Lord Kelvin (1824-1907)

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XV.—On the Dynamical Theory of Heat, with numerical results deduced from Mr
JOULE's equivalent of a Thermal Unit, and M. REGNAULT's Observations on
Steam. By WILLIAM THOMSON, M.A., Fellow of St Peter's College, Cam-
bridge, and Professor of Natural Philosophy in the University of Glasgow.

                                                     (Read 17th March 1851.)

                                     INTRODUCTORY NOTICE.

1. SIR HUMPHREY DAVY, by his experiment of melting two pieces of ice by
rubbing them together, established the following proposition:—“The phenomena
of repulsion are not dependent on a peculiar elastic fluid for their existence, or
caloric does not exist.” And he concludes that heat consists of a motion excited
among the particles of bodies. “To distinguish this motion from others, and to
signify the cause of our sensation of heat,” and of the expansion or expansive
pressure produced in matter by heat, “the name repulsive motion has been
adopted.”*

2. The Dynamical Theory of Heat, thus established by SIR HUMPHREY DAVY,
is extended to radiant heat by the discovery of phenomena, especially those of
the polarization of radiant heat, which render it excessively probable that heat
propagated through vacant space, or through diathermane substances, consists of
waves of transverse vibrations in an all-pervading medium.

3. The recent discoveries made by MAYER and JOULE,† of the generation of
heat through the friction of fluids in motion, and by the magneto-electric excita-
tion of galvanic currents, would, either of them be sufficient to demonstrate the
immateriality of heat; and would so afford, if required, a perfect confirmation of
SIR HUMPHREY DAVY's views.

* From DAVY's first work, entitled “An Essay on Heat, Light, and the Combinations of
Light,” published in 1799, in “Contributions to Physical and Medical Knowledge, principally from
the West of England, collected by THOMAS BEDDOES, M.D.,” and republished in Dr DAVY's edition of
his brother's collected works, vol. ii. Lond. 1836.

† In May 1842, MAYER announced in the “Annalen” of WÖHLER, and LIEBIG, that he had
raised the temperature of water from 12° to 13° cent, by agitating it. In August 1843, JOULE
announced to the British Association, “That heat is evolved by the passage of water through
narrow tubes;” and that he had “obtained one degree of heat per lb. of water from a mechanical
force capable of raising 770 lbs. to the height of one foot;” and that heat is generated when
work is spent in turning a magneto-electric machine, or an electro-magnetic engine. (See his
paper “On the Calorific Effects of Magneto-Electricity, and on the Mechanical Value of Heat.”
Phil. Mag. vol. xxiii. 1843.)

VOL. XX. PART II.

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