the  7  Rays  of  Radioactive  Substances. 
591 
equality  of  the  absorption  of  the  7  rays  of  thorium  and 
radium  makes  the  7  radiation  an  accurate  comparative 
measure  of  the  joint  quantities  of  these  radioactive  substances 
in  two  given  bodies. 
Actinium. — Dr.  Godlewski  has  published  in  the  Philoso- 
phical Magazine  for  September  1905  an  account  of  some 
penetrating  rays  of  actinium,  with  a  coefficient  of  absorption 
equal  to  4' 5.  He  experimented  with  a  preparation  of  GiesePs 
actinium,  activity  about  300,  and  he  was  able  to  take  measure- 
ments for  thicknesses  ranging  up  to  about  3  mms.  of  lead. 
Using  a  stronger  preparation  of  Debierne's  actinium, 
activity  about  700,  it  was  possible  to  carry  Godlewski's  work 
farther.  When  the  rays  passed  only  through  *45  mm.  of 
the  zinc  which  formed  the  base  of  the  electroscope,  the 
activity  was  measured  by  2*35  scale-divisions  per  minute. 
On  adding  successively  *15,  *30,  *45  mm.  of  lead,  the  ob- 
served activity  fell  by  an  exponential  law,  with  X  =  10*5. 
These  rays  probably  consist  partly  of  homogeneous  ft  rays, 
and  they  are  not  plotted  in  the  diagram  (fig.  2).     Further 
Fier.  2. 
,3  ~     W~~  ~~  W 
i-eac/f7~h/c/cness  in    mm. 
sheets  of  lead,  *15  mm.  thick,  were  added,  until  a  total 
thickness  of  2 '85  mm.  was  reached.  The  results  plotted  on 
logarithm  paper  gave  a  straight  line,  so  that  the  rays  are 
homogeneous  between  these  limits.  This  result  confirms 
Dr.  Godlewski'' s  work  with  GieseFs  actinium,  but  he  found 
A,  =  4-5,  and  in  the  present  case  X=4'l.  These  values  are 
nearly  equal,  for  X  is  a  sensitive  function,  and  the  difference 
is  probably  due  to  experimental  conditions.  The  two  pre- 
parations of  actinium  have  the  same  emanation  and  excited 
activity,  and  the  same  coefficients  of  absorption  are  therefore 
to  be  expected. 
