From Michael Faraday   March 14, 1855

Royal Institution, | March 14, 1855.

MY DEAR TYNDALL,

IN relation to your letter of last month,¹ I write, not for the purpose of giving what might be taken as an answer, but to say that it seems to me expedient and proper to wait and allow the thoughts that my papers may raise, to be considered and judged of at their leisure by those who are inclined to review and advance the subject. Perhaps, after a respectful interval, I may be induced to put forth such explanations, acknowledgements, or conclusions, as the state of the subject may then seem to render necessary or useful.

In the mean time, the more we can enlarge the number of anomalous facts and consequences the better it will be for the subject; for they can only remain anomalous to us whilst we continue in error. I may say, however, that the idea you suggest presents no difficulty to me; for having on former occasions (Exp. Res. 2501)² had to consider the magnecrystallic phænomena presented by the same body in different media, and having found the magnecrystallic difference unchanged in the media, I have no difficulty in conceiving that a body (as bismuth), which in the amorphous state is of the same magnetic character as the medium around it, shall, when employed as a crystal, be paramagnetic in one direction and diamagnetic in another (3157).³ What happens in a medium may, according to my knowledge of the facts, happen in space; and is in full accordance with Thomson’s clear paper on the theory of magnetic induction in crystalline bodies.⁴

In respect of the effects of pressure, to which you refer in your letter, we cannot easily draw conclusions on either side until we know better what pressure does. I am not aware whether you consider that pressure on bismuth, whilst it makes the metal more diamagnetic in one direction than another, also makes it more diamagnetic as a whole than before; or whether you suppose it less diamagnetic in the transverse direction of the pressure than at first. Gmelin says, on the authority of Marchand and Scheerer (vol. iv. p. 428),⁵ that the density of bismuth is diminished as pressure upon it is increased, and extraordinary as the fact seems, gives densities of the following degree for increasing pressures, 9.783, 9.779, 9.655, 9.556; a change in texture at the same time occurring. If the statement be true, then the line of pressure in your beautiful experiments may be the line of least density or of least approximation, though I hardly know how to think so; still it becomes difficult for us to draw reasons from the constitution of a compressed body, until we know what happens during the compression, although no difficulty arises in considering it, after compression in one direction, like to a magnecrystallic substance.

You are aware (and I hope others will remember) that I give the lines of force⁶ only as representations of the magnetic power, and do not profess to say to what physical idea they may hereafter point, or into what they will resolve themselves. Advancing no principle, I say, that the hypothetical fundamental ideas already advanced, when taken in relation to the body of facts now known, are self-contradictory and inapplicable.⁷ The following points, namely,– that the direction and polarity of lines of magnetic force are always shown truly by the electric current induced in metal moving within their influence;– that the dualities of electricity and magnetism are always respectively and essentially related;– that the dualities of an isolated magnet are not related back in straight lines through the magnet;– are to my mind not hypothetical in character, but easily proveable by experiment:– and they, with the considerations arising from the principle of the conservation of force,⁸ seem to me to be left unexplained by, and in opposition to, the usual hypotheses. No difference arises about the laws of magnetic action and their mathematical development; and that, simply because they are as yet applied only partially, and thus far are in accordance with all the views taken, including mine. When the attempt is made to apply them so as to include at once paramagnetic, diamagnetic, and electro-magnetic phænomena, and at the same time to deduce them from one hypothetical cause, then they may become so large and yet precise as to enable us to distinguish between true and false assumptions. On my part I endeavour not to assume anything, but only to draw such conclusions from the assumptions already made, and the phænomena now discovered, as seem subject to experiment and tangible by facts.

Some persons may feel surprised that I dwell upon points which are perfectly and mathematically explained by the hypothesis of two magnetic fluids, as, for instance, places of little or no action (3341 &c.⁹). My reason is, that being satisfied by the phænomena of diamagnetism, &c. that that hypothesis cannot be true, all these and such like phænomena acquire a new character and a high importance which they had not before, and amongst other philosophical uses, point most emphatically to the essential relation of the dualities and their equivalency in power. They do not contradict the old hypothesis when that is partially applied, but they are not the less strong and striking as evidence in favour of the view of lines of force.

I am, my dear Tyndall, | Yours very faithfully, | M. FARADAY.

Phil. Mag., 9 (1855), pp. 253–5.