294         Prof.  H.  A.  Bumstead  on  the  Heating  Effects 
from  this  point  of  view  that  the  problem  was  proposed  to  me 
by  Professor  J.  J.  Thomson,  during  my  stay  in  Cambridge 
]ast  year ;  and  the  experiments  which  I  am  abont  to  describe 
were  carried  out  in  the  Cavendish  Laboratory  under  his 
direction,  and  owe  much  to  his  advice  and  co-operation. 
In  considering  the  various  experimental  means  by  which 
this  problem  could  be  attacked,  the  radiometer  seemed  to 
promise  certain  advantages  over  other  heat-measuring 
instruments.  For  measuring  ordinary  radiation  with  this 
instrument  (the  development  of  which  is  chiefly  due  to 
E.  F.  Nichols),  the  usual  method  is  to  have  opaque  vanes 
with  a  transparent  wall  near  them  upon  one  side  ;  when  one 
of  the  vanes  is  illuminated,  it  is  heated  and  the  molecular 
reaction  causes  it  to  be  repelled  from  the  neighbouring  wall. 
But  it  is  quite  possible  to  reverse  this  procedure  and  have 
the  wall  opaque  and  the  vanes  transparent,  and,  although 
the  attainable  sensitiveness  is  probably  less  than  in  the  other 
case5  it  has  obvious  advantages  when  one  is  dealing  with 
Pontgen  rays  instead  of  ordinary  light.  The  walls  can  be 
made  of  the  metals  under  investigation,  and  of  suitable  thick- 
ness to  absorb  a  considerable  fraction  of  the  rays  incident  upon 
them  ;  while  the  vanes  may  be  made  very  transparent  to  the 
rays  and  be  thus  far  less  in  the  way,  and  be  less  heated  by 
the  rays  (independently  of  the  heating  of  the  substances 
under  examination)  than  would  be  possible  with  the  thermopile 
or  bolometer.  In  the  construction  of  the  radiometer  and  its 
gradual  adaption  to  the  present  purpose,  I  was  particularly 
fortunate  in  having  the  advice  and  assistance  of  Professor 
E.  F.  Nichols  who  was  also  in  Cambridge,  and  who  most 
kindly  put  at  my  disposal  the  results  of  his  long  experience 
with  the  radiometer.  I  wish  here  to  express  my  thanks  to 
him  for  a  much  shorter  period  of  apprenticeship  to  the 
instrument  than  would  have  been  possible  without  his  help. 
Apparatus. 
The  radiometer  and  its  adjuncts  passed  through  many 
preliminary  and  tentative  forms  before  a  satisfactory 
arrangement  was  obtained.  The  final  form  which  proved 
fairly  sensitive  and  manageable  is  here  described. 
The  vanes  were  made  of  aluminium-foil  which  weighed  about 
1  mg.  per  square  cm.  Each  vane  measured  8  x  10  millimetres 
with  its  greatest  dimension  vertical,  and  the  two  pieces  of 
foil  were  stretched  between  two  very  thin  horizontal  rods  of 
glass  at  top  and  bottom,  which  in  turn  were  kept  at  the 
proper  distance  apart  by  their  attachments  to  the  central  rod 
