July 23, 1908J 



NATURE 



>7i 



LETTERS TO THE EDITOR. 



[The Editor does not bold himselj responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications,] 



The Nature of the y and X-Rays. 



In a previous letter to Natl-re (January 23, p. 270) I 

 gave a brief description of some experiments made by Dr. 

 Madsen and myself on the properties of the secondary 

 radiation due to y rays. .\ fuller account is given in the 

 Transactions of the Royal Society of South Australia, 

 1908, p. I. 



The experiments have been continued, and I hope that 

 the following summary of the results will be of interest : — 



(1) When 7 radiation is diminished in quantity as it 

 passes through matter, /3 radiation appears in its place, 

 moving at the outset in the original direction of the y 

 radiation, and subsequently undergoing scattering in the 

 ordinary manner of /3 rays. 



(2) The penetration and therefore the speed of the /3 

 radiation thus produced increases with the penetration of 

 the y radiation to which it is due. 



(3) The speed of the $ radiation does not depend upon 

 the nature of the atom in which it arises. 



(4) In the case of radium at least, the speed of the 18 

 radiation is nearly equal to, perhaps a little less than, the 

 speed of the normal 3 rays emitted by radium itself. 



(5) When very hard 7 rays traverse matter their 

 absorption and therefore the production of $ rays are 

 almost independent of the atomic structure of the matter, 

 and a density law follows. Softer rays are affected by 

 atomic structure ; they are more absorbed by heavy atoms 

 than by light atoms for equal weights of absorbing 

 screen. The softer the rays, the greater is this effect. 

 Hence arises the difference in character of the logarithmic 

 curves of absorption of different substances ; heavy atoms 

 show a rapid initial fall. Hence also when soft 7 rays 

 are used the emergence radiation from heavy atoms may 

 be greater than from light atoms. And again, the relative 

 extent to which the rays produce secondary radiation from 

 different metals may be modified by passing the rays 

 through screens, as Kleeman has shown. We do not, 

 however, find any true selective absorption such as Kleeman 

 suggests. 



(6) If there are any secondary 7 rays, the ionisation 

 which they produce is negligible compared with that pro- 

 duced by the secondary radiation, at least within a 

 moderate distance of the radiator, say a metre in air. 



.Ml these facts can be explained very simply and directly 

 on the neutral-pair theory ; indeed, the theory guided us 

 to the verification of most of them. 



As regards (i), we have simply to suppose that the 

 negative and positive passing united into an atom are 

 separated if they happen to traverse a very strong field 

 anywhere therein ; the negative flies on, and the positive 

 becomes ineffective. 



The second property is also an obvious consequence of 

 the hypothesis. The faster the 7 particle is moving, the 

 greater the initial speed of the negative. 



The third is readily explainable : the electric field of the 

 atom is merely the solvent of the bonds that connect the 

 pair. It is not able to affect the speed of the negative set 

 free. 



The fourth may be taken to imply that the radio-active 

 atom (say Ra C) ejects electrons at a certain speed, some 

 of which start off in company with a positive counterpart, 

 some without. The former constitute the 7 rays, the latter 

 the 6. 



The fifth would show that there are stronger fields inside 

 heavy atoms than light ones, and that the chance of 

 separation of the pair increases with (a) the strength of 

 the field, (h) the time taken to cross it. 



Turning now to the ether pulse hypothesis, it is con- 

 venient to consider it in two different forms, which are 

 irreconcilable with each other. 



In the first of these, both the electron and the electron's 

 energy are supposed to be drawn from the atom, the 7 

 ray merely pulling the trigger. This theory requires us 



NO. 2021, VOL. 78] 



to accept the extraordinary idea that the primary ray, 

 though it does no more than pull the trigger, determines 

 the direction and velocity of the shot, and it offers no 

 explanation at all of (i) and (4) (see above). We should 

 naturally expect the velocity of the electron to be a func- 

 tion of the properties of the atom from which it is drawn, 

 as in the well-known cases of true radio-activity. More- 

 over, all the radio-activity of which we have certain know- 

 ledge is not to be hurried or stayed by any external agency. 

 It is true that Prof. W. Wien (Gottingen Nachrichten, 

 1907, p. 598) has made a tentative application of a theory 

 of Planck's, and thence derived a formula i)-A = const., 

 where v is the velocity of the ejected electron, and \ the 

 thickness of the pulse. This provides a formula, but it 

 satisfies (2) and (3) only ; moreover, it seems to me that 

 the difficulties remain as great as ever, and that the 

 application of Planck's theory must be unjustifiable. 



Passing on to the second form of the pulse theory, we 

 now suppose the electron itself to be drawn from the atom, 

 but its energy from the pulse. 



I understand that this view is now held by Prof. J. J. 

 Thomson (see Camb. Phil. Soc, vol. xiv., part iv., p. 417), 

 and it is also maintained by Mr. N. R. Campbell (" Modern 

 Electrical Theory "). New works often take some time 

 to reach us here, and I have only just received a copy of 

 this admirable book, but I hope I have understood it 

 sulificiently w'cU to enable me to describe the position 

 correctly. 



Since the energy of a pulse, if spread over an ever- 

 widening surface, is utterly insufficient to provide the 

 energy required for the secondary /3 ray. Prof. Thomson 

 and Mr. Campbell suggest that the pulse does not spread, 

 but travels radially from the arrested electron along tubes 

 of force, the latter being considered as things differentiated 

 from the surrounding space. Prof. Thomson speaks of 

 bundles of pulse energy travelling with the speed of light 

 in straight lines. When a kathode particle strikes the anti- 

 kathode, bundles dart away from the point of impact ; 

 when these impinge on atoms they drive out the electrons 

 constituting the secondary rays. In this way the energy 

 difliculty is explained, and possibly also the difference 

 between the emergence and the incidence radiations. It 

 must be remembered, however, that this difference may be 

 very large. In the case of carbon under 7 rays, the one 

 radiation is five or six times the other. Since the secondary 

 ray has the same speed (nearly) as the primary kathode 

 ray which caused the X-ray, it seems to me necessary to 

 suppose that the arrest of the kathode particle must cause 

 one bundle of energy of very small and invariable volume 

 to travel out along one straight tube (and only one) con- 

 nected to that particle. This causes the ejection of one 

 electron from some atom into which it penetrates, giving 

 all its energy to that electron. Similar arguments apply to 

 $ and 7 rays. Surely it requires a very complicated struc- 

 ture of the a'ther to effect all this. I have too deep a 

 respect for Prof. Thomson's work to say it is not possible 

 to construct a theory on these lines, but I think I may 

 fairlv claim that the neutral-pair theory explains all the 

 known properties of the 7 rays much more simply and 

 completely. 



Perhaps I oug-ht to add that the theory, although it may 

 require a detachable positive electron, does not require a 

 free positive electron. 



I have scarcely mentioned the X-rays. I am glad to see 

 that Mr. Cooksey (Nature, April" 2, p. 509) has proved 

 the difference between emergence and incidence radia- 

 tion in their case also. It can now be said, therefore, that 

 all the properties of the 7 ^ays as set out in the above 

 summarv hold for the X-rays also, mutatis mutandis. 



Universitv of -Adelaide, Mav 5. W. H. Bragg. 



Symbols for Physical Quantities. 



It is verv desirable to have a notation for the represent- 

 ation of phvsical quantities in scientific books and period- 

 icals, which shall be the same in all languages. 



The subject is under the consideration of the International 

 Electrotechnical Commission with a view to international 

 agreement, and committees in the different countries (in 

 England under the chairmanship of Lord Rayleigh, CM.) 



