268 Prof. Tyudall on the Nature of the Force by which 



of the second force, tlie bar swings like a pendulum beyond this 

 position, oscillates round it, and finally comes to i-est there. 

 Hence, if before the application of the second force the bar occupy 

 the axial position, the deflection, when the second force is applied, 

 appears to be from the axis to the equator ; but if it first occupy 

 the equatorial position, the deflection appears to be from the 

 equator to the axis. 



We have already shown that the repulsion of diamagnetic 

 bodies is to be referred to a state of excitement induced by the 

 magnet which acts upon them : it has been long known that the 

 attraction of paramagnetic bodies is due to the same cause. The 

 experiments just described exhibit to us bars of both classes of 

 bodies moving in the magnetic field : such motions occur in 

 virtue of the induced state of the body, and the relation of that 

 state to the forces which act upon the mass. We have seen that 

 in all cases the antithesis between both classes of bodies is main- 

 tained. Whatever, therefore, the state of the paramagnetic bar 

 under magnetic excitement may be, a precisely antithetical state 

 would produce all the phsenomena of the diamagnetic bar. If the 

 bar of ix'on be polar, a reverse polarity on the part of bismuth would 

 produce the effects observed. From this point of view all the 

 movements of diamagnetic bodies become perfectly intelligible, 

 and the experiments to be recorded in the next chapter are not 

 calculated to diminish the probability of the conclusion that dia- 

 magnetic bodies possess a polarity opposed to that of magnetic 

 ones. 



The phsenomena to which we have thus far referred consist in 

 the rotations of elongated bars about their axes of suspension. 

 The same antithesis, however, presents itself when we compare 

 the motion of translation of a paramagnetic body, within the coil, 

 with that of a diamagnetic one. A paramagnetic sphere was 

 attached to the end of a horizontal beam and introduced into the 

 coil : the magnet being excited, the sphere could be made to 

 traverse the space within the coil in various directions, by pro- 

 perly varying the current through the coil. A diamagnetic 

 sphere was submitted to the same examination, and it was found 

 that the motions of both spheres, when operated on by the same 

 foi'ceSj were always in opposite directions. 



V. Further Comparison of Paramagnetic and Diamag- 

 netic PHiENOMENA : DiAMAGNETIC POLARITY. 



When an iron bar is placed within a helix, it is well known 

 that on sending a current through the latter the bar is converted 

 into a magnet, one end of the bar thus excited being attracted, and 

 the other end repelled by the same magnetic pole. In this two- 

 ness of action consists what is called the polarity of the bar : we 



