January 9, 19 13] 



NATURE 



529 



RADIATIONS OLD AND NEW.'^ 



T^HE remarkable properties of the rays from 

 -'■ radio-active substances which have been 

 examined with sucli eagerness in recent years 

 throw a curious and interesting light on 

 the older attempts to find a satisfactory theory of 

 radiation. Newton and Huygens, Young and Fres- 

 nel, and other thinkers down to our own times have 

 discussed various hypotheses, rejecting, adopting, 

 0-- amending, and each has given his reasons for his 

 final choice. It is instructive at the present time to 

 examine some of those reasons, and to consider the 

 influences which prompted them to make their great 

 discoveries. More particularly is this the case because 

 some expressed their ideas in the language of a 

 corpuscular theory, and we have now had for some 

 time the opportunity of examining radiations which 

 we know to be corpuscular. 



Let me first of all set out some of the facts of the 

 new radiations. Thanks to the recent beautiful ex- 



FjG. I. — a-Rays from radium. Some of the 

 tides have traversed the :iir before tlie i 

 sion, others after the expansion. 



periments of Mr. C. T. R. Wilson, I am able to 

 illustrate my statement by a method which would 

 have been beyond my power even a few months ago. 

 We have been for some years laboriously investigat- 

 ing the paths of the a, e, and y rays through gases 

 and other material substances. Our work has been 

 conducted in the dark, so to speak, for we have been 

 obliged to rely mainly on electrical methods, to feel 

 our way along those paths in some cases, and in 

 others to arrive at their form by indirect reasoning. 

 Mr. Wilson has .shown us how to obtain a clear 

 photographic representation of the whole path of an 

 a or /3 ray. The ocular demonstration is helpful from 

 a scientific point of view, not only because of the 

 confirmation w-hich it has given of the work we have 

 already done, but also because of its suggestiveness 

 for the future. It is, if I may say so, invaluable 

 from a lecturer's point of view, because it enables me 



1 Evening discourse delivered on September 6 before the British Associa- 

 tion at Dundee by Prof. VV. H. Bragg, F.R.S. 



to dispense with difficult explanations of the methods 

 by which recent advances have been made, and to 

 show you, on the screen, direct illustrations of the 

 main points that I wish to emphasise. 



The a ray is, as is well known, an atom of helium 

 projected by the exploding radio-active atom with a 

 speed of some ten or twenty thousand miles a second. 

 .•\khough it moves off at this excessive rate it is able to 

 penetrate only two or three inches of air in its ordinary 

 state, or one or two thousandths of an inch of heavier 

 substances, like aluminium or gold. When it comes 

 to the end of its range, it has spent practically the 

 whole of its energy, it has lost its distinction, and 

 sinks to the level of an atom moving with ordinary 

 speed. Some years ago I showed that it moved in an 

 almost perfectly straight line from start to finish ; 

 and it then became evident that on its way through a 

 gas or a metal or any other substance it passed 

 through every atom which it met. It does not push 

 them out of its way, for it meets hundreds of 

 thousands of atoms, each one, as 

 a rule, far heavier than itself ; 

 and it does not thread its way 

 between them, for it has no in- 

 telligence, and cannot recover a 

 line once lost. In 1907 it was 

 shown by Mr. Geiger, working 

 at Manchester, that the track of 

 the a particle was not absolutely 

 straight, but that the particle 

 was liable to slight deflections, 

 especially when near the end of 

 its path. 



On the screen there is now one 

 of Mr. Wilson's photographs of 

 the tracks of a particles radiating 

 from a minute speck of radium 

 (Fig. 1). You will see how 

 straight they are for the most 

 part, and yet a closer examina- 

 tion will show slight, very 

 sudden, deflections. 



The next slide is an enlarge- 

 ment of two tracks, one of which 

 shows the deflections very well 

 (Fig. 2). 



It is difficult to realise that we 

 are looking at a picture of the 

 path of a single atom through 

 the air, recorded by its own 

 efforts ; and we may well ask 

 how Mr. W'ilson has managed to 

 obtain so wonderful a result. 

 .\s a matter of fact his 

 method is an improvement 



on one which he had used and explained 

 some vears ago, but it will be well to describe it 

 briefly once more. A short glass cylinder of about 

 six inches in diameter — its outline can be seen in 

 Fig. I — is closed at one end by a glass plate ; at the 

 other end is a movable piston. The chamber is filled 

 with moist air, which is chilled if the chamber is 

 suddenly enlarged by the withdrawal of the piston. 

 A fog is then formed, which settles in the first place 

 on any "ions" which may be present. In the track 

 of o and $ rays there are trails of ions formed by 

 the ravs. It is only necessary therefore to illuminate 

 the fog, and to photograph it, and we have such a 

 picture as that on the screen. 



The picture confirms so far as it goes the main 

 conclusions we had already drawn as to the path of 

 the a ray. On the screen is now a copy of a drawing 

 which I made a year or two ago to show the various 

 forms of the path as we then pictured them to our- 



NO. 2254, VOL. go] 



