1  rarious  Gases  hy  the  a.  Particles  of  Radium.  619 
The  factor  to  be  applied  to  a  ten-second  leak  varies  from 
about  1-03  at  40°  C.  to  about  MO  at  70°  C.  ;  at  90°  C.  it  is 
much  higher. 
The  total  ionization  is  measured  in  terms  of  the  product  R  T. 
The  ordinate  R  is  the  range  of  the  a  particle,  due  to  that 
product  of  radium  whose  speed  is  next  to  that  of  Ra  C.  In 
air  at  760  mm.  pressure  and  20°  C,  R  ==  4*83  cm.  very 
nearly.  The  abscissa  I  is  the  ionization  produced  in  the 
chamber  described  when  the  radium  layer  is  at  a  distance  of 
4*83  cm.  from  the  middle  of  the  chamber :  or,  more  correctly, 
it  is  proportional  to  the  ionization  that  would  be  produced  in 
a  very  shallow  chamber  at  that  distance.  The  effect  is  wholly 
due  to  the  particles  from  RaC,  the  chamber  being  out  of  range 
of  all  the  others. 
These  two  quantities  R  and  I  differ  materially  from  each 
other  in  two  respects.  To  take  the  less  important  con- 
sideration first,  the  former  quantity  lends  itself  readily  to 
exact  measurement,  the  latter  does  not.  The  range  of  the 
a.  particle  in  a  gas  can  be  measured  to  an  accuracy  of  one  or 
two  per  cent,  by  a  few  minutes'  observation,  and  to  a  much 
higher  degree  with  greater  care  :  it  is  perhaps  the  easiest  of 
-the  measurements  made  in  these  experiments.  By  far  the 
greatest  difficulties  which  I  find  in  the  determination  of  the 
stopping-power  of  a  gas  lie  in  the  purification  and  analysis  of 
the  gas. 
On  the  other  hand,  the  abscissa  I  is  much  more  difficult  to 
measure.  It  is  affected  by  variation  in  the  sensitiveness  of 
the  electrometer,  by  leakage  through  the  insulators,  by 
variation  of  the  dimensions  of  the  apparatus,  and  its  true 
value  is  not  given  unless  enough  electric  force  is  applied. 
None  of  these  things  affects  the  range.  But  it  is  not  merely 
in  the  details  of  measurement  that  these  two  quantities  differ. 
They  appear  as  physical  constants  to  be  in  distinct  categories : 
so  far,  that  is  to  say,  as  can  be  observed  at  present.  The 
stopping-power  of  an  atom  is  a  constant  of  the  atom,  un- 
affected by  its  association  with  other  atoms  in  molecular 
structure,  independent  of  pressure  and  temperature.  In  a 
paper  by  Mr.  Kleeman  and  myself  (Phil.  Mag.  Sept.  1905), 
we  gave  a  list  of  the  stopping-powers  of  various  substances, 
and  since  then  we  have  made  many  other  experiments  in  the 
same  direction.  In  no  case  have  we  found  a  departure  from 
the  additive  law  which  was  not  within  the  errors  of  experiment. 
That  is  to  say,  the  range  of  the  a  particle  in  a  given  gas  can 
always  be  predicted  from  the  composition  of  the  gas  molecule. 
Not  only  so,  but  the  stopping-powers  of  the  various  atoms 
-are  very   nearly  proportional   to  the   square  roots   of    their 
2  S2 
