336 PLtJCKER ON THE THEORY OF DIAM AGNETISM. 



uninduced bismuth must be regarded as a magnitude of the 

 third order, that has a theoretical but no experimental signi- 

 ficance. 



15. From considerations of which we shall speak afterwards, it 

 appeared to me probable that bismuth not only assumes polarity in 

 the vicinity of a magnetic pole, but that it also retains the polarity 

 for some time after the excitation has taken place ; or in other 

 words, that bismuth retains a portion of its magnetism perma- 

 nently, as steel, unlike soft iron, retains a portion of the mag- 

 netism excited in it by induction. My conjecture has been 

 corroborated by experiment. 



16. I hung a bar of bismuth 15 millims. long and 5 millims. 

 thick, between the pointed poles of the large electro-magnet ; 

 it was suspended horizontally from a double cocoon thread 

 (Plate V. fig. 5). The distance between the points was diminished 

 until the bar could barely swing freely between them. A little 

 bar of glass was brought near to one of the points, so that the 

 bismuth bar, before the magnetism was excited, in consequence 

 of the torsion of the fibre, leaned against the glass and was held 

 by the latter. On exciting the magnet by a current from three 

 of Grove's elements, the bismuth, prevented from assuming 

 the axial position, pressed more forcibly against the glass rod ; 

 when the current was interrupted the bar remained still in con- 

 tact with the glass rod, while its free end vibrated round its 

 position of equilibrium. The current was closed anew, and then 

 reversed by means of a gyrotrope. In consequence of this re- 

 version the bar of bismuth loosing from the glass rod moved 

 tow ards the axial position, but soon returned and pressed against 

 the glass as before, or in some cases having passed quite through 

 the axial position was driven round with reversed ends into the 

 equatorial. 



When the current was interrupted and then slowly closed 

 so as to reverse its direction, two or three seconds being devoted 

 to this purpose, the observed pha&nomenon did not exhibit itself. 



17. This experiment, which was made with some care, was 

 repeated with the same result, when a battery of ten of Grove's 

 elements was made use of. It proves that the bismuth requires 

 time to reverse its polarity. That end of the bar, which, being near 

 the north pole of the magnet, became itself a north pole by indue- 



