CONSTITUTION AND TEMPEEATURE ON MAGNETIC SUSCEPTIBILITY. 95 



account for the observed specific heat, JEANS, in his " Report on Radiation and the 

 Quantum Theory," published by the Physical Society of London, refers on p. 77 to 

 the necessity of the rotational term, which was pointed out by the author in ' Proc. 

 Camb. Phil. Soc.,' vol. XVII., p. 450, 1914. JEANS adds: "The absence of a 

 noticeable contribution to the specific heats is accounted for, on the quantum theory, 

 by supposing that the forces opposing rotational movements of the atoms inside the 

 solid are so large that the corresponding vibrations are of very high frequency, and 

 so, normally, possess very little energy. As far as pure theory goes, there is no 

 question that to the terms in the specific heat contemplated by NERNST'S theory 

 there ought to be added an additional term of a form exactly similar to the Einstein 



term, but having x = =j?L where v 3 is the frequency (or average frequency) of the 



-L v L 



vibrations which depend on the rotations of the atoms. 



'' It is worthy of note that sodium and mercury show an increase, beyond that 

 accounted for by the theories we have considered, in the specific heats as the fusion 

 points is approached, when, presumably, the intensity of the forces which prevent the 

 atom from rotating is relaxed, and NERNST and LINDEMANN find that in general the 

 same is true for the substances they have examined."] 



Before passing on to further experimental work and the extension of our results to 

 crystalline diamagnetic media in general, it will be convenient to collect the results 

 which have been obtained in the preceding pages. The work contained in Parts I. 

 and II. has received full support and been confirmed with regard to the enormous 

 intensity of the local molecular field in about 40 diamagnetic substances which show 

 a measurable change of x on crystallization. Evidence that the magnitude of this 

 field is comparable with that of the ferro-magnetic field has been obtained from the 

 following independent sources : 



(l.) The change of susceptibility observed on crystallization demands a local 

 molecular field of this order of intensity. 



(2.) The natural double refraction of a 'crystalline substance as compared with the 

 artificial double refraction which can be induced in a liquid by the strongest magnetic 

 field at our disposal is consistent with the value of the local molecular field implied 

 by (l) for diamagnetic crystalline media. 



(3.) (l) and (2) together imply that the aggregate of the local intensity of 

 magnetization per unit volume of a diamagnetic substance is comparable with the 

 saturation intensity of magnetization of a ferro-magnetic substance. 



(4. ) The above results lead to a correct estimate of the energy (potential) associated 

 with the crystalline structure, in virtue of the molecular grouping, as tested by the 

 magnitude of the latent heat. 



(5.) Lastly, unless the forces binding the diamagnetic molecules together were of 

 the order of magnitude stated, we should not be able to detect a departure of the 

 experimental value of the specific heat near the fusion point from the value calculated 



