772  Prof.  J.  J.  Thomson  on  the 
cannot  be  small  compared  with  ?im,  the  mass  of  the  carriers  of 
negative  electricity.  Hence  we  infer  that  n,  the  number  of 
corpuscles  in  a  hydrogen  atom,  is  not  much  greater  than  unity. 
This  result  has  been  deduced  from  the  consideration  of  the 
properties  of  a  diatomic  molecule  :  and  if  the  atoms  in  the 
molecule  were  charged,  the  expression  for  Qi2—  1)/(/lt  +  2) 
would  have  to  be  modified  ;  but  since  the  dispersion  of  helium 
is  by  Lord  Rayleiglr's  result  comparable  with  that  of  hydrogen, 
we  see.  since  the  dispersion  is  proportional  to  -,-f- — that 
L  r     L  M  +  nm  iv 
there  cannot  be  a  very  large  number  of  corpuscles  in  the 
helium  atom  ;  for  if  n  were  large,  the  dispersion  of  helium 
would  be  far  too  small. 
'2nd  Method.  Scattering  of  Rontgen  Radiation  by  Gases. — 
It  is  shown  in  my  '  Conduction  of  Electricity  through  Gases ' 
that  when  Rontgen  rays  pass  through  a  medium  in  which 
there  are  N  corpuscles  per  cubic  centimetre,  the  energy  in 
the  radiation  scattered  per  cubic  centimetre  of  the  medium  is 
~ ^E,  where  E  is  the   energy  of  the  primary  radiation 
6    m~  c"  r 
passing  through  the  cubic  centimetre,  e  the  charge,  and  m  the 
mass  of  the  corpuscle.  Barkia  has  shown  that  in  the  case  of 
gases  the  energy  in  the  scattered  radiation  always  bears,  for 
the  same  gas,  a  constant  ratio  to  the  energy  in  the  primary 
whatever  be  the  nature  of  the  rays,  I.  e.  whether  they  are 
hard  or  soft;  and  secondly,  that  the  scattered  energy  is  pro- 
portional to  the  mass  of  the  gas.  The  first  of  these  results  is 
a  confirmation  of  the  theory,  as  the  ratio  of  the  energy  scattered 
to  that  in  the  primary  rays  is  -~-  X(V/??r),  and  is  independent 
of  the  nature  of  the  rays;  the  second  result  shows  that  the 
number  of  corpuscles  per  cub.  centim.  is  proportional  to 
the  mass  of  the  gas  :  from  this  it  follows  that  the  number 
of  corpuscles  in  an  atom  is  proportional  to  the  mass  of  the 
atonij  i.  e.  to  the  atomic  weight.  Barkia  measured  the  ratio 
of  the  energy  in  the  scattered  radiation  to  that  in  the  primary 
in  the  case  of  air,  and  found  that  it  was  equal  to  2"-±X  10-4. 
Thus,  for  air 
6    mr 
Now  e/m  =  l'l  x  10"  and  e  =  \'\  x  10~20;  hence 
Ne=10. 
