ELEMENTARY MAGNETISM. 367 



As an expansion of the note tit p. 357, 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 ex- 

 erted is a repulsive one. By coiling wires into spirals, Ampere 

 was enabled to make them produce all the phenomena of attrac- 

 tion and repulsion exhibited by magnets, and from this it was but 

 a step to his celebrated theory of molecular currents. He sup- 

 posed the molecules of a magnetic body to be surrounded by such 

 currents, which, however, in the natural state of the body mutu- 

 ally neutralised each other, on account of their confused grouping. 

 The act of magnetisation 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 operated on by magnetic forces possess a po- 

 larity " 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 I Accord- 

 ing to Coulomb's and Poisson's theory, the act of magnetisation 

 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 towards it, and with it the material particles 

 with which the fluid is associated. To account for diumagnetic 

 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 towards 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, those currents would. set 

 themselves parallel to, and in the same direction as, those of tho 

 magnet, and hence attraction, and not repulsion, would be tho 

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

 these molecular currents are not the mechanism by which dia- 

 magnotic induction is effected. The consciousness of this, 1 

 doubt not, drove M. Weber to the assumption that tho phenomena 



