Electron Theory of Chemistry to Solids. 755 



other lines parallel to PQR; the displacements o£ electrons in 

 the same vertical line are equal, but the displacement of any 

 electron is of opposite sign to those of its three nearest 

 neighbours on a horizontal plane. 



The force due to the other electrons tending to increase p, 

 the vertical displacement of P is 



due to the electrons QRS, 



due to the electrons Q'R'S', 



pins 4^/1 _1 + JL + - 3 ( ] + J^ + \\ 

 a 3 \2 3 2 ^ 1 QW 26*!* ^ '" \2 5 >" 2 10 5 ' 2 /; 



= — r —„ x *ob8, 

 a° 



due to the electrons Q"R"S", 

 plus 2pe*/^ (1 I 1 v /l /J 



O 3 \v3;2 + ^ 6 3,2-i- l8 3/2-t- 38 3 / 2-+----; ^^5/2 + ^gft/8 



due to the electrons Q"'R"'S"\ 

 Taking these together, the force tending to increase p is 



i- 5-950. 



pe* 



Now consider the forces due to the positive atoms tending 

 o diminish p ; this is 



/.+ •■;. 



, E*( 1 1 1 



^P^l 3 3,,2+ 11 3,2-t- 27 3;2 



- 3 (S572 + Ir572- + 2W 2+ "/j 



, E^.cf 1 1 1 ,/l 1 \1 



Since E = 4? and c'a='58, this equals 



pe 2 



8 x 4-28. 

 a 3 02 



