Julius Plücker to Faraday   20 May 1849

Allow me, Sir, to communicate to you several new facts, which, I hope, will spread some light over the action of the Magnet upon the optic and magnecrystallic axes.

I. The first and general law, I deduced from my last experiments is the following one

“There will be either repulsion or attraction of the optic axes by the poles of a Magnet, according to the crystalline structure of the crystal. If the crystal is a negative one, there will be repulsion, if it is a positive one there will be attraction“

The crystals most fitted to give the evidence of this law are diopside (a positive crystal) cyanite, topaze (both negatives) and other ones, crystallising in a similar way. In these crystals the line (A) bisecting the acute angles, made by the two optic axes, is neither perpendicular nor parallel to the axis (B) of the prism. Such a crystal, suspended horizontally like a prism of turmaline, staurotite or “cyanure rouge de fer & potasse” in my former experiments, will point neither axially nor equatorially, but will take allways a fixed intermediate direction. This direction will continually change if the prisme will be turned round its own axis B. It may be proved by a simple geometrical construction, which shows, that during one revolution of the prism round its axis (B), this axis without passing out of the two fixed limits C & D, will go through all intermediate positions. The directions C & D, where the crystall returns, make either whith [sic] the line joining the two poles, or with the line perpendicular to it, on both sides of these lines, angles equal to the angle included by A and B: the first being the case, if the crystal is a positive one, the last if a negative one. There it follows that if the crystal by any kind of horizontal suspension may point to the poles of the Magnet, it is a positive one; if it may point equatorially it is a negative one. This last reasoning conducted me at first to the law mentioned above.

The magnecrystallic axis, I think, is, optically speaking, the line bisecting the (acute) angles made by the two optic axes, or in the case of one single axis, the axis itself. The crystals of bismuth and arsenic are positive crystals, Antimony, according to my experiments, is a negative one. All are uniaxal.

II The cyanite is by far the most interesting crystal, I examined till now. If suspended horizontally it points very nicely, only by the magnetic power of the Earth, to the north. It is a true compass needle, and more than that, you may co<bar>mand over its declination. If for instance, you suspend it in such a way that the line A bisecting the two optic axes of the crystal, be in the vertical plane passing through the axis B of the prism, the crystal will point exactly as a compass needle does. By turning the crystal round the line B you may make it point exactly to the north of the Earth &c[.] The crystal does non [sic] point according to the Magnetisme of its substance, but only by following the magnetic action upon its optical axes. This is in full concordance to the different law of diminution by distance of the pure magnetic and the optomagnetic action. If you approach to the north end of the suspended crystal the south pole of a permanent magnetic bar, strong enough to overpowering the magnetism of the Earth, the axis B of the prism will make with the axis of the bar (this bar having any direction whatever in the horizontal plane) an angle exactly the same it made before with the meridian plane: the crystal being directed either more towards the East or more towards the West.

The crystal showed, resembling for that also to a magnetic needle, strong polarity: the same end being allways directed to the north. I dare say, if it may be a polarity of the optomagnetic power. Two questions too may easily be answered. 1˚ Is the north pole indicated by the form of crystallisation 2˚ did the crystal get, when formed, its polarity by the magnetism of the Earth. Between the poles of the strong Electromagnet the permanent polarity disappeared as long as the Magnetism was excited.

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I am obliged by the new facts, mentioned above, to take up my former memoir¹, I must reproduce it under a quite new shape. I’ll examine again the rock crystal, which being acted upon weakly by a magnet induced me to deny in that memoir, what I ascertain now and what I thought most probably, as soon as I got the first notice of your recent researches². (That you will find in the Memoir given to Mr Poggendorf[f] 2 or 3 months ago³.) Perhaps the exceptional molecular condition of rock crystal, as indicated by the passage of light t[h]rough it, will produce a particular magnetic action.

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I should be very obliged to you, if you would give notice of the contents of my present letter to Mr. de la Rive, when he calls on you, as he intended to do. I showed him several of my experiments when he passed through Bonn the 12th of Mai. The following day I got the different results, mentioned above.

My best whishes [sic] for your health!

Very truly yours | Plücker

Bonn, the 20th of May | 1849.

Endorsed by Faraday: On the Magnetic relations of the Positive & Negative optic axes of crystals by Professor Plucker of Bonn in a letter to, and communicated by Dr. Faraday⁴.

Address: Professor M. Faraday | &c &c | Royal Institution | London