PROGRESS IN PHYSICS—THOMSON. 193 
A simple calculation shows that one-half of this mass is con- 
tained in a volume seven times that of a corpuscle. Since we know 
the volume of the corpuscle as well as the mass, we can calculate the 
density of the ether attached to the corpuscle; doing so, we find it 
amounts to the prodigious value of about 510", or about 2,000 mil- 
lion times that of lead. Sir Oliver Lodge, by somewhat different con- 
siderations, has arrived at a value of the same order of magnitude. 
Thus around the corpuscle ether must have an extravagant density ; 
whether the density is as great as this in other places depends upon 
whether the ether is compressible or not. If it is compressible, then 
it may be condensed round the corpuscles, and there have an abnor- 
mally great density; if it is not compressible, then the density in free 
space can not be less than the number I have just mentioned. 
With respect to this point we must remember that the forces acting 
on the ether close to the corpuscle are prodigious. If the ether were, 
for example, an ideal gas whose density increased in proportion to 
the pressure, however great the pressure might be, then if, when 
exposed to the pressures which exist in some directions close to the 
corpuscle, it had the density stated above, its density under atmos- 
pheric pressure would only be about 810-'%, or a cubic kilometer 
would have a mass less than a gram; so that instead of being almost 
incomparably denser than lead, it would be almost incomparably 
rarer than the lightest gas. 
I do not know at present of any effect which would enable us to 
determine whether ether is compressible or not. And although at 
first sight the idea that we are immersed in a medium almost infinitely 
denser than lead might seem inconceivable, it is not so if we remem- 
ber that in all probability matter is composed mainly of holes. We 
may, in fact, regard matter as possessing a bird-cage kind of struc- 
ture in which the volume of the ether disturbed by the wires when 
the structure is moved is infinitesimal in comparison with the volume 
inclosed by them. If we do this, no difficulty arises from the great 
density of the ether; all we have to do is to increase the distance 
between the wires in proportion as we increase the density of the 
ether. 
Let us now consider how much ether is carried along by ordinary 
matter, and what effects this might be expected to produce. 
The simplest electrical system we know, an electrified sphere, has 
attached to it a mass of ether proportional to its potential energy, 
and such that if the mass were to move with the velocity of light its 
kinetic energy would equal the electrostatic potential energy of the 
particle. This result can be extended to any electrified system, and 
it can be shown that such a system binds a mass of the ether propor- 
tional to its potential energy. Thus a part of the mass of any system 
is proportional to the potential energy of the system. 
