_Röntgen or X-Rays_, the most famous, and up to now by far the most
useful, kind of rays associated with high vacuum tubes, were discovered
by Professor W. K. Röntgen in 1895. His first observation was that a
photographic plate, which was enclosed in an opaque material and which
was lying by chance near the apparatus, was affected just as if it had
been exposed to ordinary light. This caused him to conclude that the
effect must be due to some unknown kind of rays, and the uncertainty as
to their character led him to provisionally apply to them the name of
X-rays, for _x_ in algebra generally denotes the unknown quantity.
The later sensational part of his discovery was that the property
possessed by a highly exhausted bulb of glass, fitted with suitable
electrodes, sends out rays or electric discharges capable of passing
through many bodies which are quite opaque to ordinary light, and of
either affecting a photographic plate or causing a screen coated with
certain chemicals to fluoresce or light up under their influence.
_How are X-rays produced?_
X-rays are thus produced by the discharge of a high-potential current
through a special form of vacuum tube, known as a Crookes’ tube. The
positive terminal of an induction coil or Wimshurst machine is connected
to the anode and the negative to the cathode of the tube. The
anticathode is connected to the anode and is also positive. The vacuum
of a tube is not perfect, and the current is conveyed through the tube
by the infinitesimal quantity of air contained therein.
[Illustration: =DIAGRAM SHOWING PARTS OF X-RAY TUBE=]
The “cathodal rays” which pass from the cathode to the anticathode
consist of infinitesimal particles traveling at a high rate of speed;
when the progress of these minute bodies is arrested, X-rays are
produced. The green fluorescence on the sides of the tube opposite the
anticathode, though not caused by the X-rays, demonstrate their
presence.
WHAT THE X-RAYS ARE.--The X-rays are ethereal vibrations traveling with
much the same velocity as light. They travel in a straight line in all
directions from the point of origin, and are almost incapable of
reflection or refraction.
X-rays are invisible to the eye, but have the property of rendering
fluorescent certain substances--for example, calcium tungstate and
barium platino-cyanide. When a screen coated with these substances is
placed near the X-ray tube in a darkened room, the tungstate or barium
surface emits a fairly bright fluorescence. If an object such as the
hand or a lead pencil is placed between the screen and the tube, the
denser parts (the bones or the graphite) appear as black shadows in a
gray background.
X-rays penetrate all substances to a greater or less degree, although
heavy metals, such as lead and mercury, are, for photographic or visual
purposes, practically opaque to the rays.
The greater part of X-ray examination is conducted by photographic
methods, as the image given by the rays on a dry plate or film show far
more detail than can be seen by visual examination with the fluorescent
screen.
APPARATUS.--The apparatus required consists of a suitable source of
electrical energy, such as a battery or dynamo, etc., and a powerful
induction or a large electrostatic influence machine, combined of course
in either case with an X-ray tube and special X-ray photographic
plates. Ordinary photographic plates can be used, but do not give such
brilliant results. If we wish to take a radiograph of the hand, we must
first of all use a plate slightly larger than the hand, and enclose it
in an opaque envelope. Two such are usually employed, one red and the
other black. This is placed on the table or stand, film side uppermost,
and the hand is placed upon it, and a short distance above the hand is
located the X-ray tube. Since what we really take is a shadowgraph
picture, to give a good sharp outline, the hand should be placed as flat
as possible on the plate, and the tube some six to eight inches from it.
With some of the most powerful apparatus now in use, even the human
trunk can be radiographed in a single flash, which is an improvement on
the exposure necessary in the early days of its use, when ten, twenty,
or even forty minutes’ exposure was no uncommon practice.
THE FLUORESCENT SCREEN.--When the X-rays impinge on certain substances
they cause them to light up or “fluoresce” under their action. The
number of bodies or chemicals which do so is very large, but for
practical purposes only one or two are of any use. The best, and the one
always employed, is a chemical known as barium platino-cyanide. The
screen-holder consists of a box, preferably of pyramidal form, with a
flattened apex or top. Inserted in this apex are two tubes, like
opera-glass tubes but without lenses; through these we can look into the
box in such a manner as to prevent any outside light from entering. The
bottom of the box consists of the screen proper, a piece of cardboard or
other suitable substance, one side (the inner) of which is coated with
the substance mentioned above, because the light rays given off by the
barium platino-cyanide under the action of the X-rays cannot of course
penetrate an object opaque to light. The box should be absolutely
light-tight except for the eye-tubes.
If such a screen be held in the neighborhood of an X-ray tube, opposite
the most brilliantly phosphorescing half of the tube, it will be found
to be lighted up under the action of the X-rays. If now we place between
the tube and the screen an object such as the hand, putting it in as
close proximity to the tube as possible, we obtain a shadowgram on the
screen, varying in intensity according to the relative transparency of
the different parts of the hand to the X-rays. Since the bones are far
less transparent than the flesh, they cast a much denser shadow and are
very distinct. On such a screen it is possible to see the beats of the
heart, the rising and falling of the diaphragm, etc.
X-RAYS AT WORK.--In medical X-ray work, the patient is placed upon a
couch consisting of a wooden frame covered with canvas. A box containing
the tube moves on wheels and rails beneath the couch; it is lined with
metal to shield the operator from the X-rays. The time of exposure
depends upon the strength of current used, the power of the coil, and
the condition of the tube. A “hard” tube--that is, a tube with an
extremely high vacuum--requires less exposure than a “soft” or
low-vacuum tube.
The condition of the tube is ascertained by finding its “equivalent
spark gap.” While the coil and tube are working, the terminal points of
the induction coil are slowly brought together. If a spark passes
between the points while they are six inches or more apart, the vacuum
is too high. If no sparking takes place between the terminals till they
are within three inches of each other, the tube is low. A good working
spark gap distance is four and one-half inches. A soft, or low-vacuum,
tube gives better definition than a hard, or high-vacuum, tube, as the
rays pass less easily through dense substances and show greater
differentiation of tissue. A very high vacuum tube may show but little
difference between the bones and flesh, while a soft tube should give
the minute structure of the bones.
TIME OF EXPOSURE.--With a current of five amperes at one hundred volts
passing through the primary winding of a ten-inch coil, the exposure for
a hand or foot would be from three to fifteen seconds. The exposure for
the thicker portions of the body would be from twenty seconds to two
minutes. If an electrolytic break is used, about half the exposure would
be required. Dry plates with extra thick sensitive films are specially
prepared for radiography, the development and fixation being the same as
in ordinary photography. The image is sometimes barely visible on the
surface of the plate during development, but when fixed the negative may
give good density and definition owing to the penetration into the film
of the X-rays.
KINDS OF X-RAYS.--It is now known that these rays are not all by any
means of the same kind or of the same penetrative power. Moreover, these
differences can be still augmented by altering what is known as the
induction in the circuit, the degree of exhaustion in the tube, and the
nature of the emitting surface. The emitting surface is not the glass
walls of the tube, as many suppose; and the canary colored light emitted
by the tube is not the X-rays, which are themselves invisible. They
originate from the anode of the tube owing to the fierce bombardment to
which the cathode rays subject it. Where the cathode rays, which travel
in straight lines, first strike any material object, from that same
object the X-rays originate.
USES OF X-RAYS.--In the early days of radiography the X-rays in medical
work were confined almost solely to the detection of fractured or
injured bones, and abnormal bone growth. At the present time, however,
even a careful examination on the fluorescent screen is sufficient to
enable an expert medical radiographist to diagnose with a considerable
degree of exactitude the condition of the heart, the lungs, and the
stomach. In making such examination a tube must be chosen which has the
lowest vacuum, in order to obtain the maximum amount of contrast between
fleshy tissue not differing greatly in density.
In some cases even the liver has been outlined and part of the kidneys.
Still more important is the fact that the rays have been applied
successfully in the treatment of certain diseases which by other means
have been deemed, if not incurable, at any rate extremely difficult to
cure. Claims have been made for cancer cures by means of these same
rays; whether these have really been complete cures or not is perhaps
open to question.
X-RAY DERMATITIS.--A painful and incurable disease, of a cancerous
nature, to which radiographers are liable, caused by frequent and
prolonged exposure to X-rays. Many of the pioneers of radiography have
fallen victims to this complaint, but greater precautions are now taken
to protect the operators from the X-rays. There is little danger of
contracting this disease in X-ray photography, as the exposures are
short and the operator need not stand directly in front of the tube. The
chief risk is entailed by visual examination with the fluorescent
screen. The disease first makes its appearance in the hands and
gradually spreads to the arms and body. The skin at first appears as if
it had been burned, hence the term “X-ray burning.”
[Illustration: =THE LIGHT THAT REVEALS THE UNSEEN IN THE HUMAN BODY=
Illustration and diagram showing the apparatus ordinarily used in X-ray
photography, together with the course of the electric circuits, and a
radiograph of the hand.]
=LIQUID AIR AND ITS MARVELS OF LOW TEMPERATURE=
[Illustration: Liquid Air in water. The silvery bubbles are liquid
oxygen; the nitrogen boils away.]
[Illustration: Filtered Liquid Air in a Dewar bulb, and Liquid Air in an
ordinary glass bulb (which has collected a coating of frost).]
[Illustration: Driving a nail with a hammer made of mercury frozen by
Liquid Air.]
[Illustration: Liquid Air boiling on a block of ice, caused by the
difference of temperature.]
[Illustration: Burning steel in an ice tumbler partly filled with Liquid
Air.]
[Illustration: Liquid Air is simply its gaseous form brought into liquid
condition by the combined effect of lowering its temperature and
subjecting it to an extreme expansion. When protected from external heat
and highly exhausted it becomes a transparent, jelly-like, mass. By
means of liquid hydrogen it may be condensed into a white solid with a
faint blue tint.]