280 FRAGMENTS OF SCIENCE. 



confer upon his pupils a profit and a joy which the mere 

 exhibition of 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 p. 272, 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 repul- 

 sive 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 dia- 

 magnetic 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 decomposi- 

 tion 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 toward the influenc- 

 ing 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 molecular cur- 

 rents 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 diauiagnetism are produced 

 by molecular currents, not directed, but actually excited in the bismuth 

 by the magnet. Such induced currents would, according to known 

 laws, have a direction opposed to those of the inducing magnet; 

 and hence would produce the phenomena of repulsion. To carry 



