Electric Origin of Rigidity and Consequences. 443 
square of the velocity and half the constant inertia. Thus 
in (34), when w=V the total mutual energy is nil. The 
kinetic and potential parts of the self-energy become equal if 
pra b=y2e/2V 2a, ie. Wee ous. . 3 (09) 
The velocity of light then is such, that if possessed by the 
electrons of a neutron it would make their mutual energy 
nil, and, subject to (35), would make their total energy 
consist x two equal parts, kinetic and potential. I have 
shown in “ The Electric Origin of Molecular Attraction” that 
the energy of an electrostatic field, according to the neutron 
theory of the ether, is stored in the ether half as kinetic and 
half as potential energy. It would seem then as though the 
velocity of light through the ether is connected with the 
velocity of its electrons in much the same way as the velocity 
of sound through a perfect gas is related to the translatory 
mean velocity of its molecules, a possibility contemplated by 
the founders of the kinetic theor y of gases, with the sether a 
gas. 
One other point demands immediate attention. According 
to the electromagnetic theory the velocity of light is (Ky)~ af 
and much discussion has centred round the dimensions and 
the nature of Kandy. FitzGerald suggested (Phil. Mag. |5] 
xxvil.) that both K and p are the 1 inverse of a velocity. Let 
us express this by putting 
Bee ye 1 fe"... 2x. in electrostatic units (36) 
_ Cee ie UP eo in electromagnetic es 
The ratio of the two units is c, and for the free ether we 
have e=U=U’=V._ But in the ether of matter, by which 
we mean the zether enclosed by the smallest spheres circum- 
scribing each atom, we cannot write U=U’, but if v is the 
velocity of light through matter, we must have o®=UU’. 
FitzGerald considered that possibly the velocities 1/K and 
1/y are proportional to the square root of the mean turbulence 
ot the ether. So we shall take U to be the velocity of the 
electrons in the ether. In the ether of matter the velocity of 
light is different from that in free ether, so that for it 
we cannot write c=U=V. But for most substances 
retains nearly the same value as in ether, so that U’, even in 
the xther of matter, generally has a value close to that of U 
in free ether. We might provisionally regard U’ asa velocity 
derived from the angular velccity of rotation of electrons 
round their own centres. In free ether it is equal to the 
translatory velocity U of the electrons, and in the ether of 
matter retains the same value as in free eether, because 
