32 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
obtained by other inquirers. In his recent memoirs upon this subject, M. v. Feilitsch 
states that he has found the deflection of diamagnetic bodies, under the circumstances 
here described, to be precisely the same as that of paramagnetic bodies ; this result 
is of course opposed to mine ; but when it is remembered that the learned German 
worked confessedly with the “ roughest apparatus,” and possessed no means of elimi- 
nating the effects of structure, there seems little difficulty in referring the discrepancy 
between us to its proper cause. 
The same perfect antithesis will be observed in the case of the abnormal bars, on a 
comparison of the third and fourth columns. In all cases then, whether we apply the 
magnet singly, or the current singly, or the magnet and current combined, the 
deportment of the normal diamagnetic bar is opposed to that of the normal para- 
magnetic one, and the deportment of the abnormal paramagnetic bar is opposed to 
that of the abnormal diamagnetic one. But if we compare the normal paramagnetic 
with the abnormal diamagnetic bar, we see that the deportment of one is identical 
with that of the other The same identity of action is observed when the normal 
diamagnetic bar is compared with the abnormal paramagnetic one. The necessity 
of taking molecular structure into account in experiments of this nature could not, I 
think, be more strikingly exhibited. 
For each of the bars, under the operation of the two forces, there is an oblique posi- 
tion of equilibrium : on the application of the second force, the bar swings like a 
pendulum beyond this position, oscillates round it, and finally comes to rest there. 
Hence, if before the application of the second force the bar occupy the axial posi- 
tion, the deflection, when the second force is applied, appears to be from the axis to 
the equator; but if it first occupy the equatorial position, the deflection appears to 
be from the equator to the axis. 
We have already shown that the repulsion of diamagnetic bodies is to be referred 
to a state of excitement induced by the magnet which acts upon them : it has been 
long known that the attraction of paramagnetic bodies is due to the same cause. 
The experiments just described exhibit to us bars of both classes of bodies moving in 
the magnetic field : such motions occur in virtue of the induced state of the body, 
and the relation of that state to the forces which act upon the mass. We have seen 
that in all cases the antithesis between both classes of bodies is maintained. What- 
ever therefore the state of the paramagnetic bar, under magnetic excitement, may be, 
a precisely antithetical state would produce all the phenomena of the diamagnetic 
bar. If the bar of iron be polar, a reverse polarity on the part of bismuth would 
* Identical to the eye, but not to the mind . The notion appears to be entertained by some, that, by changing 
molecular structure, I had actually converted paramagnetic substances into diamagnetic ones, and vice versd. 
No such change, however, can cause the mass of a diamagnetic body suspended by its centre of gravity to be 
attracted, or the mass of a paramagnetic body to be repelled. But by a change of molecular structure one of 
the forces may be so caused to apply itself that it shall present to the eye all the directive phenomena exhibited 
by the other. — J. T., May 5, 1855. 
