[     769     ] 
LXX.  On  the  Number  of  Corpuscles  in  an  Atom. 
By  Prof.  J.  J.  Thomson,  M.A.,  F.R.S.* 
I  CONSIDER  in  this  paper  three  methods  o£  determining 
the  number  of  corpuscles  in  an  atom  of  an  elementary 
substance,  all  of  which  lead  to  the  conclusion  that  this  number 
is  of  the  same  order  as  the  atomic  weight  of  the  substance. 
Two  of  these  methods  show  in  addition  that  the  ratio  of  the 
number  of  corpuscles  in  the  atom  to  the  atomic  weight  of  the 
element  is  the  same  for  all  elements.  The  data  at  present 
available  indicate  that  the  number  of  corpuscles  in  the  atom 
is  equal  to  the  atomic  weight.  As,  however,  the  evidence  is 
rather  indirect  and  the  data'  are  not  very  numerous,  further 
investigation  is  necessary  before  we  can  be  sure  of  this  equality; 
the  evidence  at  present  available  seems,  however,  sufficient  to 
establish  the  conclusion  that  the  number  of  corpuscles  is  not 
greatly  different  from  the  atomic  weight. 
It  will  be  seen  that  the  methods  are  very  different  and  deal 
with  widely  separated  physical  phenomena ;  and  although  no 
one  of  the  methods  can,  I  think,  be  regarded  as  quite  con- 
clusive by  itself,  the  evidence  becomes  very  strong  when  we 
find  that  such  different  methods  lead  to  practically  identical 
results. 
To  enable  the  argument  to  be  more  easily  followed,  I 
shall  begin  with  a  general  description  of  the  methods  and 
the  results  to  which  they  lead,  and  postpone  the  details  of 
the  theory  of  each  of  the  methods  to  the  latter  part  of  the 
paper. 
The  first  method  is  founded  on  the  dispersion  of  light  by 
gases.  If  we  regard  an  atom  as  consisting  of  a  number  of 
corpuscles  dispersed  through  a  sphere  of  uniform  positive 
electrification,  it  is  evident  that  the  dispersive  power  of  a 
medium  consisting  of  these  atoms  will  depend  upon  the  mass 
of  the  positive  electrification  as  well  as  upon  the  mass  of  the 
corpuscles,  and  will  vanish  if  either  of  these  masses  is  zero. 
For  consider  what  takes  place  when  the  electric  force  in  the 
light-wave  strikes  the  atom.  Since  the  wave-length  is  large 
compared  with  an  atom,  the  latter  may  be  regarded  as  being 
in  a  uniform  electric  field  ;  under  this  field  the  corpuscles 
will  be  displaced  in  one  direction,  the  positive  sphere  in  the 
opposite  ;  and  if  the  force  persists  long  enough,  this  displace- 
ment will  go  on  until  the  force  exerted  on  the  corpuscles  in 
their  displaced  position  by  the  positive  electricity  is  equal  and 
*  Commimicated  bv  the  Author. 
