in Faraday's experiment
it is the magnetic strain that produces the rotation of the plane of
polarization.[2]

Footnotes to Chapter 10

[1] 'By a diamagnetic,' says Faraday, 'I mean a body through
which lines of magnetic force are passing, and which does
not by their action assume the usual magnetic state of iron
or loadstone.' Faraday subsequently used this term in a
different sense from that here given, as will immediately
appear.

[2] The power of double refraction conferred on the centre
of a glass rod, when it is caused to sound the fundamental
note due to its longitudinal vibration, and the absence of
the same power in the case of vibrating air (enclosed in a
glass organ-pipe), seems to be analogous to the presence and
absence of Faraday's effect in the same two substances.
Faraday never, to my knowledge, attempted to give, even in
conversation, a picture of the molecular condition of his
heavy glass when subjected to magnetic influence. In a
mathematical investigation of the subject, published in the
Proceedings of the Royal Society for 1856, Sir William
Thomson arrives at the conclusion that the 'diamagnetic' is
in a state of molecular rotation.

Chapter 11.

Discovery of diamagnetism--researches on magne-crystallic
action.

Faraday's next great step in discovery was announced in a memoir on the
'Magnetic Condition of all matter,' communicated to the Royal Society on
December 18, 1845. One great source of his success was the employment
of extraordinary power. As already stated, he never accepted a negative
answer to an experiment until he had brought to bear upon it all the
force at his command. He had over and over again tried steel magnets and
ordinary electro-magnets on various substances, but without detecting
anything different from the ordinary attraction exhibited by a few of
them. Stronger coercion, however, developed a new action. Before the
pole of an electro-magnet, he suspended a fragment of his famous heavy
glass; and observed that when the magnet was powerfully excited the
glass fairly retreated from the pole. It was a clear case of magnetic
repulsion. He then suspended a bar of the glass between two poles;
the bar retreated when the poles were excited, and set its length
equatorially or at right angles to the line joining them. When an
ordinary magnetic body was similarly su