ON ETHER AND GRAVITATIONAL MATTER THROUGH 

 INFINITE SPACE.'' 



By Lord Kelvin. 



NOTE ox THE POSSIBLE DENSITY OF THE LUMINIFEROUS MEDIU:M AND ON 

 THE MECHANICAL VALUE OF A CUBIC MILE '' '' OF SUNLIGHT. '' 



Section 1. That there must be a raedium forming a continuous mate- 

 rial communication throughout space to the remotest visible body is 

 a fundamental assumption in the undulatorv theoiy of light. Whether 

 or not this medium is (as appears*" to me most probable) a continuation 

 of our own atmosphere, its existence is a fact that can not be ques- 

 tioned when the overwhelming evidence in favor of the undulatorj^ 

 theory is considered; and the investigation of its properties in every 

 possible wa}^ becomes an object of the greatest interest. A first ques- 

 tion would natural!}" occur, What is the absolute density of the lumi- 

 niferous ether in any part of space? I am not aware of an}' attempt 

 having hitherto been made to answer this question, and the present 

 state of science does not in fact afford sufficient data. It has, however, 

 occurred to me that we may assign an inferior limit to the density of 

 the luminiferous medium in interplanetary space by considering 

 the mechanical value of sunlight as deduced in preceding communica- 

 tions to the Royal Society ^ from Pouillet's data on solar radiation and 



"Reprinted from the London, Edinburgh, and Dublin Philosophical Magazine and 

 Journal of Science [sixth series], August, 1901, pp. 161-177. [This is an amplifica- 

 tion of Lecture XVI, Baltimore, October 15, 1884, now being prepared for print in a 

 volume on Molecular Dynamics and the Wave Theory of Light, which I hope may be 

 published within a year from the present time.] 



''Xote of December 22, 1892. — The brain-wasting perversity of the insular inertia 

 which still condemns British engineers to reckonings of miles and yards and feet and 

 inches and grains and pounds and ounces and acres is curiously illustrated by the 

 title and numerical results of this article a.s originally published. 



■^October 13, 1899. — In the present reproduction, as part of my Lecture XVI, of 

 Baltimore, 1884, I suggest cubic kilometer instead of "cubic mile" in the title, and 

 use the French metrical system exclusively in the article. 



-^From Edin. Royal Sod Trans., Vol. XXI, Part I, May, 1854; Phil. Mag., IX, 1854; 

 Comptes Rendus, XXXIX, Sept., 1854; Art. LXVII of Math, and Phys. Papers. 



''October 13, 1899. — Not so now. I did not in 1854 know the kinetic theory of 

 gases. 



'Trans. R. S. E. ; Mechanical Energies of the Solar System; republished as Art. 

 LXVI of Math, and Phvs. Papers. 



215 



