Page:The New International Encyclopædia 1st ed. v. 20.djvu/817

X-RAYS. 695 X-RAYS. Professor Riintgcn (q.v.) of the University of Wiirzlmrg, who was pcrfoniiinn; some ex|wriiiii'iit3 involving tlie tlisohargc of cU'Ctrieitv , through glass hulbs exhausted until they eontaine<l only traces of air. Tliis radiation was diseovered, and, as far as could he told, dill'ered in all respects from light and other known radiations. To it Professor Riintgen gave tlie nanu? of X-rays, signifying that its cause was unknown. Professor Riintgen discovered that the essen- tial feature in its production was an exhausted bulb or Crookes tube (q.v.) into which entered two metal conductors serving to introduce and to withdraw the electric current. The conductor which introduces the current may be an ordinary wire or it may have at its end inside the l)ulb a small metal plate; it is called the 'anode.' The conductor by which the current leaves the ex- hausted bulb consists of a wire carrying at its end inside the bulb a concave metal surface ; it is called the 'cathode.' Professor Rijntgcn found that the centre from which the X-rays proceeded was the points of the solid walls of the bulb which were struck by the cathode rays proceed- ing out from the cathode, and he therefore de- vised the plan of introducing in the bulb a square plate of platinum directly facing the cathoda so that it received practically all the cathode rays, and served, tlierefore, as the source of X-rays for all outside points. It was found later that the intensity of the radiation could be increased by joining the anode to this small piece of plati- num, which is called the 'anti-cathode.' RiJntgen observed that as the electrical discharge Was con- tinued through such a tube the intensity and character of the radiation of X-rays varied. This is owing in the main to a change in the vacuum, i.e. in the pressure of the traces of gas left in the bulb, occasioned by the electric current. In order, therefore, to maintain constant conditions in the tube it is necessary to keep the gas at a constant pressure, and the principle involved in the construction of all modern forms of X-ray tubes is to have such side attachments to the main bnlb as will permit the evolution of minute amounts of gas whenever the vacvuim in the bvilb becomes too perfect. Bulbs satisfying all these requirements are furnished by instrument-makers under the name of 'X-ray self-acting tubes.' The main properties of X-rays were discovered by Professor Riintgen himself immediately after his first observation. These may be described briefly under the following heads: ( 1 ) Optical. X-rays do not experience regu- lar reilcction. refraction, diffraction, interference, or polarization; they pass in straight lines through any medium which does not absorb them. It is thus proved that they are not due to trains of waves, otherwise they could be made to inter- fere and to experience diffraction. (2) Absorption. The most striking feature of X-rays at first sight is the fact that they penetrate with great ease certain substances ■which are opaque tn light, and. on the other hand, are absorbed by certain substances which are verj' transparent to light. Tlius the X-rays are absorbed largely by glass, but are transmitted most freely by aluminuiji. by wood, by human flesh, etc. It is owing to these facts, combined ■with those stated in the preceding paragraph, that photographs may be obtained of many objects hidden from view, by allowing X-rajs to cast a shadow picture of them on a photographic plate. In this manner iihotograjihs of the bones of tile body, of metal oljjects contained in wooden bo.xes, etc., may easily be obtained. Wlien a careful study is made of the absorp- tion ])roduced in X-rays by various substances, it is found that there is a close connection be- tween the intensity of absorption and the density of the absorbing body; and it is a general law that the greater the density of the body, the greater is its absorptive pow'er. This fact is of the utmost importance in the interpretation of photographs of portions of the human body and of other objects taken , by the X-rays: the in- tensity on the photographic plate is in reality a measure of the density of the absorbing sub- stance whose photograph is taken. (3) Photoor.vphic and Fluorescent Action. X-rays are observed to affect a photographic plate in the same manner as ordinary light, ami this fact is made use of in nearly all applica- tions of these rays. It was found also — in fact, it was the fundamental observation of Rontgen — that X-ra.ys excite the lluorescent action of cer- tain substances, so that when excited by the rays tiiey emit light. A careful study has been made of various substances which are affected in this way, and certain of them are used in making so- called lluorescent screens, which may be used to receive the shadows cast by an X-ray tube in- stead of receiving them on a photographic plate. (4) Physiological Action. Rontgen himself observed that if the X-rays were allowed to enter the eye of an observer who is situated in a room entirely dark, the retina of the eye received a stimulus and light was perceived. It is ex- tremely probable that this is due to the fluores- cent action of the X-rays on certain portions of the eye. It was observed, a few months after the discovery of the X-rays, that if the radiation is too intense its action on the skin of an observer might produce most serious changes and cause "what are known as 'X-ray burns.' Some years later it was found, as a result of careful experi- ments, that in the case of certain kinds of cancer and of a limited number of skin diseases the ac- tion of X-rays was extremelv beneficial. See X-Iiai/s ill ilcdicine and Surgeii/ below. (.5) Ionization. Riintgen made the important observation that if an electrified body were placed near an X-ray tube, so that the radiations fell upon it. the electrical charge disajipeared. This was found to be true of both positively and negatively electrified bodies. It was later shown, however, by Professor J. .J. Thomson of the Uni- versity of Cambridge. England, that the real ex- planation of this pbenomennn was not any direct action of the X-rays on the electrified body itself, but was that in their passage through the air around the electrified body the X-rays 'ionized' the gas, i.e. made it a conductor for electricity. (See Ionization and Radioactivity.) This dis- covery of Professor Thomson's was, from a scien- tific point of view, the most important one in re- gard to the properties of X-rays, for it led at once to a study of the conditions under which ioniza- tion can be produced and to the discovery of other methods of causing it. (When a gas is ionized, experiments show that its minute parts are broken up into smaller ones, some charged positively and some negatively, and that it is owing to this fact that the gas becomes a conductor.)