the case of isomorphous crystals. It is not, I think, questioned at present, that the deportment of crystals in the magnetic field depends upon their molecular structure; nor will it, I imagine, be doubted, that the molecular structure of a complete crystal of carbonate of iron is the same as that of an isomorphous crystal of carbonate of lime. In the architecture of the latter crystal, calcium simply takes the place which iron occupies in the former. Now a crystal of carbonate of iron is attracted most forcibly when the attracting force acts parallel to the crystallographic axis.* Let such a crystal be supposed to diminish gradually in magnetic capacity, until finally it attains a magnetic weight, in a direction parallel to its axis, equal to that of the medium in which we assume it to be immersed. Such a crystal would be indifferent, if the force acted parallel to its axis, but would be repelled, if the force acted in any other direction. If the magnetic weight of the crystal diminish a little further, it will be repelled in all directions, or, in other words, will become diamagnetic; but it will then follow, that the repulsion in the direction of the axis, if the nature of the excitement remain unchanged, will be less than in any other direction. In other words, a diamagnetic crystal of the form of carbonate of iron will, supposing magnetism and diamagnetism to be the same, be repelled with a minimum force when the repulsion acts parallel to the axis. Here, as before, we arrive at a conclusion which is controverted by experiment; for the repulsion of a crystal of carbonate of lime is a maximum when the repelling force acts along the axis of the crystal. Hence I would infer that the excitement of carbonate of iron cannot be the same as that of carbonate of lime. Such are the reflections which presented themselves to my mind on the evening to which I have referred. I now submit them to you as a fraction of that thought which your last memoir upon this great question will assuredly awaken. III.-MAGNETIC REMARKS BY PROFESSOR FARADAY. [To the foregoing letter Faraday returned the following reply. Phil. Mag. vol. ix. p. 253.] * MY DEAR TYNDALL,-In relation to your letter of last month, 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, acknowledgments, or conclusions, as the state of the subject may then seem to render necessary or useful. In the meantime, 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) had to consider the magne-crystallic phenomena presented by the same body in different media, and having found the magne-crystallic 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). 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.† In respect of the effects of pressure, to which you refer in Phil. Mag. 1855, vol. ix. 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), 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 magne-crystallic substance. You are aware (and I hope others will remember) that I give the lines of force* 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. 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 provable by experiment:—and they, with the considerations arising from the principle of the conservation of force, 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 It is nearly twenty-four years since I first called attention to these lines (Exp. Res. 114, note). 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 phenomena, 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 phenomena 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.). My reason is, that being satisfied by the phenomena of diamagnetism, &c. that that hypothesis cannot be true, all these and such like phenomena 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 to 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, M. FARADAY. ROYAL INSTITUTION: [The subject of a Magnetic Medium was further discussed by Professor Williamson, Phil. Mag. vol. ix. p. 541, and by Professor Hirst, Phil. Mag. vol. x. p. 442. I may say that I strongly lean towards the view that the luminiferous ether is concerned in magnetical and electrical phenomena. Some remarks on this subject are made further on.] IV. OBSERVATIONS ON THE MAGNETIC MEDIUM,' AND ON THE EFFECTS OF COMPRESSION. BY PROFESSOR WILLIAM THOMSON. [I did myself the pleasure of forwarding to Professor, now Sir William Thomson, a copy of my letter to Faraday, and received from him the following observations in reply. They were too interesting to be kept private, and at my request were published in the 'Phil. Mag.' for April, 1855.-J. T., 1870.] 2 College, Glasgow, March 12, 1855. MY DEAR SIR,-Allow me to thank you for the abstract of your lecture on magnetism, and the copy of your letter to Mr. Faraday, which I have recently received from you, and have read with much interest. I am still strongly disposed to believe in the magnetic character of the medium occupying space, and I am not sure but that your last argument in favour of the reverse bodily polarity of diamagnetics may be turned to support the theory of universally direct polarity. There is no doubt but that the medium occupying interplanetary space, and the best approximations to vacuum which we can make, have perfectly decided mechanical qualities, and among others, that of being able to transmit mechanical energy in enormous quantities (a platinum wire, for instance, kept incandescent by a galvanic current in the receiver of an air-pump, emits to the glass and external bodies the whole mechanical value of the energy of current spent in overcoming its galvanic resistance). Some of these properties differ but little from those of air or oxygen at an ordinary barometric pressure. Why not, then, the magnetic property? (of which we know so little that we have no right to pronounce a negative.) Displace the interplanetary medium by oxygen, and you have a slight increase of magnetic polarity in the locality with a drawing in of the lines |