128 Prof, Tyndall on the relation of Diamagnetic Polarity 



stand with its length equatorial, or its magnecrystallic axis parallel 

 to the lines of force. When one pole was removed and the experi- 

 ment repeated, the same effect took place, but not so strongly as 

 before ; finally, when the pole was brought as near to the crystal 

 as it could be without touching it, the same i-esult occurred, and 

 with more strength than in the last case. In the two latter 

 experiments, therefore, the ciystal of sulphate of iron, though a 

 magnetic body, and strongly attracted by such a magnet as that 

 used, actually receded from the pole of the magnet under the 

 influence of the magnecrystallic condition. 



"If the pole S* be removed and that marked N be retained 

 for action on the crystal, then the latter approaches the pole 

 urged by both the magnetic and magnecrystallic foi'ces ; but if 

 the crystal be revolve'd 90° to the left, or 180° to the right, 

 round the silken axis, so as to come into the contrary or oppo- 

 site position, then this pole rei)els or rather causes the removal 

 to a distance of the crystal, just as the former did. The experi- 

 ment requires care, and I find that conical poles are not good ; 

 but with attention I could obtain the results with the utmost 

 readiness. 



" The sulphate of iron was then replaced by a crystalline plate 

 of bismuth, placed, as before, on one side of the silk suspender, 

 andwithits magnecrystallic axis horizontalf. Makingthe position 

 the same as that which the crystal had in relation to the N pole 

 in the former experiment, so that to place its axis parallel to the 

 lines of magnetic force it must approach this magnetic ])ole, and 

 then throwing the magnet into an active state, the bismuth 

 moved accordingly and did approach the pole, against its dia- 

 magnetic tendency, but under the influence of the magnecry- 

 stallic force. 



" Hence a proof that neither attraction nor repulsion governs 

 the set. ..... This force, then, is distinct in its character and 



effects fi'om the magnetic and diamagnetic forms of force." 



These experiments present grave mechanical difficulties, and 

 are quite sufficient to justify the conclusion drawn from them, 

 namely, that the force which produces them is neither attractive 

 nor repulsive. We will now endeavour to apply the idea of a force 

 which is botli attractive and repulsive, or in other words of a polar 

 force, to the solution of the difficulty. 



* The fiijures will be given and explained further on. 



t It will be borne in mind that Mr. Faraday calls the line in a crystal 

 which sets from pole to pole, the magnecrystallic axis of the crystal, whether 

 the latter is paramat^netie or diamagnetic. In bodies of the former class, 

 however, the " axis " sets fiora pole to pole because the attraction along it 

 is a maximum ; while in bodies of the latter class, the " axis " sets from 

 pole to pole because the repulsion along the line perpendicular to it is a 

 maximimi. 



