ELEMENTARY MAGNETISM 387 



or the appeal to the bodily senses and the power of 

 memory alone, could never inspire. 



As an expansion of the note at p. 377, the following extract may find a 

 place here : 



"It is well known that a voltaic current exerts an attractive force upon 

 a second current, flowing in the same direction ; and that when the directions 

 are opposed to each other the force exerted is a repulsive one. By coiling wires 

 into spirals, Ampere was enabled to make them produce all the phenomena of 

 attraction and repulsion exhibited by magnets, and from this it was but a step 

 to his celebrated theory of molecular currents. He supposed the molecules of 

 a magnetic body to be surrounded by such currents, which, however, in the 

 natural state of the body mutually neutralized each other, on account of their 

 confused grouping. The act of magnetization he supposed to consist in setting 

 these molecular currents parallel to each other ; and, starting from this principle, 

 he reduced all the phenomena of magnetism to the mutual action of electric 

 currents. 



"If we reflect upon the experiments recorded in the foregoing pages from 

 first to last, we can hardly fail to be convinced that diamagnetic bodies oper- 

 ated on by magnetic forces possess a polarity 'the same in kind as, but the 

 reverse in direction of, that acquired by magnetic bodies. ' But if this be the 

 case, how are we to conceive the physical mechanism of this polarity? Accord- 

 ing to Coulomb's and Poisson's theory, the act of magnetization consists in the 

 decomposition of a neutral magnetic fluid ; the north pole of a magnet, for ex- 

 ample, possesses an attraction for the south fluid of a piece of soft iron sub- 

 mitted to its influence, draws the said fluid toward it, and with it the material 

 particles with which the fluid is associated. To account for diamagnetic phe- 

 nomena this theory seems to fail altogether; according to it, indeed, the oft-used 

 phrase, 'a north pole exciting a north pole, and a south pole a south pole, ' in- 

 volves a contradiction. For if the north fluid be supposed to be attracted 

 toward the influencing north pole, it is absurd to suppose that its presence there 

 could produce repulsion. The theory of Ampere is equally at a loss to explain 

 diamagnetic action ; for if we suppose the particles of bismuth surrounded by 

 molecular currents, then, according to all that is known of electro-dynamic 

 laws, these currents would set themselves parallel to, and in the same direction 

 as, those of the magnet, and hence attraction, and not repulsion, would be the 

 result. The fact, however, of this not being the case, proves that these molec- 

 ular currents are not the mechanism by which diamagnetic induction is effected. 

 The consciousness of this, I doubt not, drove M. Weber to the assumption that 

 the phenomena of diamagnetism are produced by molecular currents, not directed, 

 but actually excited in the bismuth by the magnet. Such induced currents 



