and  Composition  of  Uranium  Compounds.  177 
wholly  due,  by  the  uranium  compound  itself.  If  this 
absorption  occurs  according  to  the  simple  absorption  law, 
it  is  readily  shown  *  that 
,         2-303  s ,  1 
L2  = log , 
w  1  —  x 
where  &2  is  the  absorption  coefficient  for  unit  weight  on  unit 
area ;  x  is  the  ratio  of  the  observed  activity  of  any  very  thin 
film  of  a  uranium  compound  to  that  of  a  film  of  the  same 
substance  sufficiently  thick  to  be  of  maximum  activity  ;  s  is 
the  common  area  of  the  films  ;  and  w  is  the  weight  of  the 
thinner  film.  The  films  were  from  *001  to  *04  mm.  in 
thickness,  the  thicker  films  showing  maximum  activity.  h2 
is  a  good  constant  for  any  pure  uranium  compound.  If  the 
observed  radioactivity  of  one  sq.  cm.  of  any  film  of  maximum 
activity  is  called  a.  it  is  shown  f  that  kl  =  2k2  a,  where  k1 
is  the  total  activity  of  1  g.  of  the  uranium  compound.  The 
ratio,  K,  of  the  total  activity,  so  determined,  to  the  uranium 
content  of  the  compound  is  a  constant,  within  the  limit  of  error 
of  +  1  per  cent.,  and  represents  the  total  alpha  ray  activity 
of  1  g.  of  pure  uranium,  independently  of  its  form  of  chemical 
combination.  If  a  is  taken  as  unity  for  pure  U308,  K  is 
equal  to  791.  This  means  that  the  total  activity  of  1  g.  of 
pure  uranium  is  791  times  that  of  1  sq.  cm.  of  a  layer  of 
pure  U308  sufficiently  thick  to  be  of  maximum  activity. 
Preparation  of  a  Standard  of  Radioactivity. 
The  above  results  were  obtained  by  means  of  compounds 
prepared  directly  from  "  chemically  pure  "  uranium  nitrate 
as  furnished  by  the  firm  of  0.  A.  F.  Kahlbaum.  Since  all 
uranium  ores  contain  radium  J,  it  was  necessary  to  determine 
with  certainty  whether  the  material  used  was  free  from 
radium  in  amounts  sufficient  to  appreciably  alter  its  activity. 
This  was  done  by  starting  with  a  uranium  ore,  which  of 
course  contained  the  maximum  of  such  impurities,  and  con- 
tinuing the  process  of  purification  until  no  further  change  of 
activity  occurred.  The  activity  of  the  U308  so  prepared  did 
not  differ  by  a  measurable  amount  from  that  of  my  old 
"  standard  "  oxide,  which  had  been  made  by  direct  ignition 
of  Kahlbaum 's  nitrate.  The  measurements  of  the  activity 
were  made  electrically  in  the  manner  previously  described. 
The  error  of  comparison  was  about  02  per  cent. 
*  McCoy,  J.  Amer.  Oliem.  Soc.  xxvii.  p.  391  (1905). 
t  McCoy,  loc.  cit. 
X  McCov,  Ber.  d.  chem.  Ges.  xxxvii.  p.  2611  (1901).  Boltwood, 
Am.  J.  Sci.  xviii.  p.  97  (1901) ;  Phil.  Mag.  [6]  ix.  p.  599  (1905). 
Phil  Mao.  S.  6.  Vol.  11.  No.  61.  Jan.  1906.  JS 
