1900.] On the Dynamical Theory of Heat and Light. 363 



WEEKLY EVENING MEETING, 



Friday, April 27, 1900. 



Sir Frederick Bramwell, Bart., D.C.L. LL.D. F.R.S., 

 Vice-President, in the Chair. 



The Right Hon. Lord Kelvik, G.C.V.O. D.C.L. 

 LL.D. F.R.S. M.R.L 



Nineteenth Century Clouds over the Dynamical Theory of 

 Heat and Light. 



[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 con- 

 clusions 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 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 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 op 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 6lowly as even the most rapid of the 

 heavenlv bodies. There are, however, many very serious objections 



2 b 2 



