812 
Dr.  0.  G.  Barkla 
does  not  give  off  a  particles  or,  at  any  rate,  none  of  sufficient 
velocity  to  ionize  air. 
Fiar.  4. 
800 
•700 
600 
'500 
|  *oo 
N 
|     300 
I 00 
100 
0 
y 
/ 
/ 
^ 
V 
/ 
/ 
^ 
/ 
/       / 
/ 
// 
/ 
1 
! 
10  30  40  50 
Pressure  //v  cms. 
60 
70 
In  conclusion  1  desire  to  express  my  indebtedness  to 
Professor  Rutherford  for  suggesting  and  kindly  supervising 
this  investigation. 
Macdonald  Physics  Building,  McGill  University, 
Montreal,  April  7,  1906. 
LXXYI.  Secondary  Rontgen  Radiation.  By  Charles  Gr. 
Barkla,  JJ.Sc.  (Liverpool),  M.Sc.  (  Victoria),  B.A.  [King's 
College,  Cambridge) ;  Lecturer  in  Advanced  Electricity, 
University  of  Liverpool  * '. 
Lntroduction. 
SINCE  the  discovery  by  Rontgen  of  secondary  radiation 
from  substances  exposed  to  X-rays,  the  subject  has  been 
investigated  by  a  number  of  experimenters,  including  Perriu, 
Townsend,  Dorn,  Curie,  Sagnac,  Langevin,  and  Bumstead  f. 
*  Communicated  by  the  Physical  Society  :  read  February  23,  1906. 
f  Perrin,  Annates  de  Chimie  et  de  Physique,  [7]  xi.  p.  496  (1897). 
Townsend,  Proc.  Camb.  Phil.  Soc.  x.  p.  217  (1899).  Dorn,  Abhand.  d. 
naturf.  Ges.  zu  Halle,  xxii.  p.  39  (1900).  Sagnac,  Annates  de  Chimie 
et  de  Physique,  [71  xxii.  p.  493  (1901).  Curie  &  Sagnac,  Journal  de 
Physique,  [4]  i.  p.  13  (1902).  Langevin,  Pecherches  sur  les  gaz  ionises. 
Bumstead,  Phil.  Mag.  xi.  pp.  292-317,  Feb.  1906. 
