So the story went on from year to year. We shall see in a moment to what
it led. Meanwhile the next great step was when, in 1895, Röntgen
discovered the X-rays, which are now known to everybody. He was
following up the work of Lenard, and he one day covered a "Crookes tube"
with some black stuff. To his astonishment a prepared chemical screen
which was near the tube began to glow. _The rays had gone through the
black stuff; and on further experiment he found that they would go
through stone, living flesh, and all sorts of "opaque" substances._ In a
short time the world was astonished to learn that we could photograph
the skeleton in a living man's body, locate a penny in the interior of a
child that had swallowed one, or take an impression of a coin through a
slab of stone.
And what are these X-rays? They are not a form of matter; they are not
material particles. X-rays were found to be a new variety of _light_
with a remarkable power of penetration. We have seen what the
spectroscope reveals about the varying nature of light wave-lengths.
Light-waves are set up by vibrations in ether,[2] and, as we shall see,
these ether disturbances are all of the same kind; they only differ as
regards wave-lengths. The X-rays which Röntgen discovered, then, are
light, but a variety of light previously unknown to us; they are ether
waves of very short length. X-rays have proved of great value in many
directions, as all the world knows, but that we need not discuss at this
point. Let us see what followed Röntgen's discovery.
[2] We refer throughout to the "ether" because, although modern
theories dispense largely with this conception, the theories of
physics are so inextricably interwoven with it that it is necessary,
in an elementary exposition, to assume its existence. The modern
view will be explained later in the article on Einstein's Theory.
While the world wondered at these marvels, the men of science were
eagerly following up the new clue to the mystery of matter which was
exercising the mind of Crookes and other investigators. In 1896
Becquerel brought us to the threshold of the great discovery.
Certain substances are phosphorescent--they become luminous after they
have been exposed to sunlight for some time, and Becquerel was trying to
find if any of these substances give rise to X-rays. One day he chose a
salt of the metal uranium. He was going to see if, after exposing it to
sunlight, he could photograph a cross with it through an opaque
substance. He wrapped it up and laid it aside, to wait for the sun, but
he found the uranium salt did not wait for the sun. Some strong
radiation from it went through the opaque covering and made an
impression of the cross upon the plate underneath. Light or darkness was
immaterial. The mysterious rays streamed night and day from the salt.
This was something new. Here was a substance which appeared to be
producing X-rays; the rays emitted by uranium would penetrate the same
opaque substances as the X-rays discovered by Röntgen.