220  Mr.  N.  R.  Campbell  on  the 
For  platinum  the  numbers  were — 
x=\  /jLt=S4:,  x=l  fit=-7'o,  .r  —  2  /nt  =  '5i,  oc  —  ?)  fjLt  =  36. 
These  give  a  about  12. 
For  copper  we  find 
#=J  fit  =  '$'2,  «=1  pt—'66,  <>'  =  -  A**— "44,  #  =  3  ^=-29, 
which  gives  a  about  10. 
An  attempt  was  made  to  obtain  similar  numbers  for 
sulphur,  which,  being  a  non-conductor,  could  not  be  investi- 
gated by  the  parallel  plate  method.  For  some  unknown 
reason  it  was  hard  to  get  consistent  results,  but  the  mean  of 
a  large  number  of  observations  seemed  to  indicate  that  a  was 
between  6  and  9. 
§  13.  With  the  same  apparatus  an  attempt  was  made  to 
determine  the  sign  of  the  charge  carried  by  the  intrinsic 
absorbable  rays  from  ordinary  metals.  For  this  purpose  a 
plate  of  lead  was  supported  on  ebonite  blocks  i  cm.  above 
the  aluminium  window  from  which  it  was  insulated.  A 
difference  of  potential  of  1000  volts  was  established  between 
the  plate  and  the  window,  the  field  being  first  in  one  direction 
and  then  in  the  other.  If  the  rays  be  charged,  the  field  will 
tend  to  accelerate  them  in  one  case  and  to  retard  them  in  the 
other  ;  it  might  be  expected  that  the  proportion  passing 
through  the  aluminium  window,  and  therefore  the  ionization 
in  the  box,  would  be  smaller  in  the  latter  case  than  in  the 
former. 
An  estimation  of  the  order  of  magnitude  of  the  effect  upon 
the  leak  which  is  to  be  expected  can  be  made  easily. 
If  the  rays  be  negatively  charged,  they  must  be  of  the 
nature  of  cathode  rays,  and  their  penetration  would  indicate 
that  their  velocity  was  about  109  cms.  per  second.  The 
velocity  v  acquired  by  a  particle  of  mass  m  and  carrying  a 
charge  e  in   falling   through  a  difference  of  potential  V  is 
given  by  -^-  =eV.  is  IS  x  107  ;  V  =  10n  ; 
.-.     7j2  =  3-6xl018     or     17  =  1-9  xlO9. 
Hence  the  rays  would  be  almost  stopped  before  they  reached 
the   window.     But  if   the  rays  were  positively  charged,    — 
would  be  104,  and  the  velocity  acquired  in  falling  through 
1000  volts  would  be  only  (2  x  1015)*  =  4'4  x  107.  The  velocity 
of  the  «  rays   of  radium   is  about  2x10°;  and  hence   the 
