determines the rigidity of a body? It must be the speed and the amount
of moving matter. In a gas the speed may be considerable, but the
density is exceedingly small; in a liquid the speed would be likely to
be small, though the density may be considerable; and in both cases
the inertia resistance offered to displacement is practically _nil_.
But place a gaseous (or liquid) column in an intense, rapidly
alternating electrostatic field, set the particles vibrating with
enormous speeds, then the inertia resistance asserts itself. A body
might move with more or less freedom through the vibrating mass, but
as a whole it would be rigid.

There is a subject which I must mention in connection with these
experiments: it is that of high vacua. This is a subject the study of
which is not only interesting, but useful, for it may lead to results
of great practical importance. In commercial apparatus, such as
incandescent lamps, operated from ordinary systems of distribution, a
much higher vacuum than obtained at present would not secure a very
great advantage. In such a case the work is performed on the filament
and the gas is little concerned; the improvement, therefore, would be
but trifling. But when we begin to use very high frequencies and
potentials, the action of the gas becomes all important, and the
degree of exhaustion materially modifies the results. As long as
ordinary coils, even very large ones, were used, the study of the
subject was limited, because just at a point when it became most
interesting it had to be interrupted on account of the "non-striking"
vacuum being reached. But presently we are able to obtain from a small
disruptive discharge coil potentials much higher than even the largest
coil was capable of giving, and, what is more, we can make the
potential alternate with great rapidity. Both of these results enable
us now to pass a luminous discharge through almost any vacua