’ 
Cuar. VIL, § 5.] 
acquire a true polar condition under the action of 
magnets, but opposed to that which iron and para- 
magnetics do in like circumstances. 
(828.) Like almost every other great discovery, some 
a feeble traces of this may be found amongst the vo- 
by mae. luminous records of almost forgotten experiments. 
mans and 
Brugmans observed, in 1778, the repulsion of bis- 
Lebaillif. Hee P ; 
muth by a magnet, which was rediscovered by Le- 
baillif in 1827 ; and something like the equatoreal 
pointing of shell-lac and wood was noticed in the 
same year by Becquerel, and may also be traced in 
the writings of Coulomb. The present writer recol- 
lects very distinctly to have had pointed out to him 
by M. Becquerel, at Paris, about 1835, the pointing 
of minute chips of wood placed near a common steel 
magnet. But these incidental facts having been suf- 
fered to remain in complete obscurity for so many 
years, without even an attempt to connect and ex- 
plain them, can scarcely be said even to touch the 
originality of Dr Faraday’s discovery. 
The year after the announcement of diamagnet- 
Sat ism, Father Bancalari of Genoa discovered the 
discovers POWerful diamagnetic quality of flame. It is easily 
thedia- shown by placing the flame of a wax taper between 
magnetism ¢wo blunt conical terminations of a powerful electro- 
—* magnet. The flame spreads equatoreally, becoming 
jish-tailed, Dr Faraday, zealously taking up the in- 
quiry, proved that this depends upon hot air being 
diamagnetic relatively to the surrounding cold air, 
but that atmospheric air is always absolutely para- 
magnetic, Analyzing the effect still farther, he 
found that the oxygen of the air is a very powerful 
paramagnetic, whilst nitrogen is relatively diamag- 
netic, but in all probability is a neutral substance, 
one at the real zero of this singular scale. By a most 
ingenious application of the torsion balance, he was 
enabled to compare the relative actions of magnet- 
ism on the gases with admirable skill and precision. 
(Philosophical Transactions, 1851.) 
These experiments demonstrate a paramagnetic 
(or iron -like) attraction in oxygen really aston- 
= eda ishing. A small mass of oxygen appears to be 
of oxygen. attracted at the distance of an inch from the axial 
line of the electro-magnet by a force equal to 
its own weight! Since heat diminishes this qua- 
lity, the acute perception of Dr Faraday rapidly en- 
tertained the idea that the apparent magnetism of 
the earth might partly at least reside in the atmo- 
sphere, and that the changes of terrestrial magnetic 
intensity and direction might be explained by the 
action of the sun expanding the atmosphere. The 
26th series of his researches is devoted to an elabo- 
rate exposition of this theory, which, however inge- 
nious, is still involved in great difficulty. 
(829.) 
(830,) 
Dr Fara- 
_ (831.) Professor Pliicker—Attraction and Repulsion of 
ema Optic Axis of Crystals—Soon after Dr Faraday’s dis- 
magne- covery of diamagnetism, Professor Pliicker, of Bonn, 
optic force. announced the important fact, that the optic axis of 
Iceland spar is repelled by the magnet. A sphere 
ELECTRICITY.—DR FARADAY—M. PLUCKER. 
981 
of that substance suspended by a thread, and having 
its axis horizontal, being placed between the poles, 
notwithstanding the perfect symmetry of external 
figure, the axis ranges itself in the equatoreal position. 
In some other crystals the axis is directed in the 
line of poles. The law of the phenomena is not yet 
completely made out; but so strong is the latter qua- 
lity in erystals of kyanite, that a piece of that sub- 
stance properly suspended will actually show a direc- 
tive power under the influence of the earth’s mag- 
netism. 
Probably closely allied to this fact is a similar 832.) 
directive tendency observed in well crystallized spe- Magne- 
cimens of bismuth, antimony, and arsenic, This rhe se 
M. Pliicker calls the magne-crystallic, as the for- 
mer may be termed the magne-optic force; and 
it is often so intense as to oppose and even reverse 
the directive tendency which the body would have 
had between the poles in its massive or uncrystal- 
lized state. 
Tt will be easily conceived that the interest crea- 
ted by these admirable discoveries, revealing not only 
new and general properties of matter, but also rela- 
tions between very different branches of science, soon 
became general, and raised the reputation of Dr 
Faraday to the very highest rank as an experimental 
philosopher. If we compare his two greatest works, 
that on magneto-electricity, and this on diamag- 
netism, we find in the former perhaps a more perfect 
specimen of inductive sequences ; in the latter, facts 
more independently novel and unlooked-for, and an 
unrivalled skill in the application and invention of 
experimental methods. 
Passing over many less important matters, there 834.) 
yet remains one interesting discovery to be men- Dr Fara- 
tioned, which in point of time preceded the last, 2*Y.% 
namely— SEoaews 
IV. Optical Changes induced by Magnetism.— induced by 
In his 19th Series of Researches, published in 1845, magnetism. 
Dr Faraday announced “The Magnetization of 
Light and the Illumination of the Magnetic Lines of 
Force,”—a title which, though intended to express 
exactly the author’s idea of his discovery, perhaps 
excited undue anticipations in the public mind. For 
here we have no direct, or even apparently direct, 
action of the magnetic foree on a luminous ray, but 
only that a peculiar state is induced by magnetism 
in some transparent bodies which produces an action 
on light which they did not possess before, and which, 
indeed, differed in some respects from any similar 
action previously recognised, 
Dr Faraday’s leading experiment is the following : 
—A piece of “heavy glass,” or siliceous borate of oiaeee - 
lead, was placed lengthwise between the poles of a polariza- 
powerful electro-magnet. A ray of plane-polarized of a ray of 
light was transmitted through the glass parallel to light. 
the line of the magnetic poles; when the magnetic 
energy was fully applied, the plane of polarization of 
the light was found to have twisted round, similarly 
(833.) 
(835.) 
