XG 
ON SECONDARY RADIATION. 
By J. A. McCLELLAND, M.A., 
Professor of Experimental Physics, University College, Dublin, 
[ Read, December 20, 1904; Published, Marcu 38, 1905. | 
Tue following pages give an account of an investigation of the secondary radia- 
tion 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 6 and y rays—has been considered; the nature of this second- 
ary radiation and its relative intensity im 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 radio-activity and to all phenomena produced 
by the discharge from bodies of particles charged with electricity. 
Secondary radiation under the action of cathode rays has been investigated 
by Starke,t Austin and Starke,{ 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 others. 
Apparatus. 
The secondary radiation was detected and measured by its ionising power. 
Fig. 1 gives a sketch of the apparatus used. 7’ is a brass tube, 20 cms. in length 
* When the work described in this paper was completed and the paper partly written, one on the 
same subject by Mr. Eye appeared in the Philosophical Magazine, December, 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 
haye been made. 
+ Ann, d. Physik u. Chemie (Wied.), Ixyi., 1898, p. 49. Ann. d. Physik (Drude’s), Vierte Felge 
lii., 1900, p. 75. 
+ Ann. d. Physik (Drude’s), Vierte Folge, ix., 1902, p. 271. 
§ Proc. Roy. Soc., lxiy., 1899, p. 377. 
|| Ann, de Chimie et de Physique, tom. xi., 1897, p. 496. 
q Proc. Camb. Phil. Soc., vol. x., 1899, p. 117. 
*% Comptes Rendus, tom. cxxxii., 1901, pp. 371, 734, 1286. 
TRANS, ROY, DUB, SOC., N.S., VOL. VIII., PART XIV, 2G 
