THE NEW RAYS
IT is not given to many men to make such a wonderful discovery
as fell to the lot of Professor Röntgen in the year 1895. If anyone
wishes to mention something very new and wonderful, some great
revelation of science during the nineteenth century, he usually puts
the Röntgen Rays or X-rays in the first place. This is very natural,
for these rays give men powers of vision which they could hardly expect
ever to possess. They enable us to see through many dark bodies, and
even through the human body itself. The discovery of these rays forms
one of the most romantic chapters in the history of science.
The discovery was made in the course of some work with a vacuum tube. A
vacuum tube is a bulb of glass from which all the air has been pumped.
An electric lamp is a kind of vacuum tube, but it would not be suitable
for the experiments of Professor Röntgen unless the filament were
removed and nothing but the two platinum wires remained. If the current
were sent through the bulb as usual, it would have some difficulty in
passing from one platinum wire to the other, and indeed it could not
pass at all unless some slight trace of air remained in the bulb.
[Illustration: Fig. 72.--Professor W. C. Röntgen.]
Even then a higher pressure of electricity is required than is used
for lighting the filament in the ordinary way. Instead of having the
two platinum wires side by side, they are usually placed at opposite
ends of the tube when the ordinary experiments with vacuum tubes are
being performed. Also, instead of ending in sharp points the platinum
wires usually end in small discs or caps.
When a current is sent through a tube of this kind the trace of gas
in the tube shines with a strange light. The light usually shows
some beautiful colours, and is arranged in layers. These layers are
generally in the neighbourhood of the positive wire, which is called
the anode. At the negative wire we find a layer of white, or violet,
light, and next to this a dark space, which separates it from the
glowing space beyond. When special care is taken to get rid of as much
gas as possible, the dark space expands until it fills the whole of the
tube. Then a strange thing happens. A green glow plays on the whole of
the tube opposite the cathode, and it is in this green glow that the
X-rays are born. But before we describe how the X-rays originate, we
may say a few words of what happens in the vacuum tube, and gives rise
to the green glow.
If we could see the cathode through a microscope of a magnifying power
greatly beyond that of any existing microscope, we should find it to
be the scene of a very intense activity. We should see great jets of
small particles start from the cathode at a tremendous speed, and shoot
away in straight lines until they reached the glass wall opposite,
there producing the green glow. These small particles are thought to be
the real particles of electricity which constitute the electric current.
An electric current consists of a stream of these extremely small
particles, which are called “electrons.” They are much smaller than
the atoms of a wire; and thread their way slowly between the atoms,
knocking against them every now and then, and producing a great deal
of heat. The same number of electrons passes through every section of
the whole circuit in the same time, and naturally, when a wire in the
electric circuit is very thin the electrons must pass at a greater
speed in order to get through and make room for those that come after
them. In a thin wire, therefore, the heat is very great, and when the
wire becomes a filament, the heat is great enough to make it shine with
a bright light. This gives us an electric glow-lamp.
Now, when these electrons arrive at the end of the cathode wire, they
are urged strongly to cross into an open space beyond the wire, partly
by the crowds of electrons following them, and partly by the strong
attraction which the anode opposite exerts upon them. But the atoms
of the wire themselves attract and hold the electrons, and there is
therefore always some difficulty to overcome before the electrons can
leave the wire. Once they are free from the cathode, they are impelled
forward by the repulsion from behind and the attraction in front. Like
stones falling from a great height they rush on more and more quickly.
The force which plays upon the electrons is enormously greater than the
force of gravitation. It is more than a million times greater. On the
other hand, the particles or electrons are very small, and, therefore,
do not produce any very destructive effects, however high their speed
may become. But their speed is something enormous. It is something like
fifty thousand miles per second. No wonder that when these particles
are stopped by a glass wall they produce some very curious effects.
These effects become much more intense when they are concentrated upon
a sheet of platinum placed opposite the cathode. That sheet becomes
white hot and sends out the famous Röntgen Rays, which Röntgen himself
called “X-rays.”
Now as to the history of the discovery of these rays. Professor Röntgen
was one day experimenting with such a vacuum tube, and trying to get
the electrons to go out of the tube into the open air. This experiment
had been done many times before him by using a little window made of
very thin aluminium, which can be penetrated by electrons.
[Illustration: Fig. 73.--Vacuum Tube for producing Röntgen Rays.]
He was studying the effect of these electrons upon a sheet of some
phosphorescent substance. He wished to find out whether the electrons
would still come out if the vacuum tube were enclosed completely in
black cardboard. He also wished to observe the effect in complete
darkness. He was surprised to find that the phosphorescent screen was
lighted up, and that some influence from the vacuum tube reached the
screen through the black cardboard.
Greatly struck by this observation he tried various other arrangements.
He put a deal board between the tube and the screen, but found that
the board made no difference whatever, and that whatever rays from
the tube were lighting up the screen, these rays could pass through
the deal board. He then took away the board and put in its place a
thick book, but again this made no impression whatever upon the rays.
Thinking that perhaps the rays were coming round to the screen in some
unexplained way, he held his hand between the tube and the screen.
Imagine his surprise on seeing, not the shadow of his hand, but only
the shadow of the bones in his hand on the phosphorescent screen. It
must have given him something of a shock, and we may suppose that he
quickly turned on the light to see if his hand had really shrivelled
up into a skeleton. But no, the hand looked just as usual, and yet on
holding it in the path of these wonderful rays, the shadow of the bones
was again distinctly visible, the flesh showing only in faint outline.
It was a most wonderful discovery.
The great news was not long in travelling all over the world. The
experiment was repeated thousands of times, and it was soon found out
that a great new fact had been added to the range of human knowledge.
It was found that metallic bodies such as coins, bullets, and needles
could be located in the flesh by means of the rays playing through it
on to the screen. Instead of the screen, a photographic plate could be
used, and pictures of these bodies could be obtained in a few minutes.
[Illustration: Fig. 74.--A Foot inside a Boot as seen by means of the
Röntgen Rays.]
Surgeons were not slow in using the great discovery in the hospitals,
and every large hospital has had a special staff of men trained in the
use of the Röntgen Rays.
But it often happens that the blessings of a new discovery are
endangered by evil effects which they bring in their train, and this
was very much the case with the Röntgen Rays. The men who spent all
their days taking photographs by means of the new rays found to their
dismay that wherever the rays played upon them for a long time the
skin became dry and hot. The hair fell off, and even the nails dropped
out. Worse than that, very painful sores began to appear after a time,
which it was found almost impossible to heal. In some cases death was
the result of prolonged exposure to the Röntgen Rays; but, as usual,
science eventually succeeded in separating the good from the bad
effects, and protected the skin by means of screens of lead which
cannot be pierced by X-rays. Now this new great aid to surgery and
medicine has been established properly and regulated satisfactorily.
The X-rays were not the only kind of new rays discovered towards the
close of the nineteenth century. In fact they ushered in the discovery
of quite a number of new kinds of radiation, each more wonderful than
the last. We have already mentioned the green glow on the glass, and
the white light on the platinum screen which catches the electrons.
Somebody put forward the supposition that it is the phosphorescent
light itself which sends out the X-rays. He supposed that whenever
anything shines with phosphorescent light, X-rays are sent out, and
these make an impression upon a photographic plate. This supposition
was tested by a great Frenchman of the name of Becquerel. His father
had made a great name by investigating phosphorescence, and what more
natural than that the son should study the behaviour of phosphorescent
bodies as regards X-rays!
But by a very extraordinary and lucky chance he chose among the many
phosphorescent bodies known to him the only body that could have
led him to the great new discovery which he made. That body was a
substance called uranium. He exposed some crystals containing uranium
to sunlight until they glowed with a thin blue light.
[Illustration: Fig. 75.--Henri Becquerel.]
Then he shut up those crystals in a dark box with some photographic
paper, and left them there for a few hours. At the end of that time he
found that the crystals had blackened the photographic paper. This, he
thought, was either the effect of the bluish light, or the effect of
X-rays from the crystals. He decided to try the experiment again, but
he waited in vain day after day for the sun to come out.
This, as the sequel showed, was another fortunate chance. For all this
time the crystals lay ready in a drawer in the neighbourhood of a
photographic plate, waiting to be exposed to the sun. Tired of waiting
any longer, Becquerel thought he had better close the experiment
for the present by developing the photographic plate. To his great
surprise, he found that the crystals had made a strong impression upon
the plate, although it had been protected by black paper, and although
the crystals had not been exposed to sunlight. It then struck him that
perhaps the crystals had been exposed to the sun some considerable time
before, and were now only very gradually giving up the light which they
had absorbed. He therefore kept them for months in complete darkness,
and then exposed another plate to their action. But contrary to all
expectation, he got the same result as before, so that it was clear
that the action of the uranium was quite independent of light, and
continued without interruption.
Even greater surprises were yet in store, for this power of uranium
to give out a new kind of rays which impressed a photographic plate
was found to be quite indestructible. No matter if the uranium was
melted up in a crucible, pounded to a fine powder, mixed with other
substances, melted or boiled, or treated with all kinds of acids, the
effect was always the same so long as the quantity of uranium remained
the same. The new and strange power seemed to lie in the very atoms of
the uranium themselves.
This power was quite new, and therefore had to be called by a new
name. The wife of another French professor of the name of Curie, who
greatly distinguished herself in studying this new power, called it
“radio-activity,” and this name is now used generally. Professor Curie
and his wife went to a great deal of trouble to concentrate the uranium
and make it as highly radio-active as they could. They found that some
preparations of uranium were more active than others, and, therefore,
they found that a certain sediment of the uranium ore was quite
extraordinarily radio-active. They extracted this sediment from several
tons of the ore.
It was an extremely expensive operation, but it was richly rewarded by
another marvellous discovery, for at the end of a great deal of work
they found a new substance which was a million times more radio-active
than uranium. This substance they called “radium.” So intensely active
was this new substance, that if shown in the dark, a small piece of
it made a phosphorescent screen brightly luminous. But the radium
proved as dangerous as the X-rays, for Madame Curie, who wore a small
quantity of it in her sleeve, found that it gave her a sore which took
several months to heal.
After the tragic death of her husband in a motor accident Madame Curie
was appointed Professor of Chemistry in the Paris University, being
the first lady ever appointed to such a post in that ancient city of
learning.