494 



NATURE 



[February 9, 191 1 



Two interesting points appeared in this experiment. In 

 ;the first place, the ratio between the two quantities of 

 kathode rays, whiqh appear on the two sides of a silver 

 leaf through which the " -tin rays " pass, is nearly con- 

 stant, for different thicknesses of leaf. With the thinnest 

 leaf obtainable each quantity was about half its full value. 

 It would have been desirable to have had still thinner 

 leaves ; but it is fairly clear that the ratio would be nearly 

 the same for extreme thinness. The kathode radiation, 

 which appears on the side of the leaf whence the X-rays 

 emerge, is 1-30 times that which appears on the other, 

 and we may take it that this would be the case even if 

 the leaf were but one atom thick. Thus when an X-ray 

 plunges into an atom in which its energy is converted ' 

 into that of a kathode ray, the kathode ray may emerge 

 at any jx>int, but there is a 30 per cent, greater chance 

 that it will more or less continue the line of motion of 

 the X-ray than that it will not. In previous work on the . 

 conversion of y-ray into /3-ray energy, I have found that 

 the 3 ray may practically be supposed to continue the 

 line of motion of the 7 ray, so that there is a great differ- 

 ence in behaviour of the two classes of ray in this respect. 

 It is remarkable that the scattering of the 7 rays shows 

 also a much greater dissymmetry than is found in the case 

 of the X-rays. It looks as if the )3 rays that appear when 

 7 or X-rays impinge on atoms are related rather to the 

 scattered than to the unscattered primary rays. Putting 

 it somewhat crudely, no doubt, it might be said that 

 when a 7 or X-ray is deflected in passing through an 

 atom, it runs a risk of being converted into a ;3 rav in 

 the process, so that j3 rays are found distributed about 

 the atom in rough proportions to the secondary 7 or 

 X-rays. In the case of 7 rays this practically amounts 

 to their all going straight on at first; in the case of 

 X-rays the distribution is more uniform. 



Another interesting point arises in this way. When the 

 X-rays from tin are allowed to pass into the ionisation 

 chamber through increasing thicknesses of silver foil, the 

 kathode rays grow at a rate which is not represented by 

 the exponential curve usually assumed. The amount is 

 for some time more nearly proportional to the thickness 

 of the foil. A second foil adds its own effect without 

 destroying much of the one on which it is laid. This may 

 easily be ascribed to the relation of the ionisation due to 

 the 3 particle to the energy it has to spend. The ionisa- 

 tion is nearly all at the end of the path, and the second 

 layer does not absorb the rays made in the first because 

 thev are still at the beginning of their career. 



These few experiments which I have described mav 

 serve to illustrate both the justice and the convenience of 

 placing all these rays, o, /3, 7, and X, in one class. We 

 are tempted to consider them all as corpuscular radiations 

 of some sort, and we then look upon our researches into 

 their behaviour as attempts to understand the collisions 

 of the various new corpuscles with the constituent centres 

 of force in the atoms. But if we ascribe corpuscular 

 properties to the 7 and X-rays, we are led far away from 

 the original speculations as to their nature. Stokes sup- 

 posed them to be spreading aether pulses, but in his theory 

 the energy of the pulse spreads on ever-widening surfaces 

 .as. the time passes, and is utterly insufficient to provide 

 the energy of the /3 rays which the 7 or X-rays excite. 

 Some sort of mechanism has to be devised bv which the 

 ■energy of the 7 ray moves on without spreading, so that 

 at the fateful moment it may be all handed over to the 

 5 ray, which carries it on. I had the hardihood myself 

 to propose a theory of this kind. My idea was that the 

 7 or X-ray might be considered as an electron which had 

 assumed a cloak of darkness in the form of sufficient 

 ■positive electricity to neutralise its charge. Nor do I see 

 anv reason for abandoning this idea, for it is at least a 

 good working hypothesis. It means, of course, that not 

 only does the energy of the )3 ray come from the 7 ray, 

 but the /3 ray itself. 



Manv insist that my neutral corpuscle is too material, 

 and , that something more ethereal is wanted, for it 

 appears that ultra-violet light possesses many of the 

 properties of X and 7 rays.- It can excite electrons to 

 motion, and sometimes the speed of the electron depends 

 on the quality of the light, and not on the nature of the 

 material from which it sprinjjs. They propose, therefore. 

 a quasi-corpusculnr theory of light, 7 and X-rays being 

 NO. 2154. VOL. 85I 



included- The. immediate objection to this proposal i-- 

 that it seems to throw away at once all the marvellou 

 explanations of interference and diffraction which Youi„ 

 and Fresnel founded on a theory of spreading waves, and 

 I do not think anyone has yet made good this defect. 

 The light corpuscle which is proposed is a perfectly new 

 postulate. It is to move with the velocity of light, keep- 

 ing a circumscribed and invariable form, to have energy 

 and momentum, and to be capable of replacing and being 

 replaced by an electron which possesses the same energy 

 but moves at a slower rate, and, of course, it has to do 

 all that the old light-waves did. The whole situation is 

 most remarkable and puzzling. We are working and 

 waiting for some solution which, perhaps, will come in a 

 moment unexpectedly. Meanwhile, we must just try to 

 verify and extend our facts, and be content to piec 

 together parts of the puzzle, since we cannot, as yt t 

 manage the whole. My object to-night has been to show 

 you how we may conveniently bind together a large 

 number of the phenomena of radio-activity into an easily 

 grasped bundle, using a kinetic theory which has many 

 points of resemblance to the older kinetic theory of gases. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — It is proposed to confer the degree of 

 Master of Arts, hotioris causa, upon Mr. K. J. J. 

 Mackenzie, university lecturer in agriculture. 



On Thursday next, February 16, a Grace will be offered 

 to the Senate recommending that a site on the Downing 

 Ground be assigned for a building for the department of 

 physiology, to the east of the School of Agriculture. .At 

 the same Congregation a further Grace will also be 

 brought forward recommending that a space to the south 

 of, and adjoining, the proposed building for the depart- 

 ment of physiology, be assigned as a site for a laboratory 

 of experimental psycholog}'. 



Oxford. — On February 4 Prof. T. W. Edgeworth 

 David, CM. G., F.R.S., delivered a public lecture before 

 the University, in which he described the part he had 

 taken in Sir Ernest Shackleton's Antarctic Expedition of 

 1907-9, including the ascent of Mount Erebus and thfe 

 reaching of the South Magnetic Pole. On February 7 

 the honorary degree of D.Sc. was conferred on Prof. 

 David. 



The report of the committee for anthropology for the 

 year 1910, just presented to Convocation, contains a record 

 of continuous and healthy development of the study in 

 Oxford. The salary of the curator of the Pitt-Rivers 

 Museum has been raised from 200Z. to 500?. per annum, 

 and a readership has been founded in social anthropology, 

 to which the secretary to the committee, Mr. R. R. 

 Marett, Fellow of Exeter College, has been appointed. A 

 large number of lectures have been delivered in the course 

 of the year under the general heads of physical anthro- 

 pology, psychology, geographical distribution, prehistoric 

 archaeology, technology, social anthropology, and philo- 

 logy, besides special lectures for Sudan probationers, and 

 addresses on the art of prehistoric man in France, by M. 

 Emile Cartailhac. 



The consideration of the proposed amendments to the 

 statute on faculties and boards" of faculties has been 

 resumed by Congregation. 



It is announced. in the Revue scientifique that Prof. 

 Hans Meyer has presented 150,000 marks to the Uni- 

 versity of Leipzig for the inauguration of an institute of 

 experimental psychology. 



We have received from the honorary secretary of li 

 Association of Teachers in Technical Institutions a copy 

 of a letter sent by the association to the principal of the 

 University of London directing attention " to the marked 

 inequality of the requirements of the examiners for 

 'pass' in the respective subjects" for the intermedia: 

 and final B.Sc. external examinations. Tabulated statis- 

 tics, drawn up bv the association from the Universitv 

 Calendar, show that in 1909 the following percentages of 

 candidates, entering for the various subjects of science in 

 the intermediate external examination, failed : — chemistry. 

 469 ; physics, 307 ; pure mathematics, 25-3 ; applied 



