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XXII. On Secondary Radiation. By J. A. McClelland, 

 M.A., Professor of Experimental Physics, University Col- 

 lege, Dublin*. 



THE following paper gives an account of an investigation 

 of the secondary radiation given off by bodies when they 

 are exposed to a strong primary radiation from radium. 



In the present paper, the secondary radiation produced by 

 the more penetrating radium rays only — the /3 and 7 rays — has 

 been considered ; the nature of this secondary radiation and 

 its relative intensity in the case of different substances have 

 been studied. The subject of secondary radiation seems of 

 some importance and promise, as there are many problems on 

 which it may have a possible bearing ; it is closely related to 

 the subject of spontaneous radioactivity and to all phenomena 

 produced by the discharge from bodies of particles charged 

 with electricity. 



Secondary radiation under the action of cathode ray- has 

 been investigated by Starke f, Austin and Starke J, Swinton §, 

 and others. Secondary radiation under the action of Rontgen 

 rays has been studied by Perrin ||, Townsend^[ , and others : 

 while secondary radiation due to radium rays has been 

 detected and investigated to some extent by Becquerel** and 

 other s.tt 



Apparatus. 



The secondary radiation was detected and measured by its 

 ionizing power. Fig. 1 gives a sketch of the apparatus used. 

 T is a brass tube 20 cms. in length and ;V2 cms. internal 

 diameter, connected to small storage-cells by which it could 

 be kept at any required potential ; a metal rod is fixed along 

 the axis of the tube, being insulated by paraffin and joined to 

 a Dolezalek electrometer, the joining wire being suitably 



* Communicated by the Author. From Royal Dublin Society's 

 Transactions, 1904 : read Dec. 20th, 1904. 



f Wied. Ann. vol. lxvi. p. 49 (1898) : Diude's Ann. vol. iii. p. 75 

 (1900). 



t Drude, Ann. vol. ix. p. 271 (1902). 



§ Proc. Roy. Soc. vol. lxiv. p. 377 (1899). 



|| Annates de Chimie et de Physique, 1897, p. 490. 



H Proc. Camb. Phil Soc. 1899, p. 217. 



** Camptes Rendus, 1901, pp. 371, 734, 1286. 



ft When the work described in this paper was completed and the paper 

 partly written, one on the same subject by Mr. Eve appeared in the 

 Philosophical Magazine, Dec. 1904 ; the points discussed in the two 

 papers are not, however, by any means always the same. In cases where 

 the same points are discussed the agreement is, on the whole, good, 

 although in some cases somewhat different deductions have been made. 



