, and
moving over certain minute distances, initiate chemical actions which
are necessary to some cure. Or they may go right through the body and
fall on a photographic plate, setting in operation chemical action which
forms a picture on the plate.

There is another occasion of an entirely different kind when the
electron is greatly in evidence and displays effects which are most
astonishing and significant. Every atom of radium or other radio-active
substances sooner or later meets with the catastrophe in which its life
as radium ends and atoms of other substances are formed. At that moment
occurs the emission which is the characteristic property of the
substance. One of the radiations emitted consists of high-velocity
electrons, moving, some of them, nearly as fast as light.

Now it is found that when the speed approaches that of light, 186,000
miles or 3 x 10^{10} centimetres per second, the energy is higher than
it should be if it followed the usual rule, viz. energy is equal to half
the mass multiplied by the square of the velocity. It would seem that an
electron moving with the velocity of light would have infinite energy;
or, to put the matter in another way, the experimenter in his laboratory
can never hope to observe an electron moving so fast; it would be the
end of his laboratory and of himself if ever it turned up.

Linked up with this result is the very strange fact that no one has ever
been able to find any direct evidence of the existence of the ether,
which is postulated in order to carry light-waves. It has been pictured
as a medium through which the heavenly bodies move, and to which their
motions may be referred. But when light is launched into the ether, its
apparent velocity must depend on whether it travels with or against the
drift of the ether through the laboratory where the measurement is made.
The experiment has been performed without the discovery of any such
difference, although the method was amply accurate enough to detect the
effect that might be expected. It was afterwards shown that the negative
result might be explained by supposing that a measure of length varied
in length according to whether it was travelling with or against the
ether. But the continual failure of all such experiments has led to a
remarkable hypothetical development with which the name of Einstein is
firmly connected. It is supposed that some flaw must exist in our
fundamental hypotheses, and that if this were cor