48 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
All the effects which have been described are produced with great distinctness 
when, instead of compressed bismuth, two similar bars of the crystallized substance 
are used, in which the planes of principal cleavage are parallel to the length. Such 
bars are not difficult to procure, and they ought to hang in the magnetic field with 
the planes of cleavage vertical. It is unnecessary to describe the experiments made 
with such bars ; they exhibit with promptness and decision all the effects observed 
with the compressed bismuth. 
We have hitherto operated upon elongated masses of bismuth ; but with the com- 
pressed substance, or with the substance crystallized uniformly in planes, as in the 
case last referred to, an elongation of the mass is not necessary to the production of 
the effects described. Previous, however, to the demonstration of this proposition, I 
shall introduce a kind of lemma, which will prepare the way for the complete proof. 
Diamagnetic bodies, like paramagnetic ones, vary considerably in the intensity of 
their forces. Bismuth or antimony, for example, exhibits the diamagnetic force with 
greater energy than gold or silver, just as iron or nickel exhibits the magnetic force 
with greater energy than platinum or chromium. Let two thin bars, ab, cd, fig. 4, of 
two bodies of different diamagnetic pov/ers, be placed at right angles to each other, so 
as to form a cross ; let the cross be attached to the end of a lever and suspended 
horizontally from the point x, before the flat or rounded pole N of a magnet. Let 
the continuous line ab represent the needle of the powerful dia- ^ 
magnetic body, and the broken line cd that of the feeble one. On 
the former a mechanical couple acts in the direction denoted by 
the arrows at its ends ; and on the latter a couple operates in the 
direction of the arrows at its ends. These two couples are evidently a 
opposed to each other; but the former being, by hypothesis, the 
more powerful of the two, it will overcome the latter. The mecha- c 
nical advantage possessed by the attracted end a of the more power- 
ful bar, on account of its greater distance from the axis of suspen- 
sion X, will, in an approximately uniform field of force which we 
here assume, cause the centre of gravity of the cross to move to- 
wards the pole N. 
In the formation of such a cross, however, it is not necessary to resort to two dif- 
ferent substances in order to find two needles of different diamagnetic powers ; for 
in crystallized bodies, or in bodies subjected to mechanical pressure, the diamag- 
netic force acts with very different energies in different directions. Let a mass of 
a diamagnetic body which has been forcibly compressed in one direction be imagined ; 
let two needles be taken from such a mass, the one with its length parallel, and the 
other with its length perpendicular to the line of pressure. Two such needles, 
though composed of the same chemical substance, will behave exactly as the two 
bars of the cross in the experiment last described ; that needle whose length coin- 
cides with the line of pressure will bear the same relation to the other that the 
