Am.  Jour.  Pliarm.  ) 
April,  1914.  } 
Current  Literature. 
191 
The  most  important  property  of  radium  is  its  radio-activity  upon 
which  depends  its  therapeutic  value.  Radio-activity  is  defined  as 
"  the  property  of  spontaneously  emitting  radiations  capable  of  pass- 
ing through  plates  of  metal  and  other  substances  opaque  to  ordinary 
light  and  having  the  power  of  discharging  electrified  bodies."  A 
spontaneous  disintegration  of  the  atoms  characterizes  all  the  radio- 
active elements  and  it  is  in  this  transmutation  or  splitting  of  the 
atom  that  the  rays  are  shot  out,  some  being  material  in  nature,  others 
electrical  or  of  the  nature  of  light.  This  spontaneous  transmutation 
of  radium  is  going  on  at  a  regular  rate,  which  is  independent  of  the 
state  of  combination  of  radium  in  the  molecule  of  its  compounds. 
To  determine  the  radio-active  value  of  radium,  use  is  made  of  its 
property  of  ionizing  gases.  Thus  when  radium  is  allowed  to  act  on 
the  air  in  a  charged  gold-leaf  electroscope  the  air  becomes  ionized 
and  therefore  a  conductor  of  electricity  and  allows  the  charge  to 
leak  out,  causing  the  leaf  in  the  electroscope  to  move.  By  observing 
the  rate  of  movement  of  the  leaf  in  a  calibrated  apparatus  the  radio- 
activity can  be  determined. 
Quantities  and  concentrations  of  radium  emanation  are  expressed 
in  terms  of  "  curies  "  and  Mache  units.  A  "  curie  "  is  the  amount 
of  emanation  in  equilibrium  with  1  gram  of  radium;  a  microcurie, 
one  millionth  of  a  "  curie,"  is  the  amount  of  emanation  in  equilibrium 
with  0.001  mg.  radium  and  is  equivalent  to  about  2,500  Mache  units. 
Relation  of  Radium,  -Radium  Emanation  and  Rays. — The  rays 
are  largely  derived  indirectly  from  radium  through  the  formation 
of  its  "  active  deposit,"  according  to  the  following  scheme : 
aparticle 
/ 
Rad.  aparticle 
\  / 
eman. 
\ 
ac.  dep. sh^t: Rays. 
These  rays  are  divided  into  three  groups,  the  alpha,  beta  and 
gamma,  which  differ  in  their  velocity  and  penetrative  power.  The 
alpha  and  beta  rays  consist  of  minute  particles  of  matter  electrically 
charged  and  moving  with  a  velocity  almost  equal  to  that  of  light. 
They  are  for  the  most  part  of  relatively  feeble  penetrating  power. 
The  gamma  rays  are  vibrations  in  the  ether,  very  similar  to  X-rays, 
and  of  high  penetrating  power.  Therapeutically  the  last  group  is 
the  most  useful. 
Radium  emanation  is  continuously  given  off  from  aqueous  solu- 
