A LECTURE ON MAGNETISM. 379 



facts without principles, or the appeal to the bodily senses 

 and the power of memory alone, could never inspire. 



As an expansion of the note at page 371, 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 cur- 

 rents. He supposed the molecules of a magnetic body to be surrounded 

 by such currents, which, however, in the natural state of the body mu- 

 tually neutralized each other, on account of their confused ' grouping. 

 The act of magnetization he supposed to consist in setting these molecu- 

 lar 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 operated 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 ? According 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 example, possesses an attraction 

 for the south fluid of a piece of soft iron submitted 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 phenomena 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," involves 

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

 ard 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 



