PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
39 
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 towards the influencing north pole, it is absurd to suppose that its pre- 
sence 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 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 diamag- 
netism 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 pro- 
duce the phenomena of repulsion. To carry out the assumption here made, 
M. Weber is obliged to suppose that the molecules of diamagnetic bodies are sur- 
rounded by channels, in which the induced molecular currents, once excited, continue 
to flow without resistance. 
This theory, notwithstanding its great beauty, is so extremely artificial, that I ima- 
gine the general conviction of its truth cannot be very strong ; but there is one con- 
clusion flowing from it which appears to me to be in direct opposition to experi- 
mental facts. The conclusion is, that the magnetism of two iron particles in the line 
of magnetization is increased by their reciprocal action-, hut that, on the contrary, the 
diamagnetism of two bismuth particles lying in this direction is diminished by their 
reciprocal action!'' The reciprocal action of the particles varies inversely as the cube 
of the distance between them : at a distance expressed by the number 1, for example, 
the enfeeblement is eight times what it would be at the distance 2. 
The conclusion, as regards the iron, is undoubtedly correct ; but I believe experi 
ment proves that the mutual action of diamagnetic molecules, when caused to 
approach each other, increases their repulsive action. I have had massive iron 
moulds made and coated with copper by the voltaic current; into these fine bismuth 
powder has been introduced and submitted to powerful hydraulic pressure. No sen- 
sible fact can, I think, be more certain, than that the particles of this dust are brought 
into closer proximity along the line in which the pressure is exerted, and this is the 
line strongest d'lamagnetization. If a portion of the compressed mass be placed upon 
the end of a torsion beam and the amount of repulsion measured, it will be found 
that the repulsion is a maximum when the line of magnetization coincides with the 
line of compression ; or, in other words, with that line in which the particles are 
packed most closely together: if the bismuth were fixed, and the magnet moveable, 
the former would repel the latter with a maximum force with the line of compression 
parallel to the direction of magnetization: it is a stronger diamagnet in this direction 
