764 
Prof.  W.  H.  Bragg  on  the 
applies  only  in  considering  the  relative  ranges  of:  uranium  and 
thorium;  other  considerations  enter  when  the  method  is  used 
to  determine  ranges  absolutely. 
The  falling  o£  the  first  two  results  in  column  V.  below  the 
average  of  t lie  column  may  in  part  be  due  to  the  fact  that 
the  first  layer  or  two  must  cut  off  the  easily  absorbed  radiation 
whose  existence  has  been  proved  by  J.  J.  Thomson  and  by 
Eutherford.  I  am  not  aware  of  any  measurement  of  the 
amount  of  ionization  due  to  this  radiation.  If  in  this  expe- 
riment only  4  per  cent,  of  the  whole  ionization  current  when 
the  material  is  uncovered  is  supposed  due  to  this  cause,  and 
if  the  foil  whose  pd  =  '000317  cuts  off  three  quarters  of  it 
and  the  next  addition  of  foil  the  remainder,  then  the  figures 
in  the  last  column  become,  in  order,  488,  487,  487,  496,  486, 
490,  502,  514.  Thus  the  existence  of  a  small  quantity  of 
radiation  of  this  sort  would  explain  the  present  discrepancies 
in  the  experiment.  It  will  be  seen  later  that  a  similar  effect 
occurs  with  thorium.  It  is  not  so  noticeable  in  the  uranium 
experiment,  as  will  be  seen  on  turning  back  to  the  table 
of  results.  Still  the  first  of  these  is  rather  smaller  than 
the  others  ;  and  a  special  measurement  made  with  a  very 
thin  layer  for  which  pd=  '000133  gave  a  value  for  the  full 
range  equal  to  '00426,  which  is  smaller  still. 
RaC. 
As  a  further  test  of  the  method,  I  have  used  it  to  determine 
the  range  of  the  a  particle  of  Ra  C.  The  special  difficulty  in 
this  case  was  due  to  the  rapid  decline  of  the  activity.  It  was 
avoided  by  taking  readings  of  the  current  due  to  the  uncovered 
active  plate  before  and  after  each  measurement  with  the 
covered  plate.  The  geometric  mean  of  these  was  taken  and 
compared  with  the  smaller  measurement.  The  readings  were 
properly  spaced  in  time,  so  as  to  make  this  correct.  The  leak 
due  to  ft  rays  and  external  causes  was  found  by  placing  six 
sheets  of  tinfoil,  each  equivalent  to  about  15  mm.  of  air,  over 
the  active  plate.  Tinfoil  was  used  as  the  absorbing  sheet. 
The  results  were  as  follows : — 
I. 
pel  of  tinfoil 
X  106 
II. 
ill. 
III. 
From  Curve  B. 
IV. 
pel  of  full  range 
X  105. 
479 
958 
1437 
•518 
•235 
•072 
34-2 
73-0 
112-5 
224 
210 
204 
