independent unit. There is con- 

 siderable evidence, however, that the atoms in the diamond 

 are arranged in groups. If: so, it may be necessary to take 

 the group as the unit rather than the atom ; this would 

 indicate that when the diamond is compressed the com- 

 pression is effected more by bringing the groups together 

 than by diminishing the distance between the atoms in a 

 group. This grouping of the atoms would seriously modify 

 the expression for the bulk modulus, for we have to replace 

 A/M by A/nM, where n is the number of atoms in a group. 

 This will diminish the value of k in the proportion of 

 1 to n 4/3 : thus if there were three carbon atoms grouped 

 together in the diamond, k would be 8'l/3 4 or 1*9 X 10 12 ; 

 if there were four the value t'3xl0 12 , values which are 

 much more consistent with Richard's results than that 

 deduced on the assumption that the atoms were not 

 grouped. 



A point of considerable interest with regard to the 

 constitution of salts is raised by the value of the com- 

 pressibility for electrons arranged in a spaceTcentred cube ; 

 it all the movable electrons go into the walls of the cell 

 the space-centred cube would be the cells enclosing the 

 atoms in a binary valency compound, since in these 

 compounds there are eight electrons for the two atoms. 

 The value we have obtained for k shows that if this were 

 the case these compounds would be characterized by small 

 compressibilit}' and consequently high boiling-points : the 

 oxides answer to this description, the chlorides certainly 

 •do not. Though the k has been determined for few simple 

 ^compounds, its value for NaCl and for KC1 has been deter- 

 mined by Voigt, Rontgen, and Richards. The results are 

 oiven in the following table ; the last column gives the 

 walue calculated by the preceding formula. 





A. 



k observed. 



k calculated. 



NaCl.... 



.. 2-1 



•23 xl() 12 



1-2 xlO 12 



KCl .... 



.. 1-99 



•192 x 10 12 



•76xl0 12 



It will be noticed that the values for the chlorides 

 indicated by the theory are far in excess of those found 

 iby experiment. 



The actual values are comparable with those which would be 

 .obtained from the formula (8) on p. 736, which corresponds 

 to the case of simple cubical cells with a single positive 

 charge at the centre of each. This suggests that the 

 -structure of the chloride cells is not a face-centred cube 



