the  y  Rays  of  Radioactive  Substances.  587 
When  the  activity  of  a  substance  is  measured  by  the 
a.  or  /3  rays,  the  effect  observed  depends  on  the  density  o£ 
the  active  substance.  In  the  case  of  /3  rays  it  is  necessary 
to  cut  off  the  a  rays  by  screens  of  aluminium  or  some  light 
material,  and  these  screens  absorb  the  /3  rays  of  the  different 
active  substances  to  an  unequal  extent,  so  that  the  j3  rays 
are  not  satisfactory  as  a  test  of  activity. 
There  seemed  good  reason  to  anticipate  that  the  7  rays 
would  afford  a  most  satisfactory  measure  of  the  total  active 
matter  in  a  given  body,  because  these  rays  under  the  ordinary 
experimental  conditions  are  but  slightly  absorbed  by  the 
matter  from  which  they  are  emitted.  It  is  possible  to 
measure  the  rays  of  the  substance  in  bulk,  also,  and  this  is 
an  advantage  when  considerable  quantities  of  ore  are  under 
investigation.  Moreover,  previous  experiments  indicated 
that  the  7  rays  from  thorium,  uranium,  and  radium  were 
absorbed  to  an  equal  extent  in  passing  through  the  same 
thicknesses  of  lead.  It  was  found,  however,  in  the  course 
of  these  experiments,  that  radium  and  thorium  do  emit 
7  rays  which  have  identical  coefficients  of  absorption,  whilst 
the  7  rays  from  uranium  are  weak  and  easily  absorbed. 
Thus  the  y  ray  method  can  be  employed  to  great  advantage 
in  comparing  radium  and  thorium,  but  uranium  and  actinium 
cannot  be  compared  with  them,  or  with  one  another. 
Apparatus. — An  electroscope,  30  centimetres  high  and 
20  centimetres  in  diameter,  made  of  zinc  *45  mm.  thick, 
contained  the  usual  gold-leaf  system  on  a  long  insulated 
central  rod.  The  fall  of  potential  was  measured  by  the 
movement  of  the  gold-leaf  read  by  a  microscope  with  a 
graduated  scale  in  the  eyepiece.  The  radioactive  substance 
under  examination  was  placed  on  a  platform  about  7  cm. 
below  the  electroscope.  Layers  of  lead  were  introduced 
between  the  active  body  and  the  electroscope.  Corrections 
were  made  in  the  usual  manner  for  the  natural  leak  of  the 
instrument.  In  every  case  lead  w  as  used  as  the  absorbing 
material. 
Substances  Investigated. 
(1)  Radium  Bromide. 
(2)  Uraninite  from  Joachimsthal,  Bohemia.     (1  kilo.) 
(3)  Uranium  Nitrate,   prepared  by   Eimer    and    Amend. 
(1  kilo.) 
(4)  Thorium    Nitrate,    prepared    by    Eimer  and   Amend. 
(2  kilos.) 
(5)  Radio-thorium,  lent  to  me  by  the  discoverer,  Dr.  Hahn, 
who  was  working  in  the  McGill  Physics  Building. 
(6)  Actinium,  Giesel's  preparation,  activity  about  300. 
2Q2 
