268 MAJOR A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



explanation, I think, is to be found in the finite though small angular oscillations 

 which constitute a -portion of the thermal energy of the molecules. The molecules 

 are fixed relative to one another and form a definite space lattice but they are 

 oscillating with small amplitude under the molecular field. This allows them to 

 retain a finite susceptibility. Suppose we could double the molecular field, the 

 limiting susceptibility would become one-half its former value and the saturation 

 intensity of magnetization would be slightly increased. If we could increase the 

 molecular field indefinitely, the susceptibility would get indefinitely small, the product 

 of the two however tending to a finite limit equal to the true saturation intensity of 

 magnetization. The amplitude of the molecular oscillations would, under the 

 influence of this indefinitely large forcive, be indefinitely small. This state might be 

 attained in a practical manner by cooling the substance, say in liquid hydrogen, when 

 the limiting susceptibility would become vanishingly small. 



As N is the reciprocal of the limiting susceptibility the constant of the molecular 

 field will become indefinitely large. WEISS, however, supposes N to be constant.* 

 The tendency of XL to approach a small limiting value as the temperature is lowered 

 is confirmed experimentally for ferro-magnetic substancest and is particularly noticeable 

 in the case of weak magnetic fields. The reduction of the amplitude of vibration of 

 the molecules as the absolute zero is approached merely implies a higher frequency of 

 angular oscillation under the increasing molecular field and does not necessarily imply 

 that the rotational energy becomes vanishingly small. In this case it should be noted 

 that the saturation intensity of magnetization we are considering is smaller than that 

 which would be given by the simple summation of all the magnetic moments of the 

 molecules in unit volume. In other words, the difficulty of producing this latter 

 saturation by an external field becomes increasingly difficult on account of the larger 

 molecular forcive at low temperatures, in agreement with the vanishingly small 

 susceptibility referred to above. At higher temperatures the susceptibility to an 

 external field is far greater ; the molecules are, as it were, helped over their difficulties 

 with respect to the molecular field, when the external field is applied, by the increased 

 energy of the rotational oscillations, and having passed this critical point they are held 

 in new combinations. Beyond the critical point the molecular state is chaotic, the 

 molecules being interlocked (cf. p. 265), and the external field has sufficient control to 

 produce a paramagnetic effect only. 



* Following WEISS, we have taken the molecular field proportional to I. WEISS writes the molecular 

 field NI and assumes N to be constant. This applies with sufficient accuracy in a temperature region just 

 below the critical temperature, but cannot be true over the whole region down to absolute zero, because, 

 as the molecular translational vibrations die down, the molecules approach one another more closely and 

 the molecular field must necessarily increase considerably although I remains practically constant. This 

 increase is accounted for by the increase of the coefficient N, which is the reciprocal of the limiting 

 susceptibility. 



t EWING, ' Magnetic Induction in Iron and other Metals,' p. 172 et seq., where curves are given for iron, 

 hard steel, nickel and various nickel steels. See also p. 269 infra and EWING, loc. tit., p. 354. 



