Middle of the Nineteenth Century. 235 



circuits if they were situated in a varying magnetic field ; and 

 he pointed out that such induced molecular currents would 

 confer upon the substance the properties characteristic of 

 dia magnetism. 



The difficulty with this hypothesis is to avoid explaining too 

 much ; for, if it be accepted, the inference seems to be that all 

 bodies, without exception, should be diamagnetic. Weber escaped 

 from this conclusion by supposing that in iron and other 

 magnetic substances there exist permanent molecular currents, 

 which do not owe their origin to induction, and which, under 

 the influence of the impressed magnetic force, set themselves in 

 definite orientations. Since a magnetic field tends to give such 

 a direction to a pre-existing current that its course becomes 

 opposed to that of the current which would be induced by the 

 increase of the magnetic force, it follows that a substance stored 

 with such pre-existing currents would display the phenomena 

 of paramagnetism: t The bodies ordinarily called paramagnetic 

 are, according to this hypothesis, those bodies in which the 

 paramagnetism is strong enough to mask the diamagnetism. 



The radical distinction which Weber postulated between the 

 natures of paramagnetism and diamagnetism accords with many 

 facts which have been discovered subsequently. Thus in 1895 

 P. Curie showed* that the magnetic susceptibility per gramme- 

 molecule is connected with the temperature by laws which are 

 different for paramagnetic and diamagnetic bodies. For the 

 former it varies in inverse proportion to the absolute tempe- 

 rature, whereas for diamagnetic bodies it is independent of the 

 temperature. 



The conclusions which followed from the work of Faraday 

 and Weber were adverse to the hypothesis of magnetic fluids ; 

 for according to that hypothesis the induced polarity would be 

 in the same direction whether due to a change of orientation of 

 pre-existing molecular magnets, or to a fresh separation of 

 magnetic fluids in the molecules. " Through the discovery of 



* Annales de Chimie (7) v (1845), p. 289. 



