NATURE OF. X-rays, or Röntgen rays, is the name given to the radiations which, in the general spectrum of electromagnetic waves, occur after ultra-violet rays as we pass towards the shorter wave lengths. It is difficult to define the region exactly. As is well known the visible spectrum extends from the wave lengths of the extreme red (about o•8µ) to the furthest violet (about 0.4A), next follows the ultra-violet and then a region is reached where the absorption of the radiation by all kinds of matter becomes very great.
It is in the region of r,000 Angstrom units (or, to use other units, o•r ,u, or centimetre), that we may place the beginning of the X-rays, but we must remember that the radiations in the ultra-violet series of hydrogen are undoubtedly the terms of an X-ray series (the K series) of this gas. At the short wave length end we may regard the K rays of uranium, or more exactly the K discontinuities of this element, as marking the limit of a region of radiations intimately connected with the electronic structure of the atom and from that point the gamma rays begin (i.e., for wave-lengths less than of an Angstrom unit). (See RADIA TION.) The quantum theory (q.v.) establishes a correspondence be tween each radiation of given wave-length and a certain number of volts, and when the radiations are excited by an electric discharge it is necessary that the potential of this discharge should be at least equal to this voltage. The visible and the ultra-violet radia tions then correspond to just a few volts while the X-rays range from some hundred volts to more than a hundred thousand volts; to excite the gamma rays it would be necessary to employ a poten tial of several millions of volts.
In this wide interval are found rays the properties of which vary in a continuous manner as a function of the wave length. The Bragg-Peirce law which expresses the progressive and regular character of the absorption of a radiation in terms of the atomic number of the absorbing element appears to be valid throughout a region which extends at least from A to It) A, and probably much beyond in the short wave length direction.
Towards the end of the year 1895 Professor Röntgen, in his laboratory at Wilrzburg, in Bavaria, carried out experiments on the electric discharge through rarefied gases. The experiments of Crookes had at that time attracted a great deal of attention to this branch of physics. Röntgen had covered his discharge tube with black paper and placed a phosphorescent screen near it to see if a new radiation penetrated through the paper. Not only did the screen commence to fluoresce but also a new radiation was disclosed in a neighbouring room through a closed door. It is said that a foreign scientist visiting the German physicist some years later asked "what did you think then?" and he replied "I did not think, I experimented." The history of the discovery of the X-rays by Röntgen is described under RoNTGEN, WILHELM KONRAD VON.
The extraordinary penetrative qualities of X-rays were the object of the first researches, while the attempts to discover in them optical properties, analogous to those of the radiations of ordinary light, received a check, so much so that for a long time even their undulatory nature was in doubt. It was necessary to wait until the experiments of Laue, Friedrich and Knipping in 1912 to find, in the passage of X-rays through crystalline media, a remarkably fruitful method, which by demonstrating that the new radiations show, like light, the phenomena of interference and of diffraction, enabled the wave length to be determined and at the same time the different kinds to be separated. Between these two dates, 1895-1912, the study of Röntgen rays had made great advances, thanks especially to the work of Barkla on the secondary rays which they excite by falling on different bodies, and that of Whiddington on the excitation of X-rays by means of cathode rays of increasing velocities.