2 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
The difference between this new action, and the ordinary magnetic action, was 
further manifested, when a fragment of the heavy glass was suspended before a single 
magnetic pole : the fragment was repelled when the magnetism was excited ; 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 possessed 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 pre- 
viously obtained by Brugmann and others, were made public in 1846; and in the 
following year M. Plucker made known his beautiful discovery of the action of a 
magnet upon crystallized bodies. His first result was, that when any crystal what- 
ever 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 themselves from pole to pole. 
The attraction and repulsion, here referred to, were ascribed by M. Plucker to the 
action of a new force, entirely independent of the magnetism or diamagnetism of the 
mass of the crystal Shortly after the publication of M. Plucker’s first memoir, 
Mr. Faraday observed the remarkable magnetic properties of crystallized bismuth ; 
and his researches upon this, and one or two kindred points, formed the subject of 
the Bakerian Lecture before the Royal Society for the year 1849. 
* “ 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 magnetic and dia- 
magnetic forces emanating from these poles and acting upon the crystal." — Prof. Plucker in Poggendorjf’s An- 
nalen, vol. Ivii. No. 10; Taylor’s Scientific Memoirs, vol. v. p. 353, 
The forces emanating from the poles of a magnet are thus summed up by M. Plucker ; — 
1st. The magnetic force in a strict sense. 
2nd. The diamagnetic action discovered by Faraday. 
3rd. The action exerted on the optic axis of crystals (and that producing the rotation of the plane of polar- 
ization which probably 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 (cyanite) 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 italics in all 
cases are M. Plucker’s own. 
M. De la Rive states the view of M. Plucker to be : — “ that the axis in its quality as axis, and indepen- 
dently of the very nature of the substance of the crj^stal, enjoys peculiar properties, more frequently in oppo- 
sition to those possessed by the substance itself, or which at least are altogether independent of it.”— Treatise 
on Electricity, vol. i. p. 359. 
