266 MAGNETISM Tyndall verifies the pre diction of Fara day and Thom son for Iceland spar. Hankel. Thomson belongs the credit of throwing the laws of magne- crystallic action into the appropriate mathematical form, and of showing that they range themselves quite naturally under the theory of Poisson. 1 Tyndall succeeded, where Faraday had failed, in showing the magnecrystallic phenomenon of the second kind in Iceland spar. It is particularly instructive to compare his results for carbonate of iron and carbonate of lime, both positive uniflxals, but the one magnetic and the other cliamarrnetic. His results are as follows : Substance. Axis Axial. Axis Equatorial. Magnetic Character. Carbonate of iron 100 71 + Carbonate of lime 100 90 Sulphate of iron 100 85 + Bismuth 71 100 Hankel 2 measured the repulsion on a cylinder of bismuth placed with its magnetic axis inclined at angles of 15, 45, and 75 with the lines of force, and compared his results with the theoretical formula 9O7 + 45 3 sin 2 < ; the result was as follows : 15. 45. 75 . Observed repulsion 941 113 3 132-4 Calculated repulsion 93-7 113-3 133-0 Relation of mag netic reolo- tropy to crystal line form. Effect of compres sion. Tyndall and Knob lauch. Effect of laminar structure. Stratified and fibrous bodies. Influence of Crystalline Form, Compression, &c., in pro ducing Magnetic ^Eolotropy. In general the magnetic teolotropy stands in close relation to the crystalline form, and consequently to a considerable extent also to the optical properties. Thus crystals of the regular system exhibit as a rule no magnecrystallic properties, but there appear to be exceptions in the case of certain pyroelectric crystals such as boracite. Again, crystals that have one crystallographic axis of symmetry are usually magnetically uniaxal, but the optical distinction of positive and negative does not involve the corresponding magnetic distinction, as is shown by the results of Tyndall and Knoblauch with pure Iceland spar and Iceland spar in which part of the calcium is replaced by iron. Crystals that are optically biaxal are as a rule magnetically biaxal, but the magnetic properties cannot be deduced immediately from the optical. If a small cylinder be made of a paste formed with finely ground bismuth and gum water it will point equatorially in a heterogeneous field, but if the roll be squeezed flat the plate thus formed will point axially, although its length be ten times its breadth. A roll of paste of powdered carbonate of iron, again, will point axially, the plate formed by squeezing it flat equatorially. 3 From these results Tyndall and Knoblauch concluded that, if the arrangement of the particles of any body be such Us to present different degrees of proximity in different directions, then the line of closest proximity, other circumstances being equal, will stand axial if the mass be magnetic, equatorial if the mass be diamagnetic. They constructed parallelepipeds (1 in. x ^ in. x in.), first, by gumming together rectangular slips of sandpaper (1 in. x ^ in.), secondly, by gumming together squares of the same ( in. x in.). The paper was comparatively indifferent, while the sand by itself was magnetic ; and it was found that the first model set its longest dimension axially, while the second set its longest dimension equatorially; i.e., the layers of sand set in both cases axially. Tyndall 4 has observed similar magnecrystallic actions with naturally stratified bodies such as shale, and in fibrous bodies such as wood. He was even able by 1 Phil. May., March 1851 ; or Reprint, chap. x. 2 In 1851 ; see Wied., Galv., ii. 639. 3 Tyndall and Knoblauch, Phil. May., 1850.
- Phil. May., 1851.
" a squeezing plates of bismuth to apparently reverse the magnetic character of the substance ; for the compression rendered the plates seolotropic with an axis perpendicular to their longest dimension, and in consequence they set axially like plates of a paramagnetic substance. A crystal of bismuth compressed in a direction perpendicular to the ordinary magnetic axis, i.e., parallel to the planes of principal cleavage, had its behaviour reversed as to the second class of magnecrystallic effects, the ratio of the repulsions when the crystal was set with its original axis axial and with its original axis equatorial having been changed from 71 : 100 to 112 : 100. It was also found Imita- ; possible by squeezing a ball of bismuth dough unequally tion 1lV > in two perpendicular directions to imitate a biaxal magnetic ^. crystal such as heavy spar. Tyndall and Knoblauch biaxal attempt to explain the magnetic phenomena exhibited by crystal. crystals proper by means of these results. They assume that the planes of cleavage are directions of closer aggrega tion, and therefore tend to point axially in magnetic and equatorially in diamagnetic crystals. For example, the Theory first of the above-mentioned sandpaper models would represent magnetic crystals that cleave parallel to their axis, the second magnetic crystals which cleave perpendicular Knob- to their axis. If we regard this theory merely as a way lauch. of representing the facts of observation, even if we allow it to be sufficient, it is far inferior in simplicity to the theory of Faraday and Thomson, the sufficiency of which is not disputed. Regarded as an attempt to penetrate a little farther into the relation between molecular structure and magnetic properties, it is of great interest and importance, even if we admit that like most other speculations of the kind it leads us but a little way; for the question arises immediately, How does proximity of the molecules increase specific inductive capacity 1 This last question is all the more difficult to answer that no experiment has ever yet been adduced wherein the effect of the mutual induction of the parts of a diamagnetic or weak para magnetic body plays an undoubted part. 5 .Discussion as to the Existence of Diamagnetic Polarity. Contro- Soon after Faraday s first discovery of diamagnetism, an vers y animated discussion arose as to the proper way of stating [j^ 1 . the facts involved in the new phenomenon. Faraday him- m a gne ti self inclined in the first instance to put the matter by polarity saying that under the action of an inducing force a Faraday diamagnetic body is magnetized in a direction opposite to that of soft iron ; at a later period he abandoned this form of statement in favour of what he called the theory of magnetic conduction, which fitted better with his ideas as to the part played by the surrounding medium by means of which magnetic action is transmitted from one body to another. Faraday s first theory under the name of the theory of diamagnetic polarity was immediately adopted by the Continental physicists, such as Weber, Reich, Weber and Poggendorff, who naturally found it consonant with and their favourite views as to action at a distance. It ot was also supported in England by Tyndall and others. Tyndall, Many experiments were advanced on both sides of the question, and the result was much instructive illus tration of the laws of magnetic action. But the con troversy settled nothing, because in point of fact there was nothing to settle. Either theory was perfectly sufficient, The dis- when properly applied, to represent the phenomena, cu ^, sion and each left the question of the ultimate nature of ^ t " e paramagnetic and diamagnetic action where it found it. ^. or a s This ought to have been evident after Thomson had shown merely. that the phenomena were included in a perfectly natural Thom- generalizatian of Poisson s theory, indicated in fact by son. 5 Faraday, Exp. Res., 2825, &c. There is a further difficulty in the case of diamagnetic bodies. See Thomson s letter to Tyndall,
Reprint of Papers on Eke. and May., p. 536.