ON ETHER AND GRAVITATIONAL MATTER THROUGH 



INFINITE SPACE/ 



By Lord Kelvin. 



NOTE ON THE POSSIBLE DENSITY OF THE LUMIMI EHOUS MEDIUM AND ON 

 THE MECHANICAL VALUE OF A CUBIC MILE 1 "' OF SUNLIGHT. d 



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

 rial communication throughout space to the remotest visible body is 

 a fundamental assumption in the undulatory theory 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 undulatory 

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

 possible way becomes an object of the greatest interest. A first ques- 

 tion would 'naturally occur, What is the absolute density of the lumi- 

 niferous ether in any part of space? I am not aware of any 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 f from Pouillet's data on solar radiation and 



a 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 m a 

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

 published within a year from the present time.] 



b Note of December 22, 1892.— The brain-wasting perversity of the insular inertia 

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

 inches and -rains and pounds and ounces and acres is curiously illustrated by the 

 title and numerical results of this article as originally published. 



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

 Baltimore, L884, I suggesl cubic kilometer instead of "cubic mile" in the title, and 

 use the French metrical system exclusively in the article. 



"From Edin. Royal Soc. Trans., Vol. XXI, Part I, May, 1854; Phil. Mag., IX, L854; 

 Comptes Kendus, XXXIX, Sept., 1S54; Art. LXVII of Math, and Phys. Papers. 



^October L3 L899.— Not so now. I did not in 1854 know the kinetic theory o! 



eases. 



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

 LXV1 of Math, and Phys. Papers. . M - 



