810 
Dr.  H.  L.  Bronson  on  tlte 
007  per  cent,  of  its  maximum  value.     In  fact  there  is  no 
evidence  that  any  such  ionization  exists. 
It  thus  appears  likely  that  the  a.  particle  loses  its  energy 
very  rapidly  near  the  end  of  its  path  by  collision  with  the 
gas  molecules,  and  is  completely  absorbed  by  a  thickness  of 
air  of  7*2  cms. 
Does  Radium  B  emit  a.  Rays  ? 
Schmidt  (Rliysikalisclie  Zeitsclirift,  Jan.  1906)  has  shown 
that  radium  B  gives  out  /?  rays  of  small  penetrating  power. 
This  suggests  the  possibility  that  radium  B  may  also  give  out 
a  rays  of  small  velocity.  A  slight  irregularity,  obtained  by 
the  writer,  in  the  decay  curves  of  the  excited  activity  from 
radium  also  seemed  to  indicate  that  there  might  be  a  small 
amount  of  a-ray  ionization  from  radium  B.  This  would 
mean  that  the  velocity  of  these  a  particles  was  sufficient  to 
ionize  possibly  two  or  three  millimetres  of  air,  in  which  case  the 
effect  would  be  so  small  that  generally  it  would  be  entirely 
masked  by  the  ionization  from  radium  C.  Since  radium  B 
continuously  produces  radium  C,  the  possibility  of  getting 
the  former  entirely  separated  from  the  latter,  and  keeping 
it    separated   long  enough  to  test  the  above  point,  seemed 
Fig.  3. 
To  £l£CT/FOM£T£/?. 
Pump 
and   Gauge 
AcT/VE  W/ffE 
To  Battery 
hopeless.       Therefore    the  following   method,   suggested  b}' 
Professor  Rutherford,  was  adopted. 
The  arrangement  of  the  testing-vessel  is  shown  in  fig.  3. 
