Tie  presence  of  this  energy  is  only  perceived  in  the  case  of  those
elements  which  undergo  atomic  transformation.
239.    Heating    effect  of   uranium   and  thorium.     The

heating  eflFect  of  uranium  and  thorium  and  of  the  minerals  con-
taining them  can  be  simply  calculated  on  the  assumption  that  it
is  a  direct  measure  of  the  kinetic  energy  of  the  a  particles.  From
the  data  given  in  Section  70  it  is  seen  that  the  average  energy  of
the  two  o  particles  emitted  by  uranium  itself  is  6*8  x  10~*  ergs.
Since  2*37  x  10*  a  particles  are  emitted  per  second  per  gram  of
Uranium,  the  heating  effect  should  be  '161  ergs  per  second  or
l'4x  10^  gram  calories  per  hour.  Uranium  in  equilibrium  with
its  series  of  products  in  a  mineral  emits  8  groups  of  a  particles
of  average  energy  8*8  x  10~^  ergs.  Since  9'6  x  10*  a  particles  are
emitted  per  second  per  gram  of  uranium  in  a  mineral,  the  heating
effect  of  one  gram  of  uranium  and  its  products  in  a  mineral  should
be  7*2  x  10~^  calories  per  hour.
