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I. & II. Illustrative of Dr. Meyer Wilderman's Paper on the Velocity of Reaction before Complete Equilibrium and before the Point of Transition, &c.

III. Illustrative of Prof. S. P. Langley's Paper on the New Spectrum. IV. Illustrative of Mr. J. Rose-Innes and Prof. S. Young's Paper on the Thermal Properties of Isopentane compared with those of Normal Pentane.

V. Illustrative of Prof. J. A. Fleming and Mr. A. W. Ashton's Paper on a Model which Imitates the Behaviour of Dielectrics.

VI. Illustrative of Prof. J. Trowbridge's Paper on the Spectra of Hydrogen, and some of its Compounds.

VII-IX, Illustrative of Prof. T. W. Richards and Mr. E. H. Archibald's Paper on the Study of Growing Crystals by Instantaneous Photomicrography.


Page 532, last line of footnote, for vol. xiv. read vol. xv.







JULY 1901.

I. Nineteenth Century Clouds over the Dynamical Theory of Heat and Light*. By The Right. Hon. Lord KELVIN, G.C.V.O., D.C.L., LL.D., F.R.S., M.R.I. †.

[IN the present article, the substance of the lecture is reproduced with large additions, in which work commenced at the beginning of last year and continued after the lecture, during thirteen months up to the present time, is described-with results confirming the conclusions and largely extending the illustrations which were given in the lecture. I desire to take this opportunity of expressing my obligations to Mr. William Anderson, my secretary and assistant, for the mathematical tact and skill, the accuracy of geometrical drawing, and the unfailingly faithful perseverance in the long-continued and varied series of drawings and algebraic and arithmetical calculations, explained in the following pages. The whole of this work, involving the determination of results due to more than five thousand individual impacts, has been performed by Mr. Anderson.— K., Feb. 2, 1901.]

§ 1. THE

HE beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds. I. The first came into existence with the undulatory theory of light, and

Lecture delivered at the Royal Institution of Great Britain, on Friday, April 27, 1900.

+ Communicated by the Author.

Phil. Mag. S. 6. Vol. 2. No. 7. July 1901.


was dealt with by Fresnel and Dr. Thomas Young; it involved the question, How could the earth move through an elastic solid, such as essentially is the luminiferous ether? II. The second is the Maxwell-Boltzmann doctrine regarding the partition of energy.

$2.-CLOUD I.-RELATIVE MOTION OF ETHER AND PONDERABLE BODIES; such as movable bodies at the earth's surface, stones, metals, liquids, gases; the atmosphere surrounding the earth; the earth itself as a whole; meteorites, the moon, the sun, and other celestial bodies. We might imagine the question satisfactorily answered, by supposing ether to have practically perfect elasticity for the exceedingly rapid vibrations, with exceedingly small extent of distortion, which constitute light; while it behaves almost like a fluid of very small viscosity, and yields with exceedingly small resistance, practically no resistance, to bodies moving through it as slowly as even the most rapid of the heavenly bodies. There are, however, many very serious objections to this supposition; among them one which has been most noticed, though perhaps not really the most serious, that it seems incompatible with the known phenomena of the aberration of light. Referring to it, Fresnel, in his celebrated letter to Arago, wrote as follows:


"Mais il paraît impossible d'expliquer l'aberration des "étoiles dans cette hypothèse ; je n'ai pu jusqu'à présent "du moins concevoir nettement ce phénomène qu'en supposant que l'éther passe librement au travers du globe, "et que la vitesse communiquée à ce fluide subtil n'est qu'une petite partie de celle de la terre; n'en excède pas "le centième, par exemple.



Quelque extraordinaire que paraisse cette hypothèse au "premier abord, elle n'est point en contradiction, ce me "semble, avec l'idée que les plus grands physiciens se sont "faite de l'extrème porosité des corps."

The same hypothesis was given by Thomas Young, in his celebrated statement that ether passes through among the molecules or atoms of material bodies like wind blowing through a grove of trees. It is clear that neither Fresnel nor Young had the idea that the ether of their undulatory theory of light, with its transverse vibrations, is essentially an elastic solid, that is to say, matter which resists change of shape with permanent or sub-permanent force. If they had

*Annales de Chimie, 1818; quoted in full by Larmor in his recent book, Æther and Matter,' pp. 320-322.

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