236 



NATURE 



[November 6, 1919 



been more than realised. We stand only at the 

 beginning of what the Rontgen rays promise to 

 accomplish for us. 



Knowledge of the main properties of the 

 X-rays grew rapidly under the labours of 

 Rontgen himself and the many investigators who 

 were attracted to the new field. Much was dis- 

 covered respecting the power of penetrating vari- 

 ous substances, the existence of different qualities, 

 hard or penetrating, soft or less penetrating, the 

 dependence of quality on the degree of- evacua- 

 tion, the construction and the applied potential of 

 the X-ray bulb, the action on the photographic 

 plate and on the fluorescent screen, and the power 

 of producing ions in a gas. At the same time, 

 the technique improved rapidly ; bulbs, plates 

 and screens, coils and interruptors were all 

 designed afresh to meet the demands of an ex- 

 periment which grew into an industry. 



Notable advances were made by Barkla when 

 he proved the existence of a polarisation which 

 was to be expected on the hypothesis that the 

 rays were ethereal waves or pulses, and when he 

 showed that every element emitted its own special 

 and characteristic X-rays under proper stimulus. 

 The properties of characteristic radiation are most 

 remarkable and instructive. The radiation of any 

 element can excite the corresponding character- 

 istic radiation in elements lighter than itself, but 

 never in any element which is heavier. For 

 example, " zinc rays " can excite the character- 

 istic radiation of magnesium, potassium, or nickel, 

 but not the characteristic radiation of bromine or 

 silver, nor, indeed, of zinc itself. Since energy is 

 necessarily spent in the excitation of radiations, 

 the absorption coefficients of zinc rays by the 

 various elements show a marked discontinuity ; 

 they increase steadily from magnesium upwards, 

 but there is a sudden drop at zinc, the coefficient 

 falling to about one-eighth of its previous value. 

 After that the coefficient increases steadily with 

 the atomic weight as before. 



X-rays can excite an electron radiation in any 

 substance on which they fall, and this effect has 

 also been the subject of much investigation. The 

 more penetrating the X-rays, the higher the velo- 

 city of the electron which it can cause to be 

 emitted. This effect is carried to an extreme in 

 the corresponding emission of very high-speed 

 electrons under the stimulus of the 7-rays of 

 radium, for the parallelism between all the pro- 

 perties of X-rays and y-rays is an obvious indica- 

 tion of the similarity of their nature. There is a 

 striking correspondence in the two processes — 

 that of the excitation of X-rays by the moving 

 electrons of the X-ray bulb, and that of the emis- 

 sion of electrons under the stimulus of X-rays. 

 The quality of an X-ray depends on the velocity 

 of the electron that excited it, and not at all on 

 the number of electrons in the exciting stream ; 

 conversely, the velocity of an electron due to 

 X-rays depends only on the quality of the rays, 

 and not at all on their intensity. Some kind of 

 matter is required to bring about either of the 

 • energy transformations, but the atomic weight of 

 NO. 2610, VOL. 104] 



it has no influence on the principle just stated. 

 Anomalies may appear when characteristic radia- 

 tions are excited, but they can be explained as 

 apparent only. 



Many other remarkable properties, which 

 cannot be described in so brief a notice as this, 

 were discovered in the first period of X-ray 

 investigation. All of them were examined with 

 the greatest interest, because it was recognised 

 that if X- and 7-rays were essentially of the same 

 nature as light, their study must contribute to 

 any true theory of light radiation, and, indeed, 

 must be necessary thereto. 



A new period of investigation began when von 

 Laue and his collaborators demonstrated in 191.2 

 that X-rays could be diffracted by the ordered 

 array of the atoms of a crystal. From a simple 

 interpretation of von Laue's principle, and from 

 the results of its application to the study of 

 crystals of sodium and potassium chloride, 

 W. Laurence Bragg was able to discover the 

 actual arrangement of the atoms of those crystals 

 and the distances separating the atom-bearirg 

 planes. It thus became possible to find the actual 

 length of an X-ray. The older and vaguer 

 methods of defining the quality of an X-ray were 

 at once replaced by a method of great precision. 

 Previous work can now be revised under infin- 

 itely better conditions, and much has already been 

 accomplished in that direction. 



Moseley, making a careful survey of the wave- 

 lengths of the radiations of all the elements avail- 

 able, showed that the wave-length of the charac- 

 teristic radiation marched in perfectly even step 

 with the increase of the atomic number, and, 

 therefore, that the atomic number of an element 

 defined it more fundamentally than its atomic 

 weight. So all the elements were drawn together 

 by a common tie as they had never been before ; 

 anomalies of position in the periodic table were 

 explained, and the number and places of missing 

 elements were made clear. 



The examination of the interchange of energy 

 between X-radiation and electron movement can 

 now be made so effectively that it has been 

 possible to use. the experimental results for 

 one of the best determinations of Planck's 

 constant. 



In another direction the new discoveries have 

 opened out a wide road of advance into crystallo- 

 graphy. In the first place, it is possible to deter- 

 mine the crystal lattice — that is to say, to 

 measure the sides and angles of the rhomboid 

 cell which contains the unit pattern of atomic 

 assemblage and is repeated throughout the crystal 

 without change of form or orientation. This is 

 a comparatively easy task. It is a second and 

 more difficult task to determine the arrangement 

 of the atoms within the cell ; it has been accom- 

 plished in a few single cases only. Lastly, the 

 new researches will give us information concern- 

 ing the position of the electrons or the diffracting 

 centres within the atoms and about their normal - 

 movements. Something has already been done ' 

 in this direction also. 



