74 



SCIENTIFIC THOUGHT. 



unique conception of the communication of electric and 

 magnetic phenomena, into connection with the mathe- 

 matical theory which had been founded and worked out 

 by Poisson and Green. Without attempting to give 

 a physical explanation of Faraday's lines of force, he 

 showed how they could be utilised in calculating the 

 complicated action of magnetic push -and -pull forces; 

 suggested that the newly discovered property called dia- 

 magnetism, in virtue of which bodies in the neighbour- 

 hood of powerful magnets appeared to be repelled, not 

 attracted, could be explained as a differential ^ effect of 



1 It was in the year 1845 that 

 Faraday, after having discovered 

 the "magnetisation of light," and 

 made visible the " magnetic lines 

 of force" ('Exp. Res.,' Nos. 2146- 

 2242), entered upon that remark- 

 able series of experiments and 

 speculations which led him to the 

 discovery of diamagnetism and to 

 the assertion of the " magnetic con- 

 dition of all matter" (ibid., Nos. 

 2243, &c.) In 1847 Thomson wrote : 

 " According to Mr Faraday's recent 

 researches it appears that there are 

 a great many substances susceptible 

 of magnetic induction, of such a 

 kind that for them the value of the 

 coefficient i is negative. These he 

 calls diamagnetic substances, and 

 in describing the remarkable re- 

 sults to which his experiments 

 conducted him with reference to 

 induction in diamagnetic matter, 

 he says, ' All the phenomena re- 

 solve themselves into this, that a 

 portion of such matter, When under 

 magnetic action, tends to move 

 from stronger to weaker places or 

 points of force.' This is entirely 

 in accordance with the result ob- 

 tained above ; and it appears that 

 the law of all the phenomena of 

 induction discovered by Faraday 

 with reference to diamagnetics may 

 be expressed in the same terms as 



in the case of ordinary magnetic 

 induction, by merely supposing 

 the coefficient i to have a nega- 

 tive value " (Reprint, p. 502). In 

 the Reprint (1854) of his early 

 papers (1842) on the corresponding 

 problems of magnetism and heat 

 (Reprint, p. 18) he added a note to 

 the effect that the "same demon- 

 stration is applicable to the in- 

 fluence of a piece of soft iron, or 

 other paramagnetic, or to the re- 

 verse influence of a diamagnetic 

 on the magnetic force in any 

 locality near a magnet in which it 

 can be placed, and shows that the 

 lines of magnetic force will be 

 altered by it pi-ecisely as the lines 

 of motion of heat in corresponding 

 thermal circumstances would be 

 altei-ed by introducing a body of 

 greater or less conducting power of 

 heat. Hence we see how strict 

 is the foundation for an analogy on 

 which the conducting power of a 

 magnetic medium for lines of force 

 may be spoken of, and we have a 

 perfect explanation of the con- 

 densing action of a paramagnetic, 

 and the repulsive eSect of a dia- 

 magnetic upon the lines of force of 

 a magnetic field, which have been 

 described by Faraday " (Reprint, 

 p. 33 note ; cf . Faraday, ' Exp. 

 Res.,' Nos. 2807, 2808). 



