Julius Plücker to Faraday   24 March 1856

Bonn 24/3 56.

Dear Sir!

Looking at the date of your last kind letter¹, I am very much surprised my silence had been so long a one. Being another time by election at the head of the University, I am for a year rather intirely distracted from scientific working. Therefore, that I may not fall into my former indolence, I write to you the very first day of “vacances”.

My best thanks for your last interesting paper². There is one conclusion in it, I fully adopt. The paramagnetic induction is augmented by the reciprocal action of the particles of the induced body. The diamagnetic induction must be weakened by the same reason. When the contrary would be proved by experiment, I shall be forced to give up diamagnetic polarity. But ancient experiments dont encourage myself, to take up again this “experimentum crucis”.-

Since several years doubts rose in my mind against the theory I had imagined to connect together the complicated magnetic phenomena, exhibited by crystals. In crystals with only one optic axis, acted upon by a magnetic pole, this axis may be theoretically regarded as a line of polarity, or as the direction of resulting magnetic action. In biaxial crystals I (badly) imagined two such lines of resulting magnetic action, of which depends the position of the crystal between the poles. No experiment contradicted this theory, till July last I undertook a new series of experiments. With the same flexibility of mind, I modified before the theory, when forced by new discoveries, I now altogether give it up, substituting to it a new one, which gives to the various observed facts a new interpretation. I comunicated, on Mr. Moigno’s demand, a first account of it in the Cosmos, September last³, when I was in Paris. Cutting of the introductory phrase and the Epilogue you will find my own words. Unhappily I got no copy to send it to you.- To the former two axes I substitute two new ones, depending upon the crystalline structure of the crystal. When the crystal suspended between the two poles, in such a way that one of these two axes be perpendicular, there is no extraordinary magnetic action. These two axes for instance, in the case of “cyanure rouge de fer et potasse” are placed in the plane of the two optic axes, both systems of axes have the same mid[d]le line, only the angle included by the magnetic axes is about 42°, while the angle included by the optic axes is only 20°. &c &c.

When you say any part, that my theory does not hold, you mean surely that theory “octroyée” to me by Mr. Tyndall. The phrase I at first employed may be translated allso into Englisch: “the force which produces this repulsion is independent of the magnetic or diamagnetic condition of the mass” (Phil. Trans. 1855 p 2⁴) but its real meaning was “the force exists in both cases, the mass may be either magnetic or diamagnetic.” Thus it was understood by all germain philosophers I know.

Mr Tyndalls laws printed with italic letters in his paper p.12 & 13⁵ signifie, I think, nothing at all. Indeed how may you observe the magneto-crystallic action if not by the deplacement of a diamagnetic body from the equatorial to the axial position, of a paramagnetic body from the axial to the equatorial? Mr Tyndall fights against my theory of two conflicting forces, which never did exist (p.13). The fact is that I gave [in] 1849 before he published any paper, an mathematical expression of the force (couple) emanating from the pole of a Magnet and acting upon an uniaxial crystal⁶. This expression contains two terms with opposite signs (±), indicating that the whole force may be mechanically divided into two forces, acting upon the crystall in opposite sense. You may call them conflicting forces; whether of both predominates depends on the distance from the poles. I never theoretically admitted a magnetic action on an ideal line, in conflict with the co<line>mon magnetic or diamagnetic action. After fighting against a theory, which never was mine, Mr T. gives in the Appendix (p.44 seq.)⁷ the very same explication I gave [in] 1849 to explain by a popular fact the principle of my calculus, relating to uniaxial crystals.

After having given to you, I hope in an intelligible Englisch, these incomplete explications, a stone, as we say in German, is fallen from my breast. Instead of fruitless discussions I think it more proper to present to the Roy. Society an elaborate paper, containing the general theory of meagnetocrystallic action founded on a new series of experiments⁸. I have all reason to hope that the new theory will be generally adopted; it is no more contradicted by Poisson’s⁹, Green’s¹⁰ & Thomson’s calculus based on molecular action.

Dear Sir! The extreme kindness, with which you received my very first experimental researches is that moment in my scientific life, at which I look bak with the greatest satisfaction. When recently I had the honour to be elected a Foreign membre of the Royal Society¹¹, the origin of it is to be found only in that kindness.

With all my heart and for ever | Yours | Plücker