768  The  a  Particles  of  Uranium  and  Thorium. 
the  first  place,  it  is  not  easy  to  make  the  thin  aluminium-leaf 
lie  very  close  to  the  radiating  surface  ;  and  the  layers  of  air 
close  to  the  surface  contribute  a  relatively  large  number  of 
ions.  To  make  this  error  uniform  I  have  used  a  net  of  very 
fine  wires,  with  a  mesh  of  |  of  an  inch  to  keep  the  foils  down. 
Again,  there  is  a  disturbing  effect  due  to  the  secondary 
ionization  of  the  absorbing  sheet.  Mme.  Curie  has  called 
attention  to  effects  of  this  kind  (Rutherford,  '  Radioactivity/ 
1905,  p.  189).  I  find  that  there  is  slightly  more  ionization 
when,  of  two  layers  of  foil,  Al  and  Sn,  the  latter  is  on  top. 
Using  tinfoil,  the  range  always  comes  out  rather  larger  than 
when  aluminium-foil  is  employed.  This  may  possibly  be 
because  the  tinfoil  lies  closer  to  the  surface.  All  these  effects 
are  small,  and  disappear  in  a  comparison  method. 
By  direct  measurement  I  found  that  aluminium-foil  for 
which  pd  =  '00329  was  equivalent  to  2*30  cm.  of  air.  Hence 
the  range  of  the  a  particle  of  radium  as  measured  by  this 
method  =2*30  x  466/329  =  3*26.  The  actual  value,  as  found 
directly,  is  3*54. 
One  other  difficulty  lies  in  the  way  of  a  direct  determination. 
As  has  already  been  mentioned  bv  Kleeman  and  myself  (Phil. 
Mag.  Sept.  1905),  the  loss  of  range  of  the  a  particle  of  Ra  C 
in  going  through  a  given  sheet  of  material  appears  from  the 
ionization  curves  to  be  slightly  greater  than  the  loss  of  an 
a  particle  of  RaA;  and  it  is  not  quite  clear  whether  this 
difference  is  real  or  apparent. 
I  owe  my  grateful  thanks  to  Dr.  W.  T.  Cooke  who  carried 
out  for  me  all  the  necessary  chemical  operations. 
Since  the  above  was  written  I  have  received  the  February 
number  of  the  Philosophical  Magazine  containing  an  article 
by  Mr.  N.  R.  Campbell  on  "  The  Radiation  from  Ordinary 
Materials."  In  finding  the  formulae  necessary  to  his  investi- 
gation, Mr.  Campbell  has  anticipated  part  of  the  work  in 
Part  I.  of  this  paper ;  but  as  the  fuller  treatment  which  I 
have  given  is  required  for  my  own  experiment,  I  have  thought 
it  better  to  allow  it  to  stand  without  alteration. 
In  a  footnote  Mr.  Campbell  expresses  his  inability  to 
understand  why  I  inserted  an  obliquity  factor  cos  6  into  the 
preliminary  calculations  in  my  first  paper  on  the  a  rays 
(Phil.  Mag.  Dec.  1904).  The  mistake  is  mine.  I  did  not 
discover  it  until  I  had  occasion  to  reconsider  the  matter  in 
connexion  with  the  present  investigation. 
