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 diamagnetism 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; 
ad hence would produce the phenomena of repulsion. To carry 
