142 
MR. A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 
neglecting the term a t . AH 7 /H. since a t = l/3. For the crystals, at the same tempe¬ 
rature, we have 
Hence 
Xc = - 
X C = X, 
Ne 2 A 
4 m 
AH. 
1 + a c • JJ 
1 + a c • JJ 
AH, 
or 
X c • H* = x? • [H 2 +a c . AHJ. 
I shall call a c . AH 7 the mean molecular field of the diamagnetic crystals. Now for a 
given change of the electron orbits due to crystallization, AH. will he proportional to H,. 
Hence we may neglect the mutual forces between the diamagnetic molecules when the 
substance crystallizes providing we supply a molecular field which is proportional to 
H 7 , and therefore to AM Z , or to the intensity of magnetization of the substance 
(equation (6)). For the liquid and gelatinous states this molecular field is negligibly 
small, because ct l is small, but for the crystalline state a c is large, and the molecular 
field is comparable with the external field H.. 
This is exactly analogous to the theory of ferro-magnetism developed by M. Weiss, 
in which the molecular field is Nl, where I is the magnetic moment produced by the 
external field and N is a constant determined by the intermolecular forces of the 
crystalline ferro-magnetic material. N corresponds to the polarisation constant a c of 
the diamagnetic theory. We ought, however, to compare N with a/, where the 
molecular field 
a c . AH. = a'. N . AM.p, 
P, the density of the substance, is of the order 1 . 
If a c = 10 2 , this gives (since AH. = H./2 x 10 3 = N . AM./x x 2 x 10 3 ) 
L . ■ _ a c ' = a c /x x 2 x 10 3 . 
Assuming y = 5x 1CU‘, we find aj = 10 3 a c . Hence the constant of the diamagnetic 
field is 10 5 . The corresponding values of N given by Weiss and Beck # are 0‘38 x l(fi 
for iron, 1'27 x 10 4 for nickel, and 3'32x 10 4 for magnetite. The mean molecular fields 
are enormously greater in the cases of ferro-magnetic substances on account of the 
large value of the intensity of magnetization as compared with that for diamagnetic 
substances. The values of N! (the saturation molecular field) are 6, 530, 000 gauss for 
iron, 6, 350, 000 gauss for nickel, and 14, 300, 000 gauss for magnetite, while the 
corresponding value of a/. AM. for a diamagnetic substance is of the order 150 gauss 
for an applied field of 3000 gauss, the field used in the above experiments. 
To account for the phenomena of the Zeeman effect, magnetic rotation of the plane 
of polarisation, and diamagnetism, it is necessary to assume that an atom contains 
* ‘ Journ. de Phys.,’ ser. IV., vol. 7, p. 249, 1908. 
