Lord Kelvin. Nineteenth-Century Clouds over the Dynamical Theory of Heat and Light. // Phil. Mag. S. 6. Vol. 2. No. 7. July 1901.

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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.—Oloud 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 parait impossible d’expliquer ^aberration des “ 6toiles dans cette hypothese ; je n'ai pu jusqu'a present “ du moins concevoir nettement ce phenomene qu'en sup-“ posant que F ether passe librement au travers du globe, “ et que la vitesse communiquee a ce fluide subtil n’est “ qu'une petite partie de celle de la terre ; n'en exc&de pas u le centieme, par exemple.

“ Quelque extraordinaire que paraisse cette hypothese au u premier abord, elle n’est point en contradiction, ce me “ semble, avec l’idee que les plus grands physiciens se sont

faite de l’extreme porosite 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

* Annates de Chimie, 1818; quoted in full by Larmor in his recent book, ‘ iEther and Matter/ pp. 320-322.

grasped this idea, they must have noticed the enormous difficulty presented by the laceration which the ether must experience if it moves through pores or interstices among the atoms of matter.

§ 3. It has occurred to me that, without contravening anything we know from observation of nature, we may simply deny the scholastic axiom that two portions of matter cannot jointly occupy the same space, and may assert, as an admissible hypothesis, that ether does occupy the same space as ponderable matter, and that ether is not displaced by ponderable bodies moving through space occupied by ether. But how then could matter act on ether, and ether act on matter, to produce the known phenomena of light (or radiant heat), generated by the action of ponderable bodies 011 ether, and acting on ponderable bodies to produce its visual, chemical, phosphorescent, thermal, and photographic effects ? There is no difficulty in answering this question if, as it probably is, ether is a compressible and dilatable * solid. We have only to suppose that the atom exerts force on the ether, by which condensation or rarefaction is produced within the space occupied by the atom. At present t I .confine myself, for the sake of simplicity, to the suggestion of a spherical atom producing condensation and rarefaction, with concentric spherical surfaces of equal density, but the same total quantity of ether within its boundary as the quantity in an equal volume of free undisturbed ether.

§ 4. Consider now such an atom given at rest anywhere in space occupied by ether. Let force be applied to it to cause it to move in any direction, first with gradually increasing speed, and after that with uniform speed. If this speed is anything less than the velocity of light, the force may be mathematically proved to become zero at some short time after the instant when the velocity of the atom becomes uniform, and to remain zero for ever thereafter. What takes place is this :

§ 5. Daring all the time in which the velocity of the atom is being augmented from zero, two sets of non-periodic waves, one of them equi-voluminal, the other irrotational (which is therefore condensational-rarefactional), are being sent out in

* To deny this property is to attribute to ether infinitely great resistance against forces tending to condense it or to dilate it—which seems, in truth, an infinitely difficult assumption.

t Further developments of the suggested idea have been contributed ±0 the Royal Society of Edinburgh, and to the Congres International de Physique, held in Paris in August. (Proc. R.S.E. July 1900; vol. of reports, in French, of the Cong. Inter.; and Phil. Mag., Aug:., Sept., 1900.)

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