Electric Origin of Rigidity and Consequences. 441 
purely the ratio of the inertia of an electron to its charge and 
did not involve any “mass of a particle.” The experiments 
of Kaufmann (Compt. Rend. Oct. 1902) have since proved 
that the mass associated with the electron in cathode rays is 
entirely electromagnetic. Several subsequent estimations 
have been made of a by various authors on the assumption 
that the inertia of the electron is purely electric. Kaufmann 
gives 10-* ems., while Abraham in his comprehensive paper 
(Ann. d. Ph. [4] x. 1903) writes | 
be ee EC eee aS ) 
The equation by which these values have been calculated 
is that of J. J. Thomson (to a constant prés) 
[=22 / dac®, 
where ¢ is the ratio of the two units of electricity. 
If this inertia belongs to a cube of zether of edge 2a the 
density p of the ether is 
Pecans) 8 OT a 
Now for the zther rigidity will be determined exactly as 
for a metal at absolute zero. The rigidity N=o?, where o is 
the surface-density of the laminar distributions of electricity 
equivalent to the neutrons. So, just as in section 1, we write 
ge 27 Holy - N=20e/48a8 (30) 
3 Gat)? 7 2are? ace 
The density and rigidity of the zther being found, we get 
the velocity of light by the formula 
Np Pig ea Boe aes) ots» (ab) 
With p=I—47re*/3 we would get V=cm/122?="90c. (32) 
The coefficient of ¢ ought to be 1, but numerical dithculties 
occur in connexion with the arbitrary adoption of spheres 
and cubes as standard shapes. The important point is that 
by exactly the same process as was applied to matter a 
rigidity has been found for the zther which, along with the 
density, gives the velocity of light. It should be noticed 
that N and p both vary as a~‘*, and so & disappears in V. 
With ¢ of the order 3x 10-" and ¢/[=6x 10", we have a of 
the order 2x10~. Hence from (29) and (30) we get the 
values 
p= My Nao x 10", 
_ These huge values for the density and rigidity of the ather 
result from the small value of 2 being raised to the power —4. 
They may be taken as upper limits, for an wether made of 
