MAGNETISM well vrlier suits, con- ant. Poisson himself, and demonstrated that Faraday s con ception of the phenomena was only another method of viewing the facts leading to identical conclusions. Faraday himself l seems in the end to have considered that the difference was a matter of phrases. Since Clerk Maxwell s elaborate mathematical reconstruction of the theories of Faraday this seems to be universally recognized, and the discussion has subsided. For a full account of the various interesting experiments that were made during the contro versy the reader may be referred to Wiedemann s Gal- vanismus, 558 sq., and to the reprint of Tyndall s Papers on Diamagnetism and Magnecrystallic Action, pp. 76 sq. Numerical Data respecting the Susceptibility of Weakly Magnetic Bodies. The earlier experimenters arrived for the most part at the conclusion that the susceptibility K of weakly magnetic bodies is constant. Among these may be mentioned Weber, who experimented with bismuth, E. Becquerel, 2 Tyndall, 3 Joule, 4 Reich, 5 and Matteucci, 6 who experimented on various substances by means of the torsion balance ; Christie, 7 who worked with bismuth, and Arndtsen, 8 who worked with ferric sulphate and ferric chloride, both using Weber s diamagnetometer ; and Wiedemann, 9 who experimented with solutions of a variety of salts. E. Becquerel, however, in some of his experi ments, e.g., with sulphate of nickel, found that K showed a tendency to decrease for very large values of the magnetizing force ; Pliicker, 10 who tested a great variety of substances (powdered or in solution) by measuring with a delicate balance the attraction or repulsion exerted upon them by an electromagnet, arrived at a similar conclusion ; but the methods of both these experimenters are open to suspicion. A large number of relative results were obtained by the earlier experimenters, 11 but in some cases the methods employed were not satisfactory, and in others the results so evidently depend on the state of aggregation of the material that they are of little importance. The following tables will give the reader some idea of the relative magnitudes of the susceptibilities of different substances : Pliickcr s Table for Magnetics. Iron 100,000 Magnetic iron ore 40,227 Ferric oxide 286 Hematite 134 Specular iron ore 533 Hydrated ferric oxide 156 Ferric sulphate Ill Green vitriol 78 Nitrate of iron, cone. soln. 34 The numbers here denote specific magnetic susceptibility equal weights of the substances are compared. Results of Faraday and Becquerel. Ferric chloride, cone. soln. Ferric sulphate ,, Ferrous chloride ,, Ferrous sulphate ., Nickelous oxide Hydrate of do Hydrated manganic oxide Mangano-mansanic oxide. 98 58 84 126 35 106 70 167 Ferrous chloride, cone. ) . ,, rK ! ,. > +65o solution ) Ammoniacal solution of cuprous oxide +134 Do. of cupric oxide + 120 Oxygen + 1 7 5 Air +3-4 Olefiant gas +0 6 Nitrogen +0 3 Vacuum O O Carbonic acid O O Hydrogen Ammonia gas .... Cyanogen Glass Pure zinc Ether Alcohol absolute Oil of lemons Camphor Camphire Linseed oil -0-1 -0-5 -0-9 -18-2 -75 -75 -79 -80 -82 -83 -85 1 See a letter to Matteucci dated November 2, 1855, published in Bence Jones s Life and Letters of Faraday, reprinted in Tyndall s Diamagnetism and Magnecrystallic Action, p. 180. 2 Ann. d. Chim. et d. Phys. 4 Phil. Mag., 1852. 6 Ann. d. Chim. et d. Phys., 1859. 8 Pogg. Ann., 1858. 10 Pogg. Ann., 1854. 11 E.g ., Plticker, Pogg. Ann., 1848, 1851, &c. ; E. Becquerel, Ann. d. Chim. et d. Phys., 1350, 1851, 1855, &c. ; Matteucci, Comptrs Rendus, 1853, and Cours d Induction, 1854 ; Wiedemann, Pogg. Ann., 1865, 1868, &c. 3 Phil. Mag., 1851. 5 Pogg. Ann., 1856. 7 Poyy. Ann., 1858. 9 Pogg. Ann., 1865. Chloride of arsenic - 122 Fused borate of lead - 137 Phosphorus* -167 Selenium* - 168 Pure copper* -171 Pure silver* - 235 Pure gold*.. -350 Bismuth -1967 Olive oil - 86 Wax -87 Nitric acid -88 Water - 96 6 Ammonia solution - 98 Bisulphide of carbon .... - 100 Sat. solution of nitre. ... -100 Sulphuric acid -104 Sulphur -118 The results marked with an asterisk are taken from Becquerel ; the rest are from Faraday. The numbers relate to equal volumes of the substances, and the medium is supposed to be vacuum ; so that water in air would be represented by 100. The numbers of Faraday, Becquerel, and Matteucci agree very fairly ; e.g., according to Faraday the susceptibilities of water, oxygen, and air are as - 100 : + 1 8 : + 352, according to Becquerel as - 100 : + 1 - 82 : + 382. Pliicker s results do not agree so well with those of Faraday and Becquerel ; but his method was faulty. Within the last five years a large number of absolute Recent determinations of K have been made, chiefly for bismuth and absolute ferric chloride. Toepler and Von Ettingshausen 12 in their c ^ e . ter - expsriments on bismuth used with some alteration the t j ons method of induced currents employed by Weber 13 in the Toepler earliest attempts that were made to determine the and v<.u susceptibility of bismuth. Like Weber they compare ^ bismuth with iron, an unsatisfactory procedure on account of the great variability of the susceptibility of iron for different magnetizing* forces, and for different samples with the same magnetizing force. Silow worked with ferric chloride. In his first set. of Silow. experiments 14 he observed the time of vibration of an astatic needle suspended over a cylindrical vessel filled with the solution ; in his second investigation 15 the solution was placed in a glass globe, on the outside of which insulated wire was wound so that a given current in it produced a uniform magnetic field whose strength could be calculated; 10 the deflexion of a properly astatized needle suspended inside the globe, was observed when the globe was empty and when it was full, and thence K. was calcu lated ; in his third determination 17 he used the method of Toepler and Von Ettingshausen so improved as to allow an absoluta determination of K to be obtained directly. Borgmann 18 enclosed one coil within another, and filled Borg- the hollow cylindrical space between them with the solution n.nn. of ferric chloride to be tested ; he also used the ring method of Stoletow and Rowland. Jacques, 19 following a method elaborated by Rowland, Jacques measured the repulsion of crystals of bismuth and Iceland an> l spar placed with their magnetic axes axially and equatori- r ally between the poles of a Ruhmkorff s electromagnet, the field of which was carefully explored after the manner of Verdet by means of a small coil moved through a known distance in different parts of it ; from these observations the two principal magnetic susceptibilities were calcu lated. Schuhmeister 20 experimented with ferric chloride, using grimh- the same method as Rowland and Jacques. master. In the experiments of Eaton 21 the method formerly employed by Wiedemann was adopted; the data in his paper are, however, insufficient for an absolute determina tion either of the magnetizing force or of K ; in fact he determines merely the force with which the magnetic body is attracted and the magnetic moment of the electromagnet, assuming that the strength of the magnetic field at a given point is proportional to the latter, which is not necessarily true, for the magnetic distribution in the core of the electromagnet may alter with increasing current. 12 Pogg. Ann., 1877. 13 EL Maatbcst., Thl. Hi. 14 Wied. Ann., 1877. 13 Wied. LeibL, 1879. 16 See Maxwell, EL and Mag., vol. ii. 672. 17 Wied. Ann., 1880. 18 Wied. BeiU., 1879. 19 Silliman s Jour., 1879. The published results are vitiated by some error of calculation ; but the experiments are to be repeated.
20 Wien. Ber., 1882. sl Wied. Ann., 1882.