1904 - 5 .] Magnetic Quality in Molecular Assemblages. 1053 
the effect of self-demagnetisation, amounted to about 20°. And 
the theoretical results, as exhibited in fig. 7, indicate a possible 
obliquity of about 31°, which is subject to considerable reduction 
because of the magnetisation I 0 . Thus the internal field is quite 
comparable with the external field, and is therefore still more 
comparable with — indeed, may possibly exceed — the field which is 
due to the existence of boundaries. 
20. Estimate of Size of Molecular Magnets and of Molecular 
Susceptibility . — We can now test directly the validity of the 
assumption (§ 2) that the semi-length of a molecular magnet bears 
to the average distance between molecules a ratio which is of such 
magnitude that terms, involving the fourth and higher powers, 
in the expressions for the components of force, can be neglected. 
The magnitude of the transverse component of force in the 
diagonal plane is (§ 1 4) given by 
If we take 16° as the value of 0 corresponding to maximum 
obliquity between the external field and the direction of magnetisa- 
tion, we get F = 0*237B. And if we take Weiss’s value, 20°, for 
the angle of maximum obliquity, we find that the external field, 
H, is practically three times the internal transverse component. 
by the formula of § 12 and the data of § 22. 
Now, in Weiss’s observations, the value of H was 353 c.g.s. 
units, and the value of the magnetisation, I, may be taken about 
400 c.g.s. units. So, since in this closest-packed homogeneous 
arrangement of centres the number of molecules per unit volume is 
J2 /p 3 , we get M N /2/p 3 = I, and p 2 /a 2 = 21. This gives practically 
p = 4*6a. The first of the neglected terms, though appreciable, could 
not make any fundamental change in the results. The absolute 
value obtained for the ratio is probably of little importance ; 
consistency of order is all that can be looked for. 
F = JB sin 20(3 cos 2 0 - 1) . 
Thus 
H = 0*712 B 
