166 Lord Kelvin on Ether and 



as unity according to the convenient French metrical system, 

 the preceding statements are equivalent to saying that the 

 density of ether in vacuum or space devoid of ponderable 

 matter is eveiywhere probably not less than 5 x 10~ 18 . 



Hence the rigidity (being equal to the density multiplied 

 by the square of the velocity of light) must be not less than 

 4500 dynes * per square centimetre. With this enormous 

 value as an inferior limit to the rigidity of the ether, we shall 

 see in an addition to Lecture XIX. that it is impossible to 

 arrange for a radiant molecule moving! through ether and dis- 

 'plac'ing ether by its translatory as well as by its vibratory 

 motions, consistently with any probable suppositions as to 

 magnitudes of molecules and ruptural rigidity-modulus of 

 ether ; and that it is also impossible to explain the known 

 smallness of ethereal resistance against the motions of planets 

 and comets, or of smaller ponderable bodies, such as those we 

 can handle and experiment upon in our abode on the earth's 

 surface, if the ether must be pushed aside to make way for 

 the body moving through it. We shall find ourselves forced 

 to consider the necessity of some hypothesis for the free 

 motion of ponderable bodies through ether, disturbing it only 

 by condensations and rarefactions, with no incompatibility in 

 respect to joint occupation of the same space by the two 

 substances.] See Phil. Mag. Aug. 1900, pp. 181-198. 



§ 9. I wish to make a short calculation to show how much 

 compressing force is exerted upon the luminiferous ether by 

 the sun's attraction. We are accustomed to call ether im- 

 ponderable. How do we know it is imponderable ? If we 

 had never dealt with air except by our senses, air would be 

 imponderable to us ; but we know by experiment that a 

 vacuous glass globe shows an increase of weight when air is 

 allowed to flow into it. We have not the slightest reason to 

 believe the luminiferous ether to be imponderable. [Nov. 17, 

 1899. I now see that we, have the strongest possible reason 

 to believe that ether is imponderable.] It is just as likely to 

 be attracted to the sun as air is. At all events the onus of 

 proof rests with those who assert that it is imponderable. I 

 think we shall have to modify our ideas of what gravitation is, 

 if we have a mass spreading through space with mutual gra- 

 vitations between its parts without being attracted by other 

 bodies. \_JS J ov. 17, 18.99. But is there any gravitational 

 attraction between different portions of ether ? Certainly 

 not, unless either it is infinitely resistant against condensation, 

 or there is only a finite volume of space occupied by it. 



* See Math, and Phys. Papers, vol. iii. p. 522 ; and in the last line of 

 Table 4. for " p>10~ 2 ' } " substitute " p^lO" 22 ." 



