CHAP. VII., 5.] 



ELECTRICITY. DR FARADAY M. PLUCKER. 



183 



(828.) 

 Obscure 

 anticipa- 

 by Brug- 

 mans and 

 Lebaillif. 



(829.) 

 Father 

 Bancalari 

 discovers 

 the dia- 

 magnetism 

 of flames. 



(830.) 

 Dr Fara- 

 day on the 

 magnetism 

 of oxygen. 



(831.) 

 Professor 

 PI ticker on 

 magne- 

 optic force 



acquire a true polar condition under the action of 

 magnets, but opposed to that which iron and para- 

 magnetics do in like circumstances. 



Like almost every other great discovery, some 

 feeble traces of this may be found amongst the vo- 

 luminous records of almost forgotten experiments. 

 Brugmans observed, in 1778, the repulsion of bis- 

 muth by a magnet, which was rediscovered by Le- 

 baillif in 1827 ; and something like the equatoreal 

 pointing of shell-lac and wood was noticed in the 

 same year by Becquerel, and may also be traced in 

 the writings of Coulomb. The present writer recol- 

 lects very distinctly to have had pointed out to him 

 by M. Becquerel, at Paris, about 1835, the pointing 

 of minute chips of wood placed near a common steel 

 magnet. But these incidental facts having been suf- 

 fered to remain in complete obscurity for so many 

 years, without even an attempt to connect and ex- 

 plain them, can scarcely be said even to touch the 

 originality of Dr Faraday's discovery. 



The year after the announcement of diamagnet- 

 ism, Father Bancalari of Genoa discovered the 

 powerful diamagnetic quality of flame. It is easily 

 shown by placing the flame of a wax taper between 

 two blunt conical terminations of a powerful electro- 

 magnet. The flame spreads equatoreally, becoming 

 fish-tailed. Dr Faraday, zealously taking up the in- 

 quiry, proved that this depends upon hot air being 

 diamagnetic relatively to the surrounding cold air, 

 but that atmospheric air is always absolutely para- 

 magnetic. Analyzing the effect still farther, he 

 found that the oxygen of the air is a very powerful 

 paramagnetic, whilst nitrogen is relatively diamag- 

 netic, but in all probability is a neutral substance, 

 one at the real zero of this singular scale. By a most 

 ingenious application of the torsion balance, he was 

 enabled to compare the relative actions of magnet- 

 ism on the gases with admirable skill and precision. 

 (Philosophical Transactions, 1851.) 



These experiments demonstrate a paramagnetic 

 (or iron - like) attraction in oxygen really aston- 

 ishing. A small mass of oxygen appears to be 

 attracted at the distance of an inch from the axial 

 line of the electro-magnet by a force equal to 

 its own weight ! Since heat diminishes this qua- 

 lity, the acute perception of Dr Faraday rapidly en- 

 tertained the idea that the apparent magnetism of 

 the earth might partly at least reside in the atmo- 

 sphere, and that the changes of terrestrial magnetic 

 intensity and direction might be explained by the 

 action of the sun expanding the atmosphere. The 

 26th series of his researches is devoted to an elabo- 

 rate exposition of this theory, which, however inge- 

 nious, is still involved in great difficulty. 



Professor Plilcker Attraction and Repulsion of 

 Optic Axis of Crystals. Soon after Dr Faraday's dis- 

 covery of diamagnetism, Professor Pliicker, of Bonn, 

 .announced the important fact, that the optic axis of 

 Iceland spar is repelled by the magnet. A sphere 



(833.) 



of that substance suspended by a thread, and having 

 its axis horizontal, being placed between the poles, 

 notwithstanding the perfect symmetry of external 

 figure, the axis ranges itself in the equatoreal position. 

 In some other crystals the axis is directed in the 

 line of poles. The law of the phenomena is not yet 

 completely made out ; but so strong is the latter qua- 

 lity in crystals of kyanite, that a piece of that sub- 

 stance properly suspended will actually show a direc- 

 tive power under the influence of the earth's mag- 

 netism. 



Probably closely allied to this fact is a similar (832.) 

 directive tendency observed in well crystallized spe- Magne- 

 cimens of bismuth, antimony, and arsenic. This ryst 

 M. Pliicker calls the magne-crystallic, as the for- 

 mer may be termed the magne-optic force ; and 

 it is often so intense as to oppose and even reverse 

 the directive tendency which the body would have 

 had between the poles in its massive or uncrystal- 

 lized state. 



It will be easily conceived that the interest crea- 

 ted by these admirable discoveries, revealing not only 

 new and general properties of matter, but also rela- 

 tions between very different branches of science, soon 

 became general, and raised the reputation of Dr 

 Faraday to the very highest rank as an experimental 

 philosopher. If we compare his two greatest works, 

 that on magneto-electricity, and this on diamag- 

 netism, we find in the former perhaps a more perfect 

 specimen of inductive sequences ; in the latter, facts 

 more independently novel and unlooked-for, and an 

 unrivalled skill in the application and invention of 

 experimental methods. 



Passing over many less important matters, there 

 yet remains one interesting discovery to be men- Dr Fara- 

 tioned, which in point of time preceded the last, da ^ 

 namely changes 



IV. Optical Changes induced by Magnetism. induced by 

 In his 19th Series of Researches, published in 1845, ma S netism - 

 Dr Faraday announced " The Magnetization of 

 Light and the Illumination of the Magnetic Lines of 

 Force," a title which, though intended to express 

 exactly the author's idea of his discovery, perhaps 

 excited undue anticipations in the public mind. For 

 here we have no direct, or even apparently direct, 

 action of the magnetic force on a luminous ray, but 

 only that a peculiar state is induced by magnetism 

 in some transparent bodies which produces an action 

 on light which they did not possess before, and which, 

 indeed, differed in some respects from any similar 

 action previously recognised. 



Dr Faraday's leading experiment is the following : 

 A piece of "heavy glass," or siliceous borate o 

 lead, was placed lengthwise between the poles of ap i a riza- 

 powerful electro-magnet. A ray of plane-polarized of a ray of 

 light was transmitted through the glass parallel to h S ht - 

 the line of the magnetic poles ; when the magnetic 

 energy was fully applied, the plane of polarization of 

 the light was found to have twisted round, similarly 



(834.) 



(835.) 



of 



