f Water and Steam,
in which the phenomena and principles of Sir William Armstrong's
Hydro-electric machine are described and developed; a paper on
Magnetic Rotations, and Faraday's letters in relation to the
controversy it aroused. The contribution of most permanent value
here, is that on the Source of Power in the Voltaic Pile. By it the
Contact Theory, pure and simple, was totally overthrown, and the
necessity of chemical action to the maintenance of the current
demonstrated.

The third volume of the Researches opens with a memoir entitled 'The
Magnetisation of Light,' and the Illumination of Magnetic Lines of
Force.' It is difficult even now to affix a definite meaning to this
title; but the discovery of the rotation of the plane of polarisation,
which it announced, seems pregnant with great results. The writings
of William Thomson on the theoretic aspects of the discovery; the
excellent electrodynamic measurements of Wilhelm Weber, which are
models of experimental completeness and skill; Weber's labours in
conjunction with his lamented friend Kohlrausch--above all, the
researches of Clerk Maxwell on the Electro-magnetic Theory of
Light--point to that wonderful and mysterious medium, which is the
vehicle of light and radiant heat, as the probable basis also of
magnetic and electric phenomena. The hope of such a connection was
first raised by the discovery here referred to. [Footnote: A letter
addressed to me by Professor Weber on March 18 last contains the
following reference to the connection here mentioned: 'Die Hoffnung
einer solchen Combination ist durch Faraday's Entdeckung der
Drehung er Polarisationsebene durch magnetische Directionskraft
zuerst, und sodann durch die Uebereinstimmung derjenigen
Geschwindigkeit, welche das Verhaeltniss der electro-dynamischen
Einheit zur lectro-statischen ausdrueckt, mit der Geschwindigkeit
des Lichts angeregt worden; und mir scheint von allen Versuchen,
welche zur erwirklichung dieser Hoffnung gemacht worden sind, das von
Herrn Maxwell gemachte am erfolgreichsten.'] Faraday himself seemed to
cling with particular affection to this discovery. He felt that there
was more in it than he was able to unfold. He predicted that it would
grow in meaning with the growth of science. This it has done; this it
is doing now. Its right interpretation will probably mark an epoch in
scientific history.

Rapidly following it is the discovery of Diamagnetism, or the
repulsion of matt