220 ETHER AND GRAVITATIONAL MATTER. 



by our senses, air would be imponderable to us; but we know b}^ 

 experiment that a vacuous g-lass globe shows an increase of Aveight 

 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 i-eason to 

 believe that ether is imponderable.] It is just as likel}^ 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. 1 think w^e shall have to 

 modify our ideas of what gravitation is, if we have a mass spread- 

 ing through space with mutual gravitations between its parts with- 

 out being attracted by other bodies. [Nov. IT, 1809. -But is there 

 any gravitational attraction between different portions of ether? Cer- 

 tainly not, unless either it is infinitely resistant against condensa- 

 tion, or there is onlj^a finite volume of space occupied l)y it. Suppose 

 that ether is given uniform spread through space to infinite distances 

 in all directions. Any spherical portion of it, if held with its sur- 

 face absolutely fixed, would by the mutual gravitation of its parts 

 become heterogeneous; and this tendency could certainl}' not be coun- 

 teracted by doing away with the supposed rigidity of its boundary 

 and by the attraction of. ether extending to infinit}' outside it. The 

 pressure at the center of a spherical portion of homogeneous gravita- 

 tional matter is proportional to the square of the radius, and there- 

 fore by taking the globe large enough may be made as large as we 

 please, whatever be the density. In fact, if there were mutual gravi- 

 tation between its parts, homogeneous ether extending through all 

 space would be essentially unstable unless infinitel}'' resistant against 

 compressing or dilating forces. If we admit that ether is to some 

 degree condensable and extensible, and believe that it extends through 

 all space, then we must conclude that there is no mutual gravitation 

 between its parts, and can not believe that it is gravitationally 

 attracted by the sun or the earth or any ponderable matter; that is to 

 say, we must believe ether to be a substance outside the law of uni- 

 versal gravitation.] 



Sec. 10. In the meantime it is an interesting and definite question 

 to think of what the weight of a column of luminiferous ether of infi- 

 nite height resting on the sun would be, supposing the sun cold and 

 quiet, and supposing for the moment ether to be gravitationally 

 attracted by the sun as if it were ponderable matter of density 

 5XlO~'^ You all know the theorem for mean gravity due to attrac- 

 tion inversel}' as the square of the distance from a point. It shows 

 that the heaviness of a uniform vertical column AB, of mass ir per 

 unit length and having its length in a line through the center of force 

 C, is 



mw mm mir .^ .... 

 CA-CB' or^^^,tCB=co, 



