the  7  Rays  of  Radioactive  Substances.  593 
highly  penetrating  7  rays,  and  the  low  velocity  ft  rays  pro- 
duce weak  7  rays.  In  the  case  of  uranium  and  actinium 
there  appear  to  be  no  ft  rays  sufficiently  swift  to  cause  the 
most  penetrating  type  of  7  rays,  for  the  ft  rays  are  homo- 
geneous, and  have  a  more  or  less  uniform  velocity. 
Standard  of  y  Ray  Measurement, 
The  writer  suggests  that  one  kilogram  of  pure  thorium 
nitrate  enclosed  and  tightly  sealed  in  a  thin  glass  vessel, 
16  cms.  in  diameter,  would  form  a  convenient  standard  for 
testing  the  amount  of  radium  or  thorium  in  a  given  ore  in 
bulk.  The  7  activity  could  be  tested  through  a  centimetre 
of  lead,  and  the  rays  of  actinium  or  uranium  would  then  be 
practically  excluded.  The  results  of  various  experimenters 
could  be  readily  compared  in  terms  of  such  a  standard. 
Concentration  and  Distance. 
If  the  distance  of  an  active  substance  from  the  bottom  of 
the  electroscope  is  varied,  the  fall  of  potential  of  the  gold- 
leaf  also  alters.  In  comparing  two  substances  their  centres 
of  mass  should  be  made  to  coincide  relatively  to  the  electro- 
scope. The  measured  activity  was  found,  roughly,  to  vary 
inversely  as  the  square  of  the  distance  from  the  active 
substance  to  the  centre  of  the  electroscope. 
In  order  to  see  whether  the  observed  effect  was  dependent  on 
the  degree  of  concentration  of  the  substance  under  investiga- 
tion, a  few  pieces  of  pitchblende  were  placed,  first  at  the  centre, 
then  at  the  circumference  of  a  glass  dish  16  cms.  in  diameter. 
The  activity  observed  was  three  per  cent,  stronger  when  the 
pitchblende  was  all  near  the  centre  of  the  dish.  If,  therefore, 
the  dish  were  full,  the  loss  owing  to  the  scattering  would  be 
about  one  and  a  half  per  cent.  This  loss  is  due  to  the  fact 
that  the  cone  of  rays  cut  by  the  electroscope  is  different 
when  an  active  particle  is  at  the  centre,  and  when  it  is  at  the 
circumference  of  the  glass  vessel. 
© 
Self- Ah  sorption. 
Two  methods  were  employed  to  form  an  estimate  of  the 
loss  of  activity  due  to  self-absorption  by  thorium  nitrate.  In 
the  first  case  the  electroscope  was  placed  on  a  platform  of 
lead  *64  cm.  thick.  A  kilogram  of  thorium  nitrate  was 
then  placed  beneath,  and  the  activity  measured  2'o.  A  small 
quantity  of  radium  was  placed  at  the  centre  of  the  mass,  and 
the  total  activity  was  81'4.     Next  the  activity  of  the  radium 
