SUBSTANCES OTHER THAN IRON AND STEEL 207 



is energy and there is momentum, whether it is passing through 

 ordinary matter or through the highest vacuum of which we have 

 experience. 



Ferromagnetics, paramagnetics, and diamagnetics. 

 Though Faraday only used the two classes, pararnagnetics and 

 diamagnetics, and to one or other of these referred all hodies, it is 

 usual now to separate out the three metals iron, nickel, and cobalt, 

 and to these may probably be added manganese, and to class them 

 as Ferromagnetics. Their magnetisation does not increase in 

 linear fashion with magnetising force and they show hysteresis and 

 retentivitv. The term paramagnetic is reserved for the bodies with 

 far feebler magnetic qualities. In these the magnetisation is 

 proportional to the magnetising force. Curie* showed that with 

 given magnetising force the magnetisation for paramagnetics is 

 inversely as the absolute temperature over a very wide range, thus 

 extending a result which had been found to hold in certain cases 

 over a smaller range by G. Wiedemann. This is now known as 

 Curie's Law. 



Ferromagnetics may probably be regarded as passing into 

 paramagnetics when above their critical temperature. 



Diamagnetics, if we describe them in terms of polarity, may be 

 regarded as having a like, instead of an opposite, pole formed by an 

 inducing pole, and Curie found that their magnetisation, except in 

 the case of bismuth, is very nearly independent of temperature. 

 Thediamagnetisation of bismuth decreases with rise of temperature 

 between 182 and the point of fusion very nearly linearly, and by 



about j^Vo for 1 rise -t 



This has led to the suggestion that diamagnetism is not to be 

 regarded merely as a negative paramagnetism. The temperature 

 effect on paramagnetics would appear to show that paramagnetism 

 is concerned with molecular structure, which changes with change of 

 temperature, while the absence or smallness of the effect of change 

 of temperature on diamagnetics suggests that diamagnetism is 

 concerned rather with the atomic structure, which is, we may 

 suppose, independent of temperature. 



We shall return to the forces on paramagnetic and diamagnetic 

 bodies in Chapters XX and XXII. 



* Ann. de Chim. et de Phys., 7 e serie, t. V. (1895). An account of the subject 

 is given by H. du Bois in the Congres International de Physique, vol. ii. p. 460. 

 f Conyres International, loc. cit. p. 503. 



