CHAPTER IV
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.