Faraday to William Whewell   7 November 1848¹

Royal Institution | 7 Novr 1848

My dear Sir

It is with great pleasure that I nurse the hope of having you again in one of our Friday Evenings of our coming season². You must have done good to all who heard you but I can testify of myself that the truth you laid down that truth can more easily emerge from error than from confusion³ has been to me practically useful and a source of continual pleasure in observing that in other things besides those I meddle with it is so[.]

You remember our talk about the connexion which ought to exist between crystalline & electric forces⁴. Well it is beginning to appear. I dare say you know of Pluckers papers in Poggendorf[f]s Annalen on the Magnetic repulsion of the Optic axes of crystals ⁵. I will therefore pass from them to my own matter. I find that certain crystals are subject to Magnetic force & these may be crystals either of Magnetic or diamagnetic bodies. Bismuth, Antimony, Arsenic, Sulphate of Iron, Sulphate of Nickel are such bodies. If a crystal of bismuth be suspended by a filament of cocoon silk between the horizontal poles of a horseshoe magnet (& a common magnet that will not raise more than 2 or 3 lbs by the keeper can shew the effect if sheltered) the crystal vibrates & points. If the line through the crystal which is then horizontal & equatorial be made vertical the crystal again points & now with its maximum degree of force. If the line which is now horizontal and axial be made vertical, the crystal is indifferent or it is simply repelled as a diamagnetic body ought to be. The form of the piece of bismuth goes for nothing in this experiment the effect depends altogether upon the crystalline structure. This line which places itself parallel to the magnetic axis I have called the Magnecrystallic⁶ axis to express the original condition of the crystal but as it appears to me that the power by which it points when in the Magnetic field is an induced power I have called it the Magneto crystallic force[.]

The Magnecrystallic axis in bismuth is perpendicular or nearly so to the chief plane of cleavage, it is constant for bismuth and for all other bodies in which I find this condition. Thus in Sulphate of Iron it is perpendicular or nearly so to two sides of the rhombic prism[.]

The effect upon the crystal in the magnetic field is not one of attraction or repulsion but of position only - and as yet I can find no traces of polarity though the setting force be strong. A crystal of bismuth fixed at the end of a torsion balance is repelled by a single magnetic pole whether the crystal present the end of the Magnecrystallic axis or the side towards the pole i.e whether it be in the position it would take of itself or whether it be constrained to occupy that the farthest from it or any other position[.] As a diamagnetic body it appears unchanged, & as to polarity there are no signs of it by any difference in the repulsive force. With Sulphate of Iron in which the Magnecrystallic phenomena are very strong there is no alteration in the amount of attractive force. So strong is the tendency to go into position that I can make a diamagnetic body approach when it would naturally be repelled or a magnetic body receed when it would naturally be attracted by opposing the magnecrystallic condition to the magnetic or diamagnetic condition.

As thus N is an electromagnetic pole - opposite to which is a prismatic crystal of sulphate of iron in which the Magnecrystallic axis is represented by the dotted line and which therefore at present is oblique to the lines of force issuing from the pole N. A vertical silken axis is so adjusted that it passes downwards at a a little piece of card board is fascened [sic] across it & on this the crystal is fixed - so that it can spin as radius round this axis[.] Now if the torsion be adjusted so that it shall hold naturally the more oblique position the moment the Magnetism is put on it receeds into the other position by the tendency of the Magnecrystallic axis to place itself parallel to the lines of magnetic force. The papers are at the Royal Society⁷ & I shall send you a copy as soon as I can. Come in due time and talk to us & encourage me[.]

Ever Yours | M. Faraday

Rev. Dr. Whewell | &c &c &c