Secondary  Rontgen  Radiation.  827 
o£  propagation  with  regard  to  the   plane  of  polarization  in 
the  primary  beam. 
In  heavier  atoms  each  electron  is  more  intimately  con- 
nected with  the  electrons  in  its  immediate  neighbourhood, 
and  is  therefore  subject  to  considerable  disturbing  forces 
due  to  the  displacement  of  these.  The  period  for  which  it  is 
subject  to  considerable  forces  is  much  greater  than  that  of 
passage  of  the  primary  pulse  over  it.  hence  the  secondary 
pulse  emitted  is  thicker  and  more  complex  in  character. 
The  greater  thickness  of  the  secondary  pulse  results  in 
greater  absorbability;  and  the  interference  with  the  simple 
direct  acceleration  due  to  the  primary  pulse  prevents  pure 
scattering,  and  accounts  for  the  disappearance  of  polarization 
in  the  secondary  beam  and  of  evidence  of  polarization  in  the 
primary.  An  increase  in  thickness  of  the  primary  pulse 
produces  an  increase  in  thickness  of  the  secondary  pulses; 
consequently  an  increase  in  absorbability  of  the  primary 
results  in  an  increase  in  the  absorbability  of  the  secondary. 
On  this  hypothesis,  the  penetrating  power  of  the  secondary 
radiation  is  a  measure  of  the  independence  of  motion  of 
corpuscles  or  electrons  within  the  atom;  and  the  relation 
between  absorption  and  atomic  weight  exhibiting  a  periodicity 
which  is  obviously  connected  with  the  periodicity  in  chemical 
properties,  is  evidence  of  a  connexion  between  chemical 
properties  and  distribution  of  corpuscles  in  the  atom  such  as 
Prof.  J.  J.  Thomson  suggest*.  .  « 
It  would  be  premature  at  this  stage  of  the  investigation  to 
attempt  a  more  detailed  explanation  of  the  results,  as  the 
experiments  are  still  very  far  from  completion.1 
The  theory,  however,  appears  sufficient  to  explain  the 
results  of  experiments  made  up  to  the  time  of  writing, 
without  assuming  appreciable  disintegration  of  the  atom  to 
occur.  Radiation  due  to  disintegration  may  or  may  not 
form  a  portion  of  the  secondary  radiation  emitted  by  metals 
and  detected  by  means  of  an  electroscope,  but  it  appears 
probable  that  the  radiations  studied  have  been  at  the  expense 
of  the  energy  of  primary  radiation. 
It  should  perhaps  be  recalled  that  strong  evidence  of  the 
similarity  in  nature  of  the  secondary  radiation  from  copper 
(a  substance  emitting  a  radiation  differing  considerably  in 
absorbability  from  the  primary  producing  it)  was  given  in 
a  previous  paper  *. 
The  energy  of  secondary  radiation  from  light  atoms  was 
shown  to  be  accounted  for  by  scattering  of  the  primary 
radiation  by  the  constituent  electrons. 
*  Phil.  Mag.  vii.  pp.  543-560,  May  1904. 
