154 Prof. Tyndall on the Nature of the Force by which 



and to the force which produced this repulsion Mr. Faraday 

 gave the name of diamagnetism. 



Numerous other substances were soon added to the heavy 

 glass, and, among the metals, it was found that bismuth pos- 

 sessed the new property in a comparatively exalted degree. A 

 fragment of this substance was forcibly repelled by either of the 

 poles of a magnet; while a thin bar of the substance, or a glass tube 

 containing the bismuth in fragments, or in powder, suspended 

 between the two poles of a horseshoe magnet, behaved exactly 

 like the heavy glass, and set its longest dimension equatorial. 



These exhaustive researches, which rendered manifest to the 

 scientific world the existence of a pervading natural force, 

 glimpses of which merely had been previously obtained by 

 Brugmann and others, were made public in 1846; and in the 

 follomng year M. Pliicker announced his beautiful discovery 

 of the action of a magnet upon crystallized bodies. His first 

 result was, that when any crystal whatever was suspended 

 between the poles of a magnet, with its optic axis horizontal, a 

 repulsive force was exerted on the said axis, in consequence of 

 which it receded from the poles and finally set itself at right 

 angles to the line joining them. Subsequent experiments, 

 however, led to the conclusion, that the axes of optically 

 negative crystals only experienced this repulsion, while the axes 

 of positive crystals were attracted ; or, in other words, set them- 

 selves from pole to pole. 



The attraction and repulsion, here referred to, were ascribed 

 by M. Pliicker to the action of a new force, entirely independent 

 of the magnetism or diamagnetism of the mass of the crystal*. 



* " The force which produces this repulsion is independent of the magnetic 

 or diamagnetic condition of the mass of the crystal ; it diminishes less, as 

 the distance from the poles of the magnet increases, than the ynagnetic and 

 diamagnetic forces emanating from these poles and acting upon the crystal." 

 — Prof. Pliicker in PoggendorfF's Annalen, vol. Ivii. No. 10 j Taylor's 

 Scientific Memoirs, vol. v. p. 353. 



The forces emanating from the poles of a magnet are thus summed up 

 by M. Pliicker :— 



1st. The magnetic force in a strict sense. 



2nd. The diamagnetic action discovered by Faraday. 



3rd. The action exerted on the optic axes of crystals (and that producing 

 the rotation of the plane of polarization which ])robably corresponds to it). 

 The second diminishes more with the distance than the first, and the first 

 more than the third. — Taylor's Scientific Memoirs, vol. v. p. 380. 



The crystal (cyauite) does not point according to the magnetism of its 

 substance, but only in obedience to the magnetic action upon its optic axes. — 

 Letter to Mr. Faraday, Phil. Mag. vol. xxxiv. p. 451. The itahcs in all 

 cases are M. Pliicker's own. 



M. De la Rive states the view of M. Pliicker to be : — that the axis in its 

 quality as axis, and inde])eudently of the very nature of the substance of 

 the crystal, enjoys peculiar properties, more frequently in opposition to 



