796  Dr.  0.  Halm  on  some  Properties  of 
copper  wire  was  attached  through  an  insulating  rubber  cork 
so  as  to  remain  just  over  the  radiothorium.  The  active 
deposit  was  collected  on  the  wire,  wmich  was  charged 
negatively  to  about  110  volts  by  means  of  the  lighting  circuit. 
After  about  two  and  a  half  days'  exposure  equilibrium  is 
reached  and  the  wire  has  become  strongly  radioactive,  for  it 
discharges  an  electroscope  immediately,  and  gives  distinct 
luminosity  to  sensitive  screens. 
Scintillation  Met/ ho d. 
In  order  to  find  the  maximum  range  of  the  a  particles  of 
the  thorium  B,  the  scintillation  method  was  first  employed. 
A  small  piece  of  glass  was  coated  with  zinc  sulphide  and 
placed  vertically  over  the  active  wire,  the  latter  being  fixed 
on  a  small  disk,  which  could  be  moved  up  and  down  to  any 
desired  distance  of  the  screen.  As  long  as  the  screen  was  in 
the  range  of  ionization  of  the  a  particles,  the  scintillations 
were  seen  without  difficulty  by  means  of  a  lens.  It  is,  of 
course,  necessary  for  the  eye  to  be  first  accommodated  to  the 
dark.  Beyond  the  range  of  ionization  the  scintillations  dis- 
appear fairly  abruptly.  The  maximum  range  at  which 
scintillations  were  still  visible  was  found  to  be  8'3  cm.,  while 
at  8  cm.  the  luminosity  could  be  seen  quite  easily.  This 
result  showed  at  once  that  the  a  particles  of  the  active 
thorium  deposit  are  even  more  penetrating  than  the  most 
penetrating  a  rays  of  radium,  which  were  found  by  Bragg 
and  Kleeman  to  be  those  of  radium  0.  For  comparison, 
some  radium  C  was  collected  at  a  thin  copper  wire  and  its 
range  of  ionization  determined  with  the  same  method.  It 
was  found  to  be  6'8  cm.,  Rutherford  having  found  the  same 
when  using  the  same  method  *,  while  Bragg  and  Kleeman  f 
found  7*0  cm.  when  using  the  electrical  method.  It  may  be 
mentioned  here,  that  the  electrical  method  of  determining  the 
range  of  the  a  particles  is  somewhat  more  delicate  than  the 
optical  one. 
McClung  {,  using  Bragg' s  method  of  determining  the 
ionization  curve  of  radium  C,  covered  his  active  wire  with 
thin  sheets  of  aluminium-foil  in  order  to  see  whether  the 
range  of  ionization  decreased  in  proportion  to  the  number 
of  foils  used.  He  found  that  one  aluminium-foil  about 
0*00031  cm.  thick  corresponds  to  about  0'51  cm.  of  air, 
Rutherford  §  having  found  about  0*53  cm.  for  the  same  foil. 
The  same  result  was  obtained  by  the  writer  by  the  scintilla- 
tion method.     Different  layers  of  aluminium-foil  were  placed 
*  Phil.  Ma<r.  July  190-5.  f  Phil.  Mag.  Dec.  1904. 
\  Phil.  Mag.  Jan.  1906.  §  Phil.  Mag.  Jan.  1906. 
