January 9, 19 13] 



NATURE 



side. Photographs made by weaker beams show the 

 individual rays more clearly. The tracks are of the 

 same character in the two cases, and the intensity 

 only affects the number. These are tracks such as 

 we should expect to be 

 made by slow & rays — 

 slow because they are 

 very tortuous and do not 

 go very far. Here is 

 one greatly magnified, 

 showing the individual 

 water drops deposited 

 along the track (Fig. 8). 

 They are actually a few 

 millimetres long. But 

 the X-rays are passing 

 in straight lines across 

 the chamber, and you 

 see that they leave no 

 trace behind. 



These strange results 

 were all expected from 

 previous investigation. 

 It has b_en known for 

 some lime that X or 

 7 rays can excite /3 rays 

 in matter on which they 

 fall ; I have in recent 

 \ears tried to show that 

 the X and y rays can 

 do nothing else. They 

 do not themselves ionise 

 the air or metal or other 

 substance through 

 which they pass ; they 

 merely bring to birth 

 /3 rays which do. When 

 X-rays fall upon a 

 photograohic plate and 

 bring about a chemical 

 change, or upon the animal skin and cause a "burn," 

 as we vaguely denote the physiological effect, the 

 X-rays have not been the direct agents, but the 

 6 rays, which spring from them. We may venture 

 to make a guess as to how the action takes place. 

 When an a. particle 

 passes through a 

 molecule it may 

 ionise more than 

 one atom in that 

 molecule ; in the 

 case of a very 

 complex molecule it 

 must often ionise 

 several of the 

 atoms. Such a 

 molecule might be 

 expected to break 

 up or dissociate ; 

 and it is actually 

 found that a par- 

 ticles do cause dis- 

 sociation. Now the 

 /8 particle ionises 

 but rarel}', as the 

 pictures show ; it 

 will be a very com- 

 plex molecule in 

 which the /3 par- 

 ticle causes ionisation of two or three of the atoms 

 of which it is constituted. Colwell and Russ have 

 lately shown that X-rays can break down the starch 

 molecule, a starch solution irradiated by X-rays be- 

 coming less viscous and showing the presence of 



— Portions of Fig. 7 enlarged, showing 

 : individual ions produced alon,: a por- 

 n of one of the kathode-ray tracks. 



dextrin. It is reasonable to expect the very large 

 and complex starch molecule to be broken up by the 

 3 rays which the X-rays produce ; and by this direct 

 action of the 3 rays on large molecules we may per- 

 haps be able to explain all the physiological actions 

 of radium and of X-rays. 



There is good evidence to show that each of the 

 fi rays the tracks of which you see upon the screen 

 is due to one X-ray and no more, and that the X-ray 

 in forming the /3 ray gives it all the energy which 

 it possesses. Further, if we consider the production 

 of X-rays, we find that each X-ray that comes out of 

 the bulb carries with it the energy of one, and only 

 one, of the /3 rays which are hurled against the anti- 

 kathode. Thus in the picture I have drawn (Fig. 9) /3 

 rays striking the antikathode A, X-rays move off, each 

 inheriting the energy of one of the 3 rays. The 3 rays 

 of the X-rays tube have themselves but very little 

 power of penetrating materials ; they move at only 

 one-third (or thereabouts) of the speed of the /3 rays 

 of radium ; but the X-ray carrying the same energy 

 i^ hundreds of times as penetrating, and a large 

 number of those which are produced at the anti- 

 kathode in the bulb penetrate the glass walls. Each 



-pray 

 -X ray 



of these meets its fate sooner or later. In passing 

 through some atom the reverse change takes place, 

 and the X-ray disappears, handing over its energy 

 to a 3 ray, which starts off .with a velocity equal to 

 that with which the original /3 ray finished when it 

 disappeared in favour of the X-ray. It is as if the 

 X-ray picked up the /3 ray, moved off in a straight 

 line with it, and started it again somewhere else ; 

 or as if the & xa.y disappeared like a river going 

 underground, only to reappear and continue its course. 

 The 3 ray and the X-ray are interchangeable forms 

 of energy-carrier. Further transformations may occur 

 before the energy is spent. We may consider our- 

 selves to be following the history of a small quantity 

 of energy which is carried first by a ray, then by 

 an X-ray, then by a /3 ray again, and so on. The 

 energy is kept intact in the X-ray form, but gradually 

 frittered away in the 3 ray form, until finally it 

 sinks to so low a value that it can no longer ionise 

 or record its motion in a fog picture, and it is lost 

 to view. Just as the a particle settles down to 

 ordinary life as a helium atom at the end of its royal 

 progress, so the moving electron, the 3 ray, becomes 

 at last one of a crowd of electrons which are always 



NO. 2254, VOL. 90] 



