of the plane of polarization by solutions. 
317 
°^ = N/3[v{C+A-(B + C)} te*-+p{B + C-(B + A)} Xey' 
+ X{A+B-(A+G)} 
= N-/3 {\2ex' ( B - G ) + yley' (G - A) 
v / / a dm (dZ dY\ 
+ „Zez(A-B)\(^-- SF y 
where N is the number of systems per unit volume and /3 the 
mean value of m 1 2 /i 3 2 or m£n? for a uniform distribution of axes. 
dZ dY 
Thus, if the coefficient of ^ in the preceding expression 
does not vanish, a substance formed of such atoms as we have 
been considering will rotate the plane of polarization. Unless 
the atom possesses a considerable amount of want of symmetry 
the coefficient will vanish. Thus, if the axes of x', f , z which, 
by definition, are principal axes for the electrical charges are also 
principal axes for the masses X, y, v will all vanish and there will 
be no rotation. Again, if the ‘electrical centre’ of the negative 
charges coincides with that of the positive charges, 
'Zex — Sey' = Xez' = 0, 
and again there is no rotation, and if A = B = G, i.e. if the principal 
‘ electrical moments of inertia’ are equal, the coefficient again 
vanishes. 
If the charges were in one plane, then taking this as the 
plane of z' we have Xez' = 0, and since L and M vanish we see 
that X = p — 0, hence the coefficient vanishes and there is no 
rotation. 
21—3 
