614: Dr. J. Kunz : Abrupt Limit of Distance in the 



curve would have become parallel to the .i'-axis after reaching 

 the point b ; while if only a feeble scattering of P 3 be 

 postulated, the curve droops much more rapidly towards 

 the end. 



An experimental investigation of the curve beyond the point 

 b, which I hope to make shortly, would be of interest in 

 deciding the nature of the air-radiations excited by the very 

 penetrating rays. At the present stage of the investigations 

 it would be useless to attempt to interpret the results more 

 definitely than has been done in this paper, but a plausible 

 explanation of the transformation of the harder rays would 

 seem to be that the interval between the arrival of successive 

 pulses is great enough to allow the electrons to oscillate in 

 their free periods during this interval. This assumes that 

 those free oscillations which are effective in producing 

 ionization cause a softer radiation than the average of those 

 radiations from the bulb which can penetrate through an 

 Al plate *265 cm. thick. What is wanted most in these 

 experiments is a method of producing a constant stream of 

 homogeneous Rontgen radiation. In the analogous case of 

 fluorescence in light, the results achieved by Professor Wood 

 in the fluorescence of sodium vapour stimulated by mono- 

 chromatic illumination are well known, and it seems possible 

 that similar results might be obtained with Rontgen pulses if 

 the exciting radiations could be definitely controlled and 

 varied. 



In conclusion 1 wish to express my thanks for their kindly 

 advice and interest to Professor Wilberforce and to Dr. Barkla, 

 at whose suggestion this investigation was undertaken. 



The University of Liverpool. 



LXI. An Abrupt Limit of Distance in the Power of the 

 Positive Rays to produce Phosphorescence. By Jakob Kunz, 

 Dr. phil. Privatdozent am eidg. Polytechnikum in Zurich * . 



THE experiments made by Mine. Curie f , Rutherford J, 

 Bragg and Kleeman § on the a. rays from polonium and 

 radium, showed that the absorption of these rays presents 

 some very interesting features. The a particles are able to 

 penetrate a certain distance in air without loss of their ionizing 

 power, while, after passing this distance, they suddenly lose 

 this power almost entirely ; they lose at the same time their 



* Communicated by the Author. 



t Curie, Comptes Hendus, cxxvii. p. 175 (1898). 



t Rutherford, Phil. Mag. [6] x. p. 168 (1905). 



§ Bragg and Kleeman, Phil. Mag-. [6] x. p. 318 (1905). 



