Velocity of L'ujlit in Gases, Liquids, aiul Solids. 439 



the scjuare roots of the numhers of the electrions ; provided 

 that in each case the atom is electrically neutralized by an 

 iuteoral number of electrions. Compare App. D, § 6. 



§ 2o7. I now propose an assumption which, while greatly 

 sim})lifyino- the theory of the quasi inertia-loading of ether 

 when it moves through space occupied by ponderable matter 

 as set forth in App. A, perfectly explains the practical equality 

 of the rigidity of ether through all space, whether occupied 

 also by, or void of, ponderable matter. My proposal is that 

 the radius of an electrioti is so ea'tremely small that the quantity 

 of ether within its sphere of condensation (Lee. XIX., § 1^6) 

 is exceedingly small in comparison with the quantity of un- 

 disturbed ether in a volume equal to the volume of the smallest 

 atom. 



This assumption, in connexion with §§ 164, 166 of 

 Lee. XIX., makes the density of the ether exceedingly nearly 

 constant through all space outside the spheres of condensation 

 of electrions. This is true of space whether void of atoms, 

 or occupied by closely packed, or even overlapping, atoms ; 

 and the spheres of condensation occupy but a very small pro- 

 portion of the whole space even where most densely crowded 

 with poly-electrionic atoms. The highly condensed ether 

 within the sphere of condensation close around each electrion 

 might have either greater or less rigidity than ether of 

 normal density, without perceptibly marring the agreement 

 between the normal rigidity of undisturbed ether, and the 

 working rigidity of the ether wdthin the atom. This seems 

 to me in all probability the true explanation of what everyone 

 must have felt to be one of the greatest difficulties in the 

 dynamical theory of light ; — the equality of the rigidity of 

 ether inside and outside a transparent body. 



§ 238. The smallness of the rarefaction of the ether within 

 an atom and outside the sphere or spheres of condensation, 

 around its electrions, implies exceedingly small contribution 

 to virtual inertia of vibrating ether, by that rarefaction ; so 

 small that I propose to neglect it altogether. Thus if an 

 atom is temporarily deprived of its electrion or electrions 

 (rendering it vitreously electrified to the highest degree 

 possible), ether vibrating to and fro through it, will experience 

 no inertial or other resistance. Its presence will not be felt 

 in any w\ay by the ether existing in the same place. Thus 

 the actual inertia-loading of ether to which the refraction of 

 light is due, is produced practically by the electrions, and 

 but little if at all perceptibly by the atoms, of the transparent 

 body. 



§ 239. For the present I assume an electrion to be massless, 



