10 



The point is of considerable interest, for the existence of 

 these pairs would help to explain much of the mechanism of 

 phosphorescence. They would appear to be connected with 

 the clusters of J. B. B. Burke, which were produced by ioni- 

 sation, gave rise to phosphorescent glow, contained energy, 

 yet were not electrified. It is of interest in this connection 

 that the photograph which Sir William and Lady Hugdns 

 made of the phosphorescent glow of radium showed the bands 

 of the gas in which the salt was embedded. Rutherford also 

 has shown that the a particle can no longer cause phos- 

 phorescence when ifc has lost its power of ionisation. 



The Magnetic Deflection of the a Particle. 



In the "Physikalische Zeitschrift" for October 15 is a 

 paper by M. Becquerel, "Uber einige Eigenschaften der a 

 Strahlen des Radiums." The author discusses the theory 

 that the a rays gradually lose their velocity as they spend 

 their energy on the ionisation of the media through which 

 they pass, a theory which I put forward about two years 

 ago,"^ and which has the support of much experimental evi- 

 dence accumulated by Professor Rutherford,! and by Mr. 

 Kleeman and myself, f 



He maintains that the theory is unsuccessful in explain- 

 ing the experiments which he has himself performed, and in 

 particular he describes one experiment which he has devised 

 as a crucial test, and which he considers to show that the 

 theory is incorrect. 



It is as follows (loc. cit., p. 688) : — 



The a rays from a small quantity of radium salt are 

 allowed to stream upwards through a narow slit and fall 

 upon a photographic plate. A powerful magnetic field de- 

 flects them slightly to one side. The field is reversed when 

 the experiment is halfway through, and as a result two 

 images of the slit appear, slightly separated, upon the plate. 

 Now, M. Becquerel covers half the slit with a thin sheet of 

 aluminium, and, according to the theory which I have ad- 

 vanced, the a rays which pass through the sheet are thereby 

 retarded. Consequently, M. Becquerel argues, these a rays 

 should be more bent to one side than those which have not 



* Australasian Association for the Advancement of Science, Re- 

 port, Dnnedin, January, 1904. 



t "Phil. Mag.," July, 1905. 



t "Phil. Mag.," December, 1904, and September, 1905, 



