RÖNTGEN RAYS, a peculiar kind of radiation discovered in 1895 by Professor Röntgen, and produced, under suitable conditions, by electric discharge in VACUUM TUBES (q.v.). They pass, with comparatively little absorption, through wood, flesh, paper, and other optically opaque substances, but are strongly absorbed by such substances as metals, bone, and glass. In a general way the 'transparency' of matter to these peculiar rays decreases as the density increases. Their presence is detected by their power of producing luminescence in certain fluorescent substances, such as the platino-cyanide of barium, sodium, or potassium (PHOSPHORESCENCE). A screen covered with a layer of these fluorescing crystals becomes brightly luminescent in the neighbourhood of a vacuum tube which is giving out Röntgen rays. When a slab of wood or vulcanite is interposed between the vacuum tube and the screen, the screen still continues to glow. An arm interposed casts upon the screen a shadow in which the dark central bony structure can be clearly distinguished from the lighter flesh shadows. These shadow-pictures may also be photographed on a sensitive photographic film or plate, and a permanent record secured—an application of immense service to surgery.
It should be noted that there is nothing really extraordinary in optically opaque matter being transparent to other kinds of rays. A similar group of phenomena is met with in the opacity of glass, and the transparency (or diathermancy) of rock-salt to the dark heat-rays, although optically both are transparent. Moreover, in their power of exciting fluorescent substances and in affecting the photographic plate the Röntgen rays may be compared to the actinic or ultra-violet rays. They have also the property of discharging electrified bodies, a property possessed under certain conditions by violet light. But in almost all other respects the Röntgen rays differ markedly from the recognised forms of ether radiation. They cannot be reflected or refracted; they suffer no diffraction; they have not yet been polarised.
As to their nature, many views have been advanced. The hypothesis which appears to be most satisfactory is that enunciated by Sir George Stokes, who regards the Röntgen rays as a succession of impulses communicated to the ether in an irregular or fitful manner. He gives reasons for believing that the behaviour of ordinary rays of radiant energy, when refracted, reflected, or diffracted, depends upon the fact that they are composed of trains of waves periodically associated. However this may be—and the question is one of extreme difficulty—there is no doubt that, in the impacts of rapidly moving particles on the glass walls of the vacuum tube, we have conditions favourable for the production of the impulses imagined by Stokes. There is strong evidence that such a bombardment is going on. For a particular condition of vacuum in a Crookes tube, there appears to be projected from the negative electrode a bright beam of rays, the so-called kathode rays. At a somewhat higher vacuum these rays cease to be visible as a stream of Crookes' 'radiant matter'; but their presence is indicated by the phosphorescence produced on the glass walls. By suitable contrivances the kathode rays can be focussed on a definite part of the containing glass wall; and it is there that Röntgen rays are produced in quantity.
In 1894 Philipp von Lenard (an Austrian pupil of Hertz, and from 1896 professor at Heidelberg), following out a suggestion by Hertz, observed what seemed to be a passing of the kathode rays through a thin piece of aluminium which formed part of the containing wall of the vacuum tube. These Lenard rays, in their phosphorescing effects and in their sensitiveness to an approaching magnet, have an undoubted resemblance to the kathode rays. There is no evidence, however, that the kathode rays have really passed through the metal disc. It is possible that their bombardment of the disc on the one side may produce electrical changes capable of starting similar rays on the other. This is the explanation given by Professor J. J. Thomson, whose measurement of the speed of the kathode rays quite disproves the contention (based largely on Lenard's observation) that the kathode rays are ether vibrations. This speed was found to be about one or two hundred miles per second—that is, about th the speed of light. It was when studying the properties of the kathode and Lenard rays, which seem to differ only in their mode of production, that Röntgen discovered what he himself called the X-rays. A large and increasing literature is now devoted to the discussion of their properties and applications.