'But supposing that you do not reject Ampère's theory of permanent molecular currents, but are disposed to enter upon the inner connection and true significance of the theory, you will easily recognise that it is by no means an arbitrary assumption of mine, that in bismuth molecular currents are excited, when the exciting magnetic force is augmented or diminished; but that the excitation of such molecular currents is a necessary conclusion from the theory of Ampère, which conclusion Ampère himself could not make, because the laws of voltaic induction, discovered by Faraday, were unknown to him. In all cases where molecular currents exist, by increase or diminution of the magnetic exciting force, molecular currents must be excited, which either add their action to, or subtract it from, the action of those already present.

‘Finally, permit me to make a few remarks on the following words of your memoir :

""To carry out the assumption here made, M. Weber is obliged to suppose that the molecules of diamagnetic bodies are surrounded by channels, in which the induced currents, once excited, continue to flow without resistance."

The assumption of channels which surround the molecules, and in which the electric fluids move without resistance, is an assumption contained in the theory of Ampère, and is by no means added by me for the purpose of explaining diamagnetic polarity. A permanent molecular current without such a channel involves a manifest contradiction, according to the law of Ohm.

'I may further observe, that I do not wonder that you regard a theory which is built upon the assumption of such channels, as "so extremely artificial that you imagine the general conviction of its truth cannot be very strong." In a certain sense I quite agree with you, but I only wish to convince you that this objection applies really to the theory of Ampère,* and only applies to mine in so far as it is built upon the former. (You may perhaps find less ground for objecting to the specialty of such an assumption, if you separate the simple fundamental conception, which recommends itself particularly by a certain analogy of the molecules to the heavenly bodies in space, from those additions which Ampère was forced to make, in order to apply the mathematical methods at his command, and to make

the subject one of strict calculation. He was necessitated to reduce the case to that of linear currents, which necessarily demand channel-shaped bounds, if every possibility of a lateral outspreading is to be avoided.)

6

To place my theory of diamagnetic polarity in a truer light, I am anxious also to convince you that this theory is by no means based upon new assumptions (hypotheses), but that it only rests upon such conclusions as may be drawn from the theory of Ampère, when the laws of voltaic induction discovered by Faraday, and the laws of electric currents by Ohm, are suitably connected with it. I affirm, that, even if Faraday had not discovered diamagnetism, by the combination of Ampère's theory with Faraday's laws of voltaic induction, and Ohm's laws of the electric current, as shown in my memoir, the said discovery might possibly have been made.

'In respect, however, to the artificiality of the theory of Ampère, I hope that mathematical methods may be found whereby the limitation before mentioned to the case of linear currents may be set aside, and with it the objection against channel-form beds. All our molecular theories are still very artificial. I, for my part, find less to object to in this respect in the theory of Ampère than in other artificialities of our molecular theories; and for this reason, that in Ampère's case the nature of the artificiality is placed clearly in view, and hence also a way opened towards its removal.*

'To Mr. Faraday I beg of you to present my sincerest respect.

'Believe me, dear Sir,

Professor Tyndall.'

'Most sincerely yours,

'WILHELM WEBER.'

The foregoing letter possesses more than a private interest, and I have therefore laid it before the readers of the Philo

* In Heat as a Mode of Motion, 4th edition, and elsewhere, I write thus:Whether we see rightly or wrongly-whether our insight be real or imaginary— it is of the utmost importance in science to aim at perfect clearness in the description of all that comes, or seems to come, within the range of the intellect. For if we are right, clearness of utterance forwards the cause of right; while if we are wrong, it ensures the speedy correction of error.' It is needless to say more to show how heartily I subscribe to the view of Professor Weber.-J. T., 1870.

sophical Magazine.' On one point in it only would I ask permission to make a remark, and that is the proposition, that the diminution of the excitement of a row of bismuth particles in the line of magnetisation by their reciprocal action is 'a direct consequence of diamagnetic polarity.' M. Weber (I believe) founds this proposition on the following considerations :-Let a series of bismuth particles lie in the axial line between the magnetic poles N and S: the polarity excited in these particles by the direct action of the poles will be that shown in the figure, being the reverse of that of iron particles under the same circumstances. But as the end n of the right-hand particle tends to excite a magnetism

N

ś n

O

S

S

like its own in the ends of the left-hand particle, and vice versá, this action is opposed to that of the magnet, and hence the magnetism of such a row of particles is enfeebled by their reciprocal action.

Now it appears to me that there is more assumed in this argument than experiment at present can bear out. There are no experimental grounds for the assumption, that what we call the north pole of a bismuth particle exerts upon a second bismuth particle precisely the same action that the north pole of an iron particle would exert. Magnetised iron repels bismuth; but whatever the fact may be, the conclusion is scarcely warranted, that therefore magnetised bismuth will repel bismuth. Supposing it were asserted that magnetised iron attracts iron and repels bismuth, while magnetised bismuth attracts bismuth and repels iron, would there be anything essentially impossible, self-contradictory, or absurd involved in the assertion? I think not. And yet if even the possible correctness of such an assertion be granted, the proposition above referred to becomes untenable. It will be observed that it is against a conclusion rather than a fact that I contend. With regard to the fact, I should be sorry to express a positive opinion; for this is a subject on which I am at present seeking instruction, which may lead me either to M. Weber's view or the opposite. Be that as it may, the result cannot materially affect the respect I entertain for every opinion emanating from my distinguished correspondent on this and all other scientific subjects. J. T., 1855.

VI.-ON RECIPROCAL MOLECULAR INDUCTION.*

[A letter addressed to Professor W. Thomson.]

Royal Institution, November 26, 1855.

MY DEAR SIR,-The communication from Professor Weber which appears in the present Number of the Philosophical Magazine,' has reminded me, almost too late, of your own interesting letter on the same subject published in the April Number of this Journal. A desire to finish all I have to say upon this question at present induces me to make the following remarks, which, had it not been for the circumstance just alluded to, might have been indefinitely deferred..

With reference to the mutual action of a row of bismuth particles, you say that it is perfectly easy to demonstrate that it must be such as to impair the "diamagnetisation" when the line of the row is parallel to the lines of force.' From this you infer, that in a uniform field of force a bar of bismuth would set its length along the lines of force. Further on it is stated that this action is demonstrated with as much certainty as the parallelogram of forces;' and you conclude your letter thus:- The assertions which I have made are demonstrable in every case without special experiment . . . and are absolutely incontrovertible, as well as incapable of verification, by experiment or observation on diamagnetics.'

Most of what I have to say upon this subject condenses itself into one question.

Supposing a cylinder of bismuth to be placed within a helix, and surrounded by an electric current of sufficient intensity, can you say, with certainty, what the action of either end of that cylinder would be on an external fragment of bismuth presented to it?

If you can, I, for my part, shall rejoice to learn the process by which such certainty is attained; but if you cannot, it will, I think, be evident to you that the verb 'must' is logically 'defective.'

Phil. Mag., December, 1855.

We know that magnetised iron attracts iron: we know that magnetised iron repels bismuth: this, so far as I can see, is your only experimental ground for assuming that magnetised bismuth repels bismuth, and yet you affirm that an action deduced from this assumption is demonstrated with as much certainty as the parallelogram of forces.' Do I not state the question fairly? I can, at all events, answer for my earnest wish to do so.

It is needless to remind one so well acquainted as you are with the mental experience of the scientific inquirer, that the very letters which you attach to your sketch, page 291 [223 here], may tempt us to forget the possibility of a physical difference between the n of iron and the n of bismuth, and may thus lead us very wide of the truth. The very term 'pole' often pledges us to a theoretic conception without our being conscious of it. You are also well aware of the danger of shutting the door against experimental inquiry on an unpromising subject; and when you apparently do this in your concluding paragraph, I simply accept it as a strong way of expressing your personal conviction, that the action referred to is too feeble to be rendered sensible by experiment.

Believe me, dear Sir,

Professor W. Thomson, F.R.S.

Most truly yours,

JOHN TYNDall.