RADIOACTIVITY VISUALISED 485 



recoil of the system which has caused the deviation of the 

 particle. 



The next two photographs (figs. 4, 5) show the effect pro- 

 duced in the cloud chamber by a trace of radium emanation — 

 the radioactive gas which is the first product of the disintegra- 

 tion of radium. Each cloud ray is a visible record of the 

 conversion, by expulsion of an alpha-particle, of a single atom 

 of the emanation into an atom of the next member of the radio- 

 active series. Since the rays start in the gas, it is now possible 

 to get tracks which are complete from beginning to end. The 

 ends are distinguishable by the characteristic bend or hook. 

 At the beginning there is an enlarged head, where, moreover, 

 the cloud is of greater density ; this represents ionisation by the 

 recoil of the atom from which the alpha-particle has escaped. 



It may be noticed there is a sudden bend in one of the rays 

 with which there is again associated a spur-like process. 



Radioactive substances emit beta-particles as well as alpha- 

 particles. These produce comparatively few ions along their 

 tracks, which are thus much less conspicuous when converted 

 into visible cloud rays than those of the alpha-particles. They 

 are, in consequence, more difficult to photograph and they have 

 not appeared in any of the pictures shown thus far. 



With suitable illumination, however, the droplets condensed 

 on the individual ions may be photographed, provided they are 

 not too closely packed. It is thus possible to study the path of 

 any ionising particle, however small the number of ions produced. 

 On account of the enormous velocities with which they are 

 emitted — closely approaching that of light — the beta-particles 

 are able to travel considerable distances in the air, distances 

 many times greater than the diameter of the cloud chamber. It 

 is therefore impossible to obtain a picture of the whole track of 

 a single beta-particle. 



Here, on one plate (fig. 6) are shown the final portions of 

 the tracks of an alpha- and of a beta-particle. The beta-ray 

 shows much less intense ionisation, as indicated by the compara- 

 tive densities of the clouds ; and its devious path forms a great 

 contrast to the straightness of the alpha-ray. 



The beta-particle, of course, is so much more readily diverted 

 from its course on account of its much smaller mass. 



If, however, we catch the beta-particle at a sufficiently early 

 stage of its career, we find that its immense velocity compensates 



