a 
Heating  Effects  produced  by  Rontgen  Rays  in  Metals.     21)3 
On  the  other  hand,  all  substances  when  illuminated  with 
Rontgen  rays  or  with  Becquerel  rays  of  the  y  type  give  out 
complex  secondary  radiation,  part  of  which  at  least  is 
wholly  different  in  character  from  the  primary  radiation. 
For  example,  the  secondary  radiation  due  to  Rontgen  rays 
consists  in  part  of  negatively  charged  corpuscles  or  electrons 
which  are  not  present  in  the  primary  rays.  This  suggests 
that  there  may  be  some  breaking  up  of  the  atoms  of  the 
secondary  radiator;  but  it  is  only  a  suggestion,  for  it  by 
no  means  follows  that  the  presence  of  /3  rays  involves  atomic 
disintegration.  The  modern  theories  of  electrical  conduction 
imply  the  existence  in  conductors  (and  all  bodies  are  con- 
ductors to  a  greater  or  less  extent)  of  large  numbers  of 
corpuscles  not  closely  bound  up  in  the  atomic  structure  ; 
and  it  is  quite  conceivable  that  some  of  these  may  constitute 
the  secondary  j3  rays,  the  necessary  energy  having  been, 
in  some  way,  imparted  to  them  by  the  primary  radiation. 
The  most  direct  way  of  discriminating  between  these  two 
possibilities  is  to  investigate  the  energy  relations  when,  for 
example,  Rontgen  rays  are  absorbed  by  matter.  If  none  of 
the  atoms  are  broken  up,  then  the  conservation  of  energy, 
in  the  ordinary  sense,  will  hold  :  if,  on  the  other  hand,  some 
of  the  atoms  are  exploded  by  the  Rontgen  rays,  as  dynamite 
is  exploded  by  a  shock,  then  the  total  energy  after  the 
absorption  of  the  rays  may  be  considerably  greater  than  the 
energy  of  the  rays  themselves.  This  excess  of  energy  might 
be  expected  to  manifest  itself  mainly  in  the  form  of  heat  in 
the  absorbing  body;  for  it  is  known  that  a  large  fraction  of 
the  secondary  Rontgen  rays  are  very  easily  absorbed  indeed*; 
and  Sagnac  found  that  tertiary  rays  were  more  easily  absorbed 
than  secondary  raysf.  Thus  only  the  secondary  rays  which 
are  produced  very  near  the  surface  of  the  absorber  would 
carry  their  energy  away  with  them  ;  those  which  are  set  up 
throughout  the  mass  of  the  body  would  be  absorbed  before 
reaching  the  surface,  and  eventually  would  warm  the 
absorber. 
Assuming  for  the  moment  that  Rontgen  rays  are  able  to 
cause  atoms  to  break  up,  it  is  very  improbable  that  the  atoms 
of  different  substances  are  equally  susceptible  to  this  effect  ; 
and  we  should  expect  to  find  an  inequality  in  the  amount  of 
heat  produced  when  Rontgen  rays  are  equally  absorbed  in 
different  substances.  If,  on  the  contrary,  there  is  no  atomic 
disintegration,  the  quantities  of  heat  should  be  equal.      It  was 
*  J.  J.  Thomson,  'The  Conduction  of  Electricity  through  Gases,' 
p.  263. 
t  Ibid.  p.  273. 
