ETHER AND GRAVITATIONAL MATTER. 219 



If we assume r=-V, this becomes 



„^ 981X1235X46000 , 981 X 1235 X 46000^ , 20 -CA ^^ , 



W = ys Xn^- (3Xl(Fr i:O^X '^'gm. ; 



and for the mass, in grams, ot a ciibio kilometer we have-^^ Xy;\ 



Sec. 7. It is quite impossible to iix a definite limit to the ratio 

 which V may bear to V; but it appears improbable that it could be 

 more, for instance, than one- fiftieth for any kind of light following 

 the observed laws. We may conclude that probably a cubic centi- 

 meter of the lumiiiiferous medium in the space near the sun contains 

 not less than 516x10"^" of a gram of matter; and a cubic kilometer 

 not less than 516XlO~^ of a gram. 



Sec. 8. [Non-. 16, 1899. -AVe have strong reason to believe that 

 the density of ether is constant throughout interplanetary and inter- 

 stellar space. Hence, taking the density of water 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 everywhere probably not less than 

 5 X l()-'\ 



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 centimeter. 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 displacing ether b}^ 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 bod}" moving through it. We shall find ourselves forced to 

 consider the necessity of some hypothesis for the free motion of pon- 

 derable 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. *"] 



Sec. 9. I wish to make a short calculation to show how much com- 

 pressing force is exerted upon the luminiferous ether by the sun's 

 attraction. We are accustomed to call ether imponderable. How do 

 we know it is imponderable? If we had never dealt with air except 



*See Math, and Phys. Papers, Vol. Ill, p. 522; and in the last line of table 4, for 

 "p>10-"" substitute "p<10--." 



•'See Phil. Mag., Aug., 1900, pp. 181-198. 



