r.)18] on Paths of Particles Ejected from Atoms ii78 



second of the two bends, there is a distinct spur which one can 

 hardly interpret otherwise than as being due to the recoil of the 

 system which has caused the deviation of the particle. 



The next two photographs (Figs. 5, 6) show the effect produced 

 ill the cloud chamlier by a trace of radium emanation — -the radio- 

 active gas which is the first product of the disintegration 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 radioactive series. Since the rays start in 

 the gas, it is now possible to get tracks which are complete from 

 lieginning 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 ionization 

 by the recoil of the atom from which the alpha-particle has 

 escaped. 



It may be noticed there is a sudden Ijend 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 con- 

 sequence, 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 ionizing 

 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. 7) are shown the final portions of the 

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

 less intense ionization, as indicated l)y the comparative densities of 

 the clouds ; and its devious path forms a great contrast to the straight- 

 ness 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 for its 

 very small mass and its path may be sensibly straight for distances of 

 several centimetres, in spite of the very large luimber of atoms which 

 it must traverse. This is illustrated by the next picture (Fig. 8) 

 which shows, in addition to the end of a beta-ray, a portion of the 

 trail left by a beta-particle while its velocity was still very high : it is 



