Gravitational Matter through Infinite Space. 165 



space traversed by it ; and from all we know of the mecha- 

 nical theory of undulations, it seems certain that this velocity 

 must be a very small fraction of the velocity of propagation 

 in the most intense light or radiant heat which is propagated 

 according to known laws. Denoting this velocity for the 

 case of sunlight at the earth's distance from the sun by i\ and 

 calling W the mass in grammes of any volume of the Irani- 

 niferous ether, we have for the mechanical value of the dis- 

 turbance in the same space, in terms of terrestrial gravitation 

 units, 



W 2 



7* 



where g is the number 981, measuring in (c.G.s.) absolute 



units of force, the force of gravity on a gramme. Xow, from 



Pouillet's observation, we found in the last footnote on §1 above, 



1235x46,000, ., , . , , . ,. , 



y tor the mechanical value, in centimetre-grams, 



of a cubic centimetre of sunlight in the neighbourhood of the 

 .sun ; and therefore the mass, in grammes, of a cubic centi- 

 metre of the ether, must be given by the equation, 



981x1235x46000 



w ~ ^T~ ~~ # 



If we assume v= -V, this becomes 

 n 



w 981x1235x46000 2 981x1235x46000 

 W= v ,— -x**= ___-Xl* 



20-64 

 = T0^r Xrc*gm.; 



and for the mass, in grammes, of a cubic kilometre we have 

 2064 



10' 



Xn 



§ 7. It is quite impossible to fix a definite limit to the ratio 

 which v may bear to V; but it appears improbable that it 

 could be more, for instance, than -^ for any kind of light 

 following the observed laws. We may conclude that probably 

 a cubic centimetre of the luminiferous medium in the space 

 near the sun contains not less than 516 x 10~ 20 of a gramme 

 of matter ; and a cubic kilometre not less than 516 x 10~ 5 of 

 a gramme. 



§ 8. [Nov* 16, 1899. We have strong reason to believe 

 that the density of ether is constant throughout interplanetary 

 and interstellar space. Hence, taking the density of water 



