40 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
than in any other. Cubes of bismuth, which, in virtue of their crystallization, pos- 
sessed a line of minimum magnetization, have been placed in those moulds and 
pressed closely together in the direction of the said line : the approximation of the 
particles thus effected has converted the direction spoken of from one of minimum 
ipto one of maximum magnetization. It would be difficult for me to say how many 
diamagnetic bodies I have submitted to compression, some massive, some in a state 
of powder, but in no single instance have I discovered an exception to the law that 
the line of compression of purely diamagnetic bodies is the line of strongest diamag- 
netization. The approximation of diamagnetic particles is therefore accompanied 
by an augmentation of their power, instead of a diminution of it, as supposed by the 
theory of M. Weber. 
Any hypothesis which involves the idea of the diminution of the diamagnetic action 
of a body by the approximation of its particles, is, I believe, opposed to facts. Such 
a hypothesis must, I imagine, form the basis of the following remark of Professor 
W. Thomson: — referring to “a thin bar or needle of a diamagnetic substance,” he 
says, ‘‘such a needle has no tendency to arrange itself across the lines of magnetic 
force ; but, as will be shown in a future paper, if it be very small compared with the 
dimensions and distance of the magnet, the direction it will assume, when allowed to 
turn freely round its centre of gravity, will be that of the lines of force*.” I have 
not found in any of the subsequent numbers of the Philosophical Magazine the proof 
here promised'!'. But I doubt not the conclusion involves the assumption that the 
mutual action of diamagnetic particles is to weaken each other, and hence to pro- 
duce a more feeble magnetization along a thin diamagnetic bar than across it — an 
assumption which, as already shown, is contradicted by experiment. 
It is scarcely possible to reflect upon the discovery of Faraday in all its bearings, 
without being deeply impressed with the feeling that we know absolutely nothing of 
the physical causes of magnetic action. We find the magnetic force producing, by 
processes which are evidently similar, two great classes of effects. We have a certain 
number of bodies which are attracted by the magnet, and a far greater number which 
are repelled by the same agent. Supposing these facts to have been known to 
Ampere, would he have satisfied his profound mind by founding a theory which 
accounts for only the smaller portion of them ? This theory is admirable as far as it 
goes, but the generalization is yet to come which shall show the true relationship of 
phenomena, towards whose connexion the theory of Ampere furnishes at present no 
apparent clue. 
Royal Institution, October 1854. 
* Philosophical Magazine, vol. xxxvii. p. 244. 
f This remark appears to have induced Mr. Thomson to publish the proof referred to in the last Number 
of the Philosophical Magazine. The arguments there brought forward have been long familiar to me, but I 
regret to say that I cannot attach much real value to them. At some future day I hope to be able to justify 
the scepticism which I here venture to express. — J. T., May 5, 1855. 
