484 SCIENCE PROGRESS 



to begin at some distance from the source, the older trails being 

 most closely packed in the region around the source. 



By means of a suitable shutter arrangement attached to the 

 floor of the cloud chamber, it is possible to prevent alpha-particles 

 from traversing the moist air till after the expansion. The 

 diffuse cloud trails are then absent from the photographs 

 (fig. 2). 



The most remarkable feature of the tracks of the alpha- 

 particles is their general straightness. Sudden bends are to 

 be observed, however, practically all the rays being bent 

 within a millimetre or two of their ends. In this respect, as in 

 others, the photographs confirm the conclusions arrived at by 

 less direct methods. 



In the next picture (fig. 3) an enlargement of two of the 

 tracks is shown, one of them having two sudden bends. The 

 path is otherwise straight except very near to its end. Now the 

 alpha-particle has thousands of encounters with atoms of 

 the gases of the air in each millimetre of its course by which 

 ionisation is brought about, as we know from measurements 

 made by the electrical method ; and in accordance with this, 

 the cloud particles (which are simply ions magnified by con- 

 densation of water) are so closely packed that they are not 

 separately visible in the photograph. It is remarkable that 

 only two encounters out of the many thousands occurring in 

 the course of its flight should succeed in deviating the particle 

 visibly from its course and that in these cases the deviation 

 should be quite large. 



The alpha-particle, in passing near one of the electrons of 

 an atom, may impart to it sufficient energy to cause it to escape 

 from the atom, whilst on account of its own enormous 

 momentum it is not perceptibly deviated from its course. 

 We can thus understand the general straightness of the tracks. 

 The sudden deviations must be due to encounters of a special 

 kind ; according to Rutherford's view, such large deviations 

 would be caused by the alpha-particle passing near the centre 

 of the atom, where he supposes the positive charge to be 

 concentrated. 



What is perhaps the most interesting feature of the 

 particular track I have been describing remains to be mentioned. 

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

 one can hardly interpret otherwise than as being due to the 



