15 



waves of one length. Moreover, Rutherford passed his a 

 particles for some considerable distance through a vacuum 

 whilst yet under the influence of the magnetic field. Thus 

 the evidence of the increase of curvature in their paths, 

 originally caused by the loss of velocity in penetrating matter, 

 was accumulated. But if, as in M. Bccqucrers experiment, 

 the path is in the air, then any appreciable increase of curva- 

 ture closely precedes the cessation of all evidence of motion, 

 and the result must be in any case almost beyond detection. 



M. Becquerel remarks that there is no evidence in his 

 photographs of the greater precision of the outer line of the 

 trace, which I had anticipated. But the photograph which 

 he now publishes shows that there is too much penumbra for 

 such an effect to be visible. 



THE POSITIVE CHARGE OF THE a PARTICLE. 



Considerable discussion has recently taken place as to 

 the mode in which the a particle acquires its positive charge. 

 It has been pointed out more than once that it may be 

 explained as the result of ionisation by collision (Rutherford, 

 address to St. Louis Congress, 1904; Bragg, 'Phil. Mag.," 

 December, 1904), and that the same hypothesis will explain 

 the deposit of tJie radium emanation on the negative electrode 

 (Bragg and Kleeman, December, 1904). In the case of the 

 emanation, an explanation, which I understand to be similar, 

 has been carefully worked out by Mackower ("Phil. Mag.," 

 November, 1905). 



Rutherford has shown that the a particle is charged at 

 the moment of leaving the radium salt. But I do not think 

 that this result is in any way prejudicial to the collision 

 theory. He evaporated a very weak solution of radium on a 

 plate, and supposed that as a result he had an excessively 

 thin layer, so that the particle made very few collisions before 

 emergence. But when such deposits are examined under a 

 microscope it is seen that the salt is gathered in little heaps, 

 and there is no true layer at all. The bulk of the a particles 

 pass through hundreds of atoms before emergence, and there 

 is ample opportunity for ionisation by collision. 



We find that the a particle spends energy in causing 

 the expulsion of electrons from the atoms of any gas which it 

 traverses. The tables of stopping powers given above show 

 that the expenditure of energy follows the same law when 

 the atoms are massed in a solid. We conclude that the solid 

 is ionised in the same way as the gas. We should therefore 

 expect to find slow-moving electrons issuing from radium 

 itself, and from both sides of any solid screen through which 

 the particles pass. Surely these are the effects observed by 

 J. J. Thomson, Rutherford, and others. This has already 

 been suggested by Soddy ("'Nature," March, 1905). 



