CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 257 



As the corresponding stresses and energies are of the same order in the ferro- 

 magnetic metals, and, further, since it has been shown (see Part II., pp. 143, 145 and 

 Part III., pp. 84-87) that the local molecular forcive in diamagnetic media is of the 

 same order as that in the ferro-magnetic metals, we may conclude that the local 

 intensities of magnetization in the two types of media are comparable. 



Since 



i.H c .I=2xl0 9 ergs, 

 and 



H c = 10 7 gauss (see Part III., p. 8(5), 

 we find 



I == 400, 

 and 



In a diamagnetic crystalline medium the local forcives are comparable, with those 

 in iron, and, since the latter medium shows hysteresis in a magnetic field, we may 

 enquire whether a similar phenomenon will be shown by diamagnetic media. If the 

 diamagnetic molecules are magnetically unsymmetrical, the application of an external 

 magnetic field will tend to orientate them.* But this will be a differential effect on 

 our conception of a diamagnetic molecule, and thus the tendency of the applied field 

 to produce new molecular groupings will be small. We should therefore expect that 

 hysteresis due to magnetization will be inappreciable in diamagnetic media. In iron, 

 on account of the unbalanced magnetic nature of the molecules or atoms, new 

 groupings are actually produced under fields of moderate intensity and the 

 formation of these implies a loss of energy which is measured by the area of the 

 hysteresis loop. 



If, however, we take a diamagnetic copper wire and subject it to mechanical strain, 

 the medium shows mechanical hysteresis. If sufficiently large stresses are employed, 

 a permanent set is produced within the individual crystalline grains, new groupings of 

 the molecules are formed, and a certain amount of energy is dissipated. All media, 

 whether they are ferro-, para-, or diamagnetic, will show mechanical hysteresis. The 

 difference from a magnetic point of view lies merely in the compensated nature of the 

 diamagnetic molecule as compared with the uncompensated nature of the ferro- 

 magnetic molecule, but the local forcives are comparable, so that under mechanical 

 stress the mechanical- hysteresis effects will be comparable. 



(3) A COMPARISON OF THE ELASTICITIES or SOME DIAMAGNETIC CRYSTALS WITH 

 THOSE OF CRYSTALLINE PARAMAGNETIC AND FERRO-MAGNETIC MEDIA. 



It is well known that the application of an external magnetic field alters the 

 distribution of stress in a mass of iron crystals. On our theory we see how the 



* This orientation in a diamagnetic liquid gives rise to the induced magnetic double refraction. See 

 Part III., p. 87. 



2 N 2 



