438 Lord Kelvin 07i Electro-ethereal Theory of the 



concerned with the source ; and in bringing this last of our 

 twenty lectures to an end, I must limit myself to finding the 

 effect of the presence of electrionic vibrators in ether, on the 

 velocity of light traversing it. 



§ 234. The "fundamental modes'' of which, in Lee. X., 

 p. 120, we have denoted the periods by k, k^, Kj^, . . . are 

 now modes of vibration of the electrions within a fixed atonj, 

 wdien the ether around it and within it has no other motion 

 than what is produced by vibrations of the electrions. It is 

 to be remarked, however, that a steady motion of the atom 

 through space occupied by the ether, will not affect the vibra- 

 tions of the electrions within it, relatively to the atom. 



§235. To illustrate, consider first the simple case of a 

 mono-electrionic atom having a single electrion wdthin it. 

 There is just one mode of vibration, and its period is 



K = ^iT ^-^ =^^/^' . . . (199), 



where oc denotes the radius of the atom, e the quantity of 

 resinous electricity in an electrion, and m its virtual mass ; 

 and c denotes e^a~^. This we see because the atom, being 

 mono-electrionic, has the same quantity of vitreous electricity 

 as an electrion has of resinous; and therefore (App. D, § 4) 

 the force towards the centre, experienced by an electrion held 

 at a distance x from the centre, is e^u-^x ; which is denoted 

 in § 240 by ox, 



§ 236. Consider next a group of i electrions in equilibrium, 

 or disturbed from equilibrium, within an z'-electrionic atom. 

 The force exerted by the atom on any one of the electrions is 

 ie^a"H), tow^ards the centre, if D is its distance from the 

 centre. Let now the group be held in equilibrium with its 

 constituents displaced through equal parallel distances, x, 

 from their positions of equilibrium. Parallel forces each 

 equal to ie^ar^x, applied to the electrions, wdll hold them in 

 equilibrium ^ ; and if let go, they wnll vibrate to and fro in 

 parallel lines, all in the same period 



_L^^;^ (200). 



^/i e 



This therefore is one of the fundamental modes of vibration 

 of the group; audit is clearly the mode of longest period. 

 Thus we see that the periods of the gravest vibrational modes 

 of different electrionic vibrators are directly as the square 

 roots of the cubes of the radii of the atoms and inversely as 



« Compare App. D, § 23. 



