Cambridge | Dec. 29 53.
My dear Sir,
I should much earlier have thanked you for the memoirs1 you were kind enough to send me some time ago, had I not expected to be able to read them with the attention they deserve much sooner than engagements and, latterly, indisposition have permitted. I have now had the satisfaction of reading them with considerable care, and I may add with great pleasure. Your refutation of Plücker’s views2 as opposed to your own appears to me complete, and your proofs that Magnetic and Dia-magnetic action follow the same law,3 and the getting rid of Faraday’s optic axis force4 I think excellent generalizations, but I am not satisfied with the proofs you offer of your conclusion that the greater intensity of magnetic action in a particular direction is due only to the closer proximity of the constituent molecules of the body in that direction. There is strong presumptive reason to believe it probable that slate rocks presenting on every distinct cleavage may have been subjected to enormous pressure in direction approximately perpendicular to the planes of cleavage, which would bring the constituent particles more closely in contact in that direction. But I fancy you have somewhere shown that in a piece of Coal-shale5 the action is least in that direction. In such case the influence of subsequent crystallization must overbalance that of pressure, but how this has been is the mystery. Again your reasoning p. 25 in the memoir on the Magneto-optic properties of Crystals,6 is not, I think, conclusive. Admitting the force of it, even, as you have used it, it only assigns a reason why the shorter axis of the rhombic slice should be the line of electric polarity rather than the larger one. The distance between the constituent rhombs will be smaller in directions parallel to the sides of the rhomb than along its shorter diagonal. Why should not this be the line of electric polarity? Our ignorance too of the molecular constitution of bodies throws doubt on the conclusiveness of any reasoning of this kind. But I should like to have some further correspondence with you on this part of your theory when I can devote more time to its consideration. I may possibly have suggestions to make to you respecting it. Any thing more that can be done in this part of your subject is well worth your attention and thought. That you have already established a probability, perhaps a strong one, in favour of your views, cannot I think be doubted.
I some time ago wrote to you7 respecting some experiments on the effect of great pressure on the conductive power of substances, in which you were kind enough to say you would lend me some assistance. I am still continuing my experiments and am now about to set out for Manchester for that purpose. I shall also consider how far they may be made subservient to the establishment of your views on magnetism and dia-magnetism.
Believe me | Yours very truly | W. Hopkins
RI MS JT/1/TYP/2/604–605
LT Typescript Only
the memoirs: Tyndall sent Hopkins his published papers on 26 November (Journal, JT/2/13b/633) because he discovered that Hopkins was one of the members of the Royal Society who opposed him being awarded the Royal Medal (see letter 0827, n. 5), and this despite the fact that Hopkins had apparently ‘never read a single one of my papers’ (Journal, 25 November, JT/2/13b/640). The papers Tyndall sent would have likely included J. Tyndall and H. Knoblauch, ‘On the magneto-optic properties of crystals, and the relation of magnetism and diamagnetism to molecular arrangement’, Phil. Mag. 37:247 (July 1850), pp. 1–33; Tyndall, ‘On Diamagnetism and Magnecrystallic Action’, pp. 165–88; and J. Tyndall, ‘On the polarity of bismuth, including an examination of the magnetic field’, Phil. Mag. 2:12 (November 1851), pp. 333–44.
Your refutation of Plücker’s views: Tyndall refuted Plücker’s idea that it was solely the optic axis that determined the orientation of crystal in a magnetic field. Furthermore, he and Knoblauch ‘showed that Plücker’s new law of the behavior of optically positive and negative crystals was also invalid’, and ‘demolished Plücker’s argument that the magnetic attraction decreases in a “quicker ratio” than the repulsion of the optic axis’ (R. Jackson, ‘Tyndall and the Royal Medal’, p. 154).
your proofs that Magnetic and Dia-magnetic action follow the same law: in his second Memoir, Tyndall demonstrated that ‘The repulsion of a diamagnetic substance placed at a fixed distance from the pole of a magnet is governed by the same law as the attraction of a magnetic substance’ (Tyndall, ‘On Diamagnetism and Magnecrystallic Action’, p. 188).
Faraday’s optic axis force: This concept is based on Plücker’s studies on the optic axis, and is discussed by Faraday in ‘On the crystalline polarity of bismuth and other bodies, and on its relation to the magnetic form of force’, Phil. Trans. 139 (January 1849), pp. 1–41, on p. 32.
Coal-shale: a type of rock formed next to coal deposits, sometimes containing fossils (OED).
p. 25 in the memoir on the Magneto-optic properties of Crystals: see n. 3.
I some time ago wrote to you: letter 0753.
Please cite as “Tyndall0862,” in Ɛpsilon: The John Tyndall Collection accessed on