Various  Gases  by  the  a  Particles  of  Radium.  621 
stopping-powers  are  systematic  even  in  their  slight  departure 
from  the  square-root  law.  For,  whilst  dependent  mainly  on 
the  square  roots  of  the  weights,  they  have  a  leaning  towards 
the  weights  themselves.  We  did  not  call  attention  at  the 
time  to  this  fact,  for  we  thought  it  might  be  a  spurious  effect. 
But  it  has  appeared  so  regularly  in  all  further  determinations 
that  it  seems  right  to  note  it,  and  to  attempt  an  explanation 
of  its  physical  meaning. 
If  we  assume  the  correctness  of  the  explanation  already 
given  of  the  square-root  law,  viz.,  that  the  a.  particle  spends 
energy  for  the  most  part  on  tearing  away  electrons  from 
their  attachment  at  the  edges  of  the  atom  disks,  then  the 
natural  complement  to  this  is  the  further  assumption  that 
electrons  in  all  parts  of  the  atom  disk  may  be  disturbed  to 
vibration  by  the  passage  of  the  particle,  which  latter  there- 
fore spends  a  small  amount  of  energy  in  simple  proportion 
to  the  weight  of  the  atom.  If  w  be  the  atomic  weight,  the 
stopping-power  of  an  atom  should  therefore  be  capable  of 
expression  by  the  formula 
a^/iv  +  bw; 
where  the  former  term  is  usually  by  far  the  most  important. 
The  close  agreement  of  the  figures  in  the  second  and  fifth 
columns  of  Table  A  shows  that  this  is  very  nearly  the  case. 
As  regards  pressure  and  temperature,  I  have  not  yet 
found  any  effect  produced  by  variation  of  these  conditions. 
The  quantity  RP/T  appears  to  be  a  constant,  P  being  the 
pressure  and  T  the  absolute  temperature.  This  implies  that 
the  stopping-power  of  an  atom  or  molecule  is  independent 
of  P  and  T.  Examples  of  the  fact  that  RP  is  constant 
whilst  T  is  constant  are  given  in  the  paper  "  On  the  Recom- 
bination of  Ions  in  Air  and  other  Gases. "  The  following- 
experimental  result  will  serve  as  an  illustration  of  the  fact 
that  R  is  proportional  to  T  when  P  is  constant.  The 
ionization  vessel  filled  with  air  was  raised  to  a  temperature 
of  90°  C,  the  pressure  being  763  mm.  R  was  then  found  to 
be  5-98.     Now  whenp  =  760  mm.,  and  T  =  20°  C,  R=4*83. 
4-83x363x760 
And 
5-98  x  293  x  763 
=  1-005. 
It  has  of  course  been  pointed  out  by  several  observers 
that  the  ionization  effects  of  radium  are  largely  independent 
of  pressure  and  temperature  and  of  physical  and  chemical 
conditions  generally. 
This,  however,  does  not  cover  the  present  statement, 
which  refers  to  the  stopping-power  of  the  atom,  a  quantity 
