310         Prof.  H.  A.  Buinstead  on  the  Heating  Effects 
the  scale.  For  a  variation  in  the  vane  distance  of  0*48  mm.  the 
change  in  sensitiveness  was  less  than  12  per  cent.  The 
positions  of  the  surfaces  of  the  lead  and  zinc  strips  were 
afterwards  tested  by  means  of  a  point  gauge,  and  they 
certainly  did  not  differ  by  more  than  0*1  mm. 
8.  It  might  also  be  imagined  that  the  measurement  of 
the  absorptions  by  means  of  the  electroscope  did  not  give  the 
relative  amounts  of  energy  absorbed  by  the  lead  and  zinc  : 
that  the  ionization  in  the  electroscope  wras  not  a  measure  of  the 
energy  of  the  rays.  One  may  express  this  possibility  in  slightly 
different  form  by  saying  that,  accompanying  the  rays  which 
ionize  the  gas,  there  may  be  other  rays  which  do  not  cause 
ionization  but  -which  do  carry  energy  ;  and  that  these  latter 
may  be  more  absorbed  in  the  lead  than  in  the  zinc.  In  order 
to  test  this,  measurements  of  the  absorption  were  made  with 
the  radiometer  itself.  One  strip  was  shielded  so  that  as  large 
a  deflexion  as  possible  might  be  obtained  with  the  rays,  and 
an  aluminium  screen  of  the  same  thickness  as  the  window  was 
set  up  in  front  of  the  window ;  the  lead  and  zinc  of  the  thick- 
ness of  the  strips  were  introduced  behind  this  screen.  The 
following  deflexions  in  succession  were  obtained  by  a  three- 
minute  exposure  to  the  rays  in  each  case  : — 
Al     &± 
Al  +  Pb    0-8 
Al  +  Zn    0*9 
Al  +  Pb    0-6 
Al     0-(5 
It  is  plain  that  the  rays  did  not  remain  very  constant 
during  this  somewhat  prolonged  use;  but  it  is  also  plain  that, 
whatever  agent  it  may  be  which  affects  the  radiometer,  it 
is  practically  equally  absorbed  by  the  lead  and  the  zinc. 
Taking  the  means  of  the  readings  as  they  stand,  we  find 
that,  of  the  energy  which  gets  through  the  aluminium, 
88  per  cent,  is  absorbed  by  the  zinc  and  91  per  cent,  by  the 
lead. 
9.  Even  if  one  assumed  the  substantial  correctness  of  the 
energy  measurements,  there  still  remains  the  possibility  that 
the  difference  between  the  two  metals  may  not  be  due  to 
atomic  disintegration.  It  is  known  that  secondary  rays  are 
generated  by  the  absorption  of  the  primary  rays,  and  it  may 
be  supposed  that  these  carry  away  from  the  metal  a  consider- 
able fraction  of  the  energy  of  the  primary  rays — a  much 
greater  fraction  in  the  case  of  zinc  than  in  the  case  of  lead, 
thus  giving  rise  to  the  observed  difference.  An  examination 
of  this  possibility  is  therefore  necessary. 
