CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 287 



peculiarities of the molecular configuration. It has been shown that the energy 

 change which occurs on crystallization is compatible with a volume change of the 

 same order of magnitude as that accompanying crystallization, and we may therefore 

 interpret the change of volume on crystallization as a magneto-striction effect of the 

 local molecular forcive. The magneto-striction effect depends on molecular orientation 

 which is proportional to the square of the magnetic forcive (pp. 259-263). In Part III. 

 it was shown that the double refraction of crystalline media can be interpreted as 

 due to the magnetic double refraction effect of the local molecular forcive which 

 orientates the molecules into a crystalline space lattice. This effect is also propor- 

 tional to the square of the magnetic forcive and the two effects mutually support 

 one another. 



(V.) The above results are interesting in connexion with TAMMANN'S theory of the 

 closed region of stability of the crystalline state, as represented on the pressure 

 temperature diagram. TAMMANN'S experimental work gives an alternative method of 

 determining intrinsic pressures, but the results are notably higher than those found in 

 other ways. Possibly this is due to extrapolation over a wide pressure range. The 

 pressure temperature diagrams showing the fusion curve are instructive in dealing 

 with problems relating to thermal evolution (or absorption), and volume changes and 

 possible interpretations of these peculiarities in the ferro-magnetic elements, iron, nickel 

 and cobalt, have been given (pp. 263-266). 



(VI.) A physical interpretation has been given of the large values of the 

 coefficients, N and a' c , of the molecular fields in ferro-magnetic and diamagnetic 

 crystalline media respectively. These coefficients are the reciprocals of the limiting 

 local susceptibilities of the media under field strengths equal to the respective 

 molecular fields. The 'local susceptibility of a diamagnetic molecule is comparable 

 with that of a ferro-magnetic molecule and the two vary in the same way with field 

 strength and temperature. In diamagnetic media, however, magnetic hysteresis will 

 be inappreciable, since the molecule as a whole possesses a zero magnetic moment. 

 Nevertheless, mechanical hysteresis in diamagnetic media will be of the same order as 

 in ferro-magnetic media (pp. 267-270 and p. 257). 



(VII.) From TYND ALL'S experiments on the deportment of paramagnetic and 

 diamagnetic crystals in a magnetic field, the positions of the planes of cleavage can be 

 traced. These results show that the forces responsible for crystalline symmetry are 

 very probably of a magnetic nature. If the forces are of an electrostatic nature, then, 

 since an electric field must disclose the same planes of cleavage, the electric and 

 magnetic symmetries must coincide. This is not the case however. The magnetic 

 forces are partly due to the valency or boundary electrons whose orbits are 

 controlled by the atomic nuclei. The nuclei determine the crystalline symmetry, 

 indirectly, through the medium of the magnetic forces of the electrons. This 

 conclusion is not at variance with the results of X-ray diffraction experiments ; 

 the latter determine only the positions of the diffuse diffracting cores and give no 



VOL. ccxx. A. 2 R 



