DISCOVERY AND NATURE OF X-RAYS
In March, 1923, there passed from this world one of the most beautiful
exemplars of the true scientific spirit that earth has ever seen—Dr.
William Conrad Roentgen, F.R.S., Professor of Experimental Physics in
the University of Munich, the discoverer of X- or Roentgen Rays.
Born at Lennep, on March 27, 1845, Professor Roentgen filled a number
of important posts before his death in 1923, in which year he was
awarded the Nobel Prize in Physics—an award which brought with it a
gift of $40,000. Although suffering from the poverty which resulted in
Germany as an aftermath of the World War, Professor Roentgen refused to
utilize the Nobel Prize award for his own personal uses. He gave the
entire sum to a research society to enable other students to carry on
their investigations.
While occupying the chair of Professor of Physics and Director of the
Physical Institute at Würzburg, Dr. Roentgen made the discovery—in
1895—for which his name is chiefly known—though his researches led to
important advances in several other departments of physics.
While experimenting with a highly exhausted vacuum tube on the
conductivity of electricity through gases, Dr. Roentgen noticed that
a paper screen covered with potassium platinocyanide—a phosphorescent
substance—which chanced to be lying nearby, became fluorescent under
action of some radiation emitted from the tube, which at the time
was enclosed in a box of black cardboard. Professor Roentgen then
found, by experiment, that this heretofore unknown radiation had the
power to pass through various substances which are impenetrable to
ordinary light-rays. He found that if a thick piece of metal—a coin,
for example,—were placed between the tube and a plate covered with the
phosphorescent substances, a sharp shadow was cast upon the plate. On
the other hand, thin plates of aluminum and pieces of wood cast only
partial shadows.
Thus was it demonstrated that the rays which produced the
phosphorescence on the glass of the vacuum tube could penetrate bodies
quite opaque to ordinary light-rays. Like ordinary light, these rays
affected a photographic plate; but owing to their peculiar behavior in
regard to reflection and refraction, Roentgen was led to put forward
the hypothesis that the rays were due to longitudinal, rather than
to transverse waves in the “ether.” They will ionize gases, but
they cannot be reflected, polarized or deflected by a magnetic or
electric field, as are ordinary light-rays. (It has been shown that the
_scattered_ secondary rays show polarization.)
Being in doubt as to the real nature of these penetrating rays,
Roentgen called them “X-rays.”
In 1896 Professor Roentgen was the recipient of the Rumford Medal of
the Royal Society. This honor was shared by his compatriot Philipp
Lenard. Lenard was the discoverer of the rays emanating from the outer
surface of a plate composed of (any) material permeable by cathode
rays. By impinging on solids, the cathode rays (negative electrons)
generate X-rays. “Lenard rays,” which are similar in all their known
properties to cathode rays projected from the cathode of a vacuum tube,
do not emanate from the cathode. (Unlike the X-rays, cathode rays may
be deflected from their natural course along “straight lines” by the
application of a magnetic or electric field.) Professor Lenard, as also
Hertz, discoverer of the now well-known “wireless waves,” had already
demonstrated that a portion of the cathode rays could pass through a
thin film of a metal such as aluminum.
When Roentgen rays (X-rays) are allowed to fall upon any substance, the
matter emits cathodic (or secondary Roentgen) rays. “The characteristic
secondary radiation may be compared with the phosphorescence produced
by ultra-violet light, and the cathodic secondary rays with the
photoelectric effect” (Sir J. J. Thomson).[2]
The penetrating power (“hardness”) of these rays appears to be
determined solely by the nature of the elements in the emitting
substance. The velocity of the cathodic (or secondary Roentgen) rays
seems to be quite independent of the matter exposed to the primary
rays, but increases as the hardness (penetrating power) of the primary
Roentgen rays increases.
The _character_ of the emitted rays, in brief, appears to be quite
unaffected by the chemical or physical condition of the element.
Red-hot iron, for example, exhibits the same characteristic Roentgen
radiation as iron at room temperature. But the _penetrating power_
(hardness) of this characteristic (emitting) radiation increases
gradually and continuously with increasing atomic weight of the
emitting elements. The complete independence of the penetrating power
of the characteristic Roentgen radiation from external surroundings
indicates strongly that it is closely connected with the nature of the
nuclei (“cores”) of the atoms giving rise to it.
FOOTNOTES:
[2] When ultra-violet light is allowed to fall upon a metal it causes
the metal to emit electrons and thus to acquire a positive charge, the
velocity of the emitted electrons being exactly proportional to the
frequency of the incident light. Or when light of X-ray type falls
upon the surface of almost any substance, it takes hold of an electron
in the atoms of that surface and hurls it out into space with a speed
exactly proportional to the wave length of the light. This phenomenon
is known as the photoelectric effect.