52 Prof. G. Wiedemann's Magnetic Researches. 



rigidity, which retains the molecules in their positions after 

 the cessation of the active forces. 



We have long -been acquainted with all these propositions. 

 They agree perfectly, so far as they are correct, with those 

 which I have published after the earlier calculations of Weber ; 

 as moreover is recognized by Prof. G. Hoffmann, the reporter 

 upon Mr. Hughes's experiments in the i Electrotechnical 

 Journal'*. It is only in regard to proposition (c) that he 

 thinks Mr. Hughes has the priority. This is, however, not 

 correct. In the second edition of my Galvanismus, ii. (1) 

 p. 373, § 327 (1871) I say distinctly:— " It is obvious that 

 we may imagine an infinite number of arrangements of the 

 molecular magnets in bodies, with each of which no external 

 action would result. If, for example, they are arranged in 

 circles with their unlike poles in contact, or if their poles have 

 in each space-element all possible directions, this result will 

 follow." The assumption of Mr. Hughes, that I have referred 

 the neutrality of a bar to a "heterogeneous " arrangement of 

 molecules, which is completely different from his theory of 

 neutrality, is thus opposed to facts. 



Moreover, it is incorrect that the external magnetic neu- 

 trality, whether of a new bar or in one subject to magnetizing 

 influences, is conditioned solely by the formation of closed 

 molecular rings, such as Mr. Hughes imagines. This would be 

 possible only with a power of perfectly free rotation of the 

 molecules round their centres of gravity. But since experi- 

 ence shows that they suffer a resistance to this rotation, 

 this arrangement can only be approximately attained ; the 

 axes of the molecular magnets must in like manner deviate 

 from the axis of such a ring on all sides. Moreover there is, 

 a priori, no ground why any one direction should have the 

 preference in an absolutely neutral magnet, either for the 

 position of the separate molecular magnets, or for the rings 

 approximately formed by their opposite attractions. The 

 second statement of my proposition quoted above is therefore 

 entirely justified. 



Lastly, Mr. Hughes's proposition (d), that in visible mag- 

 netization a " symmetrical " rotation and arrangement of the 

 molecular magnets takes place, is only correct in the case of 

 a symmetrical form of the body magnetized, and a symme- 

 trical distribution of the magnetizing forces. 



§2. 



Sir William Thomsonf has endeavoured to explain the 



* G. Hoffmann, fflectrotechnische ZeiUchrift, iv. p. 3G6 (1883). 

 t Proc. Roy. Soc. Lond. xvii. p. 442 (1878). 



