1913] on Paths of Particles Ejected from Atoms 069 



Each alpha- or beta-particle, in the course of its flight through a 

 gas like air, traverses large numbers of the atoms of the gas. 

 According to modern theories, such as those developed by Sir J. J. 

 'riioiiison and Rutherford, each atom may be regarded as a sort of 

 luiuiature solar system in which the planets are represented by 

 uciratively charged corpuscles or electrons ; the forces with which we 

 are concerned being of course electrical and not gravitational. 

 Wiien either an alpha- or a beta-pxirticle passes near one of the 

 members of the system, there are forces tending to deviate the flying 

 particle from its otherwise straight course and to cause distuibances 

 in the path of the planetary electron ; these may be violent enough 

 to cause the electron to escape from the system. An electron thus 

 set free will become attached finally to some other atomic system, 

 which thus acquires a negative charge, whilst the atom which 

 has lost an electron has been left with an excess of positive elec- 

 tricity. We thus get positively and negatively charged atoms or ions. 

 Xow a metliocl -of making visible the individual ions has long 

 been available. Molecules of water or of other vapours attach them- 

 selves more readily to ions than to uncharged atoms or molecules. 

 Thus, in the absence of other nuclei on which vapour can condense 

 more readily, such as those called dust particles l)y Aitken, it is 

 possible to arrange that every free ion shall act as a nucleus and 

 cause the condensation of water vapour, whilst none condenses else- 

 where. Each invisil)le ion may thus be converted into a visible 

 water drop. The supersaturated condition necessary in order that 

 water vapour may condense on the ions is most conveniently produced 

 by the sudden expansion of moist air. 



The advance which I have recently succeeded in making in the 

 condensation method of studying ionization is this. The "ions are 

 now captured and converted into visil)le w'ater drops in the positions 

 which they occupied immediately after their liberation by the ionizing 

 agent ; the cloud of drops is then at once photographed. Thus the 

 invisible trail of ions left behind along the course of any ionising 

 particle is converted into a visible line of cloud of which a photo- 

 graph is secured. In this way a record is obtained of the path of 

 each projectile by making visible the atomic wreckage it has caused 

 in its passage through the air or other gas. In many cases the indi- 

 vidual ions produced along the tracks are visible in the photographs. 

 In order that undistorted pictures showing the result of the 

 passage of the various rays may be obtained, it is essential that the 

 expansion should be effected without stirring up the gas. This 

 condition is secured by using a wide shallow cloud chamber of which 

 the floor can be made to drop suddenly and so produce the desired 

 increase of volume (Fig. 1).* 



* The apparatus is described in the Proceedings of the Eoyal Society, A., 

 vol. 87 (1912), p. 277. "■ 



