672 Mr. C. T. R. Wilson [March 7, 



cloud chamber fixes a hmit to the age and hence to the diflfuseuess of 

 the trails which are rendered visible ; under the actual conditions 

 any free ions would be driven by the electric force to the roof or 

 floor within less than a fifth of a second after being set free. None 

 of the ions made visible has had a free existence exceeding this 

 limit. 



It is clear that an ionizing particle, while traversing or even pass- 

 ing near to an older trail of ions on which a cloud has already formed, 

 will not find the vapour supersaturated to the extent necessary to 

 cause condensation on the ions ; it will therefore fail to leave a 

 visible trail in this region. This is doubtless the reason why the 

 sharply defined trails only appear 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. 3). 



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. 4) an enlargement of two of the tracks 

 is shown, one of them having two sudden bends. The path is other- 

 wise straight except very near 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 ^\hich ionization is bi'ought 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 condensation 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 



