stars in their courses is slow compared with the flight of the
electrons. Since they fly out of the system, in the conditions we
have described, at a speed of between 90,000 and 100,000 miles a
second, they must be revolving with terrific rapidity within it.
Indeed, the most extraordinary discovery of all is that of the
energy imprisoned within these tiny systems, which men have for
ages regarded as "dead" matter. Sir J. J. Thomson calculates
that, allowing only one electron to each atom in a gramme of
hydrogen, the tiny globule of gas will contain as much energy as
would be obtained by burning thirty-five tons of coal. If, he
says, an appreciable fraction of the energy that is contained in
ordinary matter were to be set free, the earth would explode and
return to its primitive nebulous condition. Mr. Fournier d'Albe
tells us that the force with which electrons repel each other is
a quadrillion times greater than the force of gravitation that
brings atoms together; and that if two grammes of pure electrons
could be placed one centimetre apart they would repel each other
with a force equal to 320 quadrillion tons. The inexpert
imagination reels, but it must be remembered that the speed of
the electron is a measured quantity, and it is within the
resources of science to estimate the force necessary to project
it at that speed.*

* See Sir J. J. Thomson, "The Corpuscular Theory of Matter"
(1907) and--for a more elementary presentment--"Light Visible and
Invisible" (1911); and Mr. Fournier d'Albe, "The Electron Theory"
(2nd. ed., 1907).


Such are the discoveries of the last fifteen years and a few of