48o SCIENCE PROGRESS 



It is plain that a great advance would be made if it were 

 possible to induce each alpha- or beta-particle to leave a visible 

 trail behind it along its whole course and to photograph this 

 trail. This is what is accomplished by the method now 

 described. 



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. Thomson and Rutherford, each atom 

 may be regarded as a sort of miniature solar system in which 

 the planets are represented by negatively charged corpuscles 

 or electrons ; the forces with which we are concerned being 

 of course electrical and not gravitational. When either an 

 alpha- or a beta-particle passes near one of the members ol 

 the system, there are forces tending to deviate the flying 

 particle from its otherwise straight course and to cause dis- 

 turbances 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 electricity. We thus get positively and 

 negatively charged atoms or ions. 



Now a method of making visible the individual ions has 

 long been available. Molecules of water or of other vapours 

 attach themselves 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 by 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 elsewhere. Each invisible 

 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 ionisation is this. 

 The ions are now captured and converted into visible water 

 drops in the positions which they occupied immediately after 

 their liberation by the ionising agent; the cloud of drops is 

 then at once photographed. Thus the invisible trail of ions 



