'produced  by  Rontyen  Rays  in  Different  Metals.        307 
when  the  strips  were  heated  by  ordinary  radiation.  For  this 
purpose,  a  glass  window  was  substituted  for  the  one  of 
aluminium  and  (everything  else  being  left  exactly  as  before) 
a  beam  of  light  was  admitted  from  an  eight- candle  power 
incandescent  lamp,  125  cm.  from  the  window.  The  result  of 
such  an  experiment  is  shown  graphically  in  fig.  5,  where  as 
before  the  abscissae  represent  time  in  minutes,  the  ordinates 
deflexions  in  centimetres ;  a  positive  deflexion  means  repulsion 
by  the  zinc,  a  negative  deflexion  repulsion  by  lead.  From 
Qm  to  ~m  ^h  strips  were  exposed  to  the  light,  and  the 
curve  clearly  shows  the  temporary  deflexion  in  favour  of  the 
Fig.  5 
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lead,  returning  to  zero  as  the  zinc  overtakes  the  lead  ;  from 
7m  to  14m  the  window  was  covered  by  the  lead  screen  so 
that  both  strips  cooled,  the  zinc  lagging  behind  as  before, 
and  so  giving  a  similar  temporary  deflexion  in  the  opposite 
direction.  From  14m  to  22m,  the  zinc  strip  was  exposed 
while  the  lead  was  shielded,  and  from  29m  to  36m  the 
lead  alone  was  exposed.  It  will  be  observed  that  the 
behaviour  is  perfectly  in  accord  with  the  above  explanation 
of  the  heating  effect,  and   that  the  deflexions,  when  the  lead 
X2 
