MAGNETISM susceptibility may be neglected, it is clear that the resultant actiou on any body is the difference between the action upon it and the portion of the medium which it displaces. This principle, which is the analogue of the Archimedean law for floating bodies, is of great use in quantitative Pliicker magnetic experiments. It was exemplified by Pliicker, 1 and and extensively applied in magnetic observations by Bec " Becquerel. 2 Becquerel found, for instance, that the querel. jigergnce.j between the couples tending to set a small rod of sulphur in water and in air, in magnesium chloride and in air, and in nickel sulphate and in air were very nearly the same as the corresponding differences for a rod of wax. Very curious qualitative illustrations of differential magnetic action are obtained by scattering drops of alcoholic solution of chloride of iron in olive oil; 3 the drops of chloride collect and displace the olive oil in the places of stronger force. Another form 4 of the same experiment consists in placing a layer of oil of violets over Different a layer of solution of chloride of iron. When a narrow cell liquids, filled in this way is placed equatorially with the interface of the two liquids in the axial line, on exciting the electro magnet the iron solution rises in the equatorial plane forming a disk-shaped mass around the axial line. Not withstanding these results the general opinion of experi menters seems to be that no separation of the parts of a solution can be effected magnetically once the constituents Constitu- have been thoroughly mixed. Thus Faraday could obtain ents of a no evidence of the concentration of an iron solution near thorough t j ie p | e O f a magne^ although it was exposed for days not sepa- together in the magnetic field, and found no separation of rated. the oxygen and nitrogen of atmospheric air, although they differ greatly in their magnetic character. Rarefac- Pliicker endeavoured to show that the air enclosed in a tion of vessel placed between the poles of an electromagnet was rare fi e( l by the magnetic action, Faraday, however, with almost identical experimental arrangements arrived at a negative result. Magnetic Elaborate investigations of the magnetism of chemical compounds have been made by G. "VViedemann with a view of chemi- to connect their magnetic properties with their composition. cal com- A full account of these researches will be found in pounds. Wiedemann s Galvanismus, Bd. ii. 590 s^. 7 The follow- Wiede- j u g are some O f the more important of his conclusions as to lm> the effect of composition, 1. The magnetic susceptibility of the dissolved salt by itself is nearly independent of the solvent, being propor tional to the concentration. 2. If the magnetic moment m induced by a field of unit intensity in a unit of weight of the salt dissolved in water be called the "specific susceptibility," and the product /JL = A?, where A is the molecular weight of the salt, the "molecular susceptibility," then the molecular susceptibility of the dissolved salt of the same metal with different acids is approximately the same. The mean molecular suscepti bilities for nickelous, cobaltous, ferrous, and mauganous salts are as 142 : 313 : 387 : 468. 3. The molecular susceptibility of cobaltous salts stands about midway between the molecular susceptibilites of nickelous and manganous salts ; and the ferrous salts stand midway between cobaltous and manganous. 4. The molecular susceptibility of dry salts (combined with water of crystallization) is for the most part nearly the same as their molecular susceptibility in solution. A similar law holds to a certain extent for insoluble 1 Pogg. Ann., 1849. - Ann. d. Ckim. tt d. Phys., 1850. 3 Matteucci, Comptes Rendns, 1853. 4 Marangoni, Wied. lieibl., 1881. 5 Exp. Res., 2757 ; see also Righi, Wied. Beibl, 1878. 6 Pogg. Ann., 3848; see also Beer s treatise referred to al>ove, p. 250. 7 gee also p hiL May ^ ^7^ btf 1
salts freshly precipitated ; and generally, with like chemical properties of the metallic molecule, the molecular suscepti bility remains the same. 8 5. Two diamagnetic elements may give a magnetic com pound ; e.g., copper and bromine, both diamagnetic, give bromide of copper, which is paramagnetic. 6. When two solutions are mixed and the salts exchange their constituents by double decomposition, the specific magnetism of the solutions taken together is unchanged. Whence the conclusion is drawn that the susceptibility of a binary compound is made up by addition of the suscepti bilities of its constituents, and that these constituents pre serve their susceptibilities unaltered when their constitution or atomic arrangement in composition is unaltered. Magnecrystallic Action. In what precedes we supposed Magne- the inductively magnetized body, whether paramagnetic or crystal! diamagnetic, to be isotropic, and all experiments on its action> magnetic properties to be conducted in a heterogeneous magnetic field. In a uniform field such a body would be acted upon neither by force of translation nor by rotational couple. The case is otherwise if the body be magnetically adolotropic. In this case, according to the mathematical theory, (1) the body ought to set in a uniform field so as Two to place its axis of greatest magnetic permeability (i.e., of ^ mds greatest paramagnetic and of least diamagnetic suscepti- l bility) parallel to the lines of force, and (2) in a hetero geneous field Faraday s translational force from places of less to places of greater resultant force in the case of para magnetic, and from places of greater to places of less in the case of diamagnetic bodies, ought to be greatest when the axis of greatest susceptibility is parallel to the lines of force, least when the axis of least susceptibility is in the same position, and intermediate for other positions of the body. In observing the first class of phenomena above men- Approx tioned, poles with flat faces are placed on the electro- ma tely magnet. Faraday recommends that the faces should be fi"^ 011 placed at a distance of about one-third of their breadth. i, e tweei He warns the experimenter, however, that the uniformity flat pol with this arrangement is by no means perfect, although in use(1 f general sufficient. The best arrangement would be to use ^ tku the magnetic field in the interior of a cylindrical coil of mat r ne - sufficient length were it not for the difficulty of attaining crystal! the requisite intensity in this way. In cases where there is action, any doubt it is well to give the body under examination a spherical or cubical shape, and so eliminate the tendency to set arising from heterogeneity of field. The first observations of the magnecrystallic couple were Magne- made by Pliicker, 9 and elaborate investigations of the crystal] phenomenon were made by him in conjunction with Beer, 10 cou P le in the course of which the magnetic properties of a large covered number of crystalline bodies were examined. Pliicker also by detected the magnecrystallic property in a rapidly cooled Pluckei cylinder of glass. Shortly after Plucker s first results were published, Faraday discovered the magnecrystallic Farada; action of crystallized bismuth. At first, misled no doubt by the language in which Pliicker stated the newly dis covered facts, he did not recognize the identity of the two phenomena ; but on further investigation he was able to class all the observations under a few simple laws, 11 which in the mathematical form given to them by Thomson con stitute the theory already given. To the observations of Pliicker and Faraday Knoblauch and Tyndall added the Knob- important discovery that bodies in which the linear density lauch in one direction is greater than in another, whether as a ^. consequence of compression or of stratification artificial or natural, exhibit magnetic seolotropy. 8 For qualifications see Wied., Galv., I.e. 9 Pliicker, Pogg. Ann., 1847, 1848, 1849, 1852. 10 Pliicker and Beer, Porjg. Ann., 1850, 1851.
11 Exp. Res., 2797 sq., 1850.