July 14, 1923] 



NA TURE 



D3 



that the photographic method of C. T. R. Wilson 

 would be the best means easily available of actually 

 testing the stability of the nuclei of atoms for the 

 different types of disintegration, particularly when 

 the particles emitted have a short range. In the 

 earlier applications of this method polonium has 

 been used almost exclusively as the source of a- 

 particles. The a-particles thus emitted have a 

 relatively small kinetic energy, so the evidence 

 obtained from the photographs is not of much value 

 in its bearing on nuclear stability. The writers 

 have therefore used the high - speed a-particles 

 from thorium C, with a velocity of 2-05 x 10* cm. /sec, 

 or 0-688 c. 



In an earlier letter (Nature, January 27, p. 114) 

 we gave a photograph showing the sharpest colli- 

 sion obtained in ten thousand exposures, the 

 a-particle being turned through an angle of about 

 125°. The sharpest collision given by Blackett (Proc. 

 Roy. Soc. A, 103, p. 79 (plate 3)) is less sharp, 

 since the a-particle is turned through an angle of 

 110° or less. Fig. i shows two views, taken from 

 directions perpendicular to each other, of a collision 

 between an a-particle and the nucleus of an atom of 

 air. This is the sharpest collision we have obtained 

 by taking twenty-one thousand photographs. In 

 this case the a-particle is turned through an angle of 

 165°, so that the lines which show the track of the 

 a-particle before and after the collision exhibit a 

 sharp angle equal to 15°. 



In an ordinary collision the initial track of the 

 a-particle splits into two branches beyond the point 

 where the collision takes place. One of these is due 

 to the rebounding a-particle, and the other to the 

 forward track of the nucleus which is hit. If this 

 nucleus were to disintegrate during the collision or 

 quickly enough afterward, an additional track would 

 emerge from the point of collision, and this would 

 be due to the fragment, such as a hydrogen nucleus 

 or an a-particle, which is ejected. It is possible, 

 too, that electrons or other additional particles might 



Fig. I. — a-ray track which splits into three branches after 

 a collision. The upward loop in the initial track is 

 ' due to the diffusion of electrons out of a region partly 



robbed of water vapour by an earlier track. 



also be emitted, so that the track might split into 

 even more than three branches. However, all of the 

 particles thrown off may not produce visible tracks. 

 . Thus the tracks given by high-speed electrons are 

 faint, and are sometimes invisible in parts of the gas 

 which have been robbed of their water vapour. 



The extremely sharp collision photographed in 

 Fig. I exhibits the very interesting phenomenon that 

 the original track splits into tlivee branches at the 

 point of collision, which is exactly the characteristic to 

 be expected if the bombarded atom disintegrates. The 

 film on which the photograph was taken shows the 



lines at the point of collision much more plainly than 

 the reproduction, and a study of the black lines on 

 the film as seen under the microscope indicates that 

 the third particle is shot diagonally upward, exactly 

 from the point of collision as nearly as this can be 

 determined by a microscopic examination of both of 

 the views (taken 

 at right angles) . 

 The great relative 

 brightness of the 

 track of this par- 

 ticle is due to the 

 fact that the 

 camera gets a 

 " head-on " view. 

 The discussion of 

 the momentum 

 relations will be 

 left to a more com- 

 plete paper, but it 

 may be stated 

 that, so far as we 

 are able to de- 

 termine, the col- 

 lision does not ex- 

 hibit conservation 

 of momentum if 

 the particle which 

 shoots upward is 

 left out of ac- 

 count. If this 

 could be definitely 

 proved it would 

 give remarkably 

 substantial evi- 

 dence, in addition to that of the number of tracks, 

 that a disintegration has occurred. 



Bumstead, and later Wilson, have secured photo- 

 graphs of the tracks of 5-rays, supposedly due to 

 electrons pulled out of the non-nuclear systems of 

 the atoms through which the a-particle passes. 

 These tracks are extremely short, and are most 

 easily seen when the expansion in the ionisation 

 chamber is not too high. Fig. 2 shows an entirely 

 new type of secondary track. Here what appear to 

 be electrons are thrown out a great many times as 

 far as in the 5-rays, and in a different direction. The 

 two electron tracks curve upward, show a backward 

 motion, and are remarkably close to being parallel. 

 They differ so markedly from those of the 5-rays that 

 they may be considered as a different type of ray. 

 They may be designated as f-rays. 



Altogether about eighty thousand tracks have been 

 photographed. From the assumed dimensions of a 

 molecule in air it may be estimated that each a- 

 particle passes through between 100 and 200 thousand 

 atoms, so approximately 10 billion atoms have been 

 shot through, with the result that only three nuclear 

 collisions have resulted in which the initial a-particle 

 has been given a retrograde motion. In only one of 

 these, as illustrated in Fig. i, has the collision been 

 very direct. The photographs show many other 

 interesting relations which cannot be discussed here. 



William D. Harkins. 

 R. W. Ryan. 



Fig. 2. — Nearly parallel curved tracks of par- 

 ticles ejected backward. Probably these 

 are due to electrons. 



Science and Economics. 



May I bring this correspondence back to earth by 

 recalling that I based my deduction that no one even 

 pretended to understand the present economic system 

 upon the fact that, although the age is as far ahead of 

 any preceding epoch in the science of producing wealth 

 as it is in astronomy or chemistry, yet millions of 



NO. 2802, VOL. I 12] 



