808  Dr.  H.  L.  Bronson  on  the 
diameter  and  height  to  include  the  entire  cone  of  rays  under 
all  circumstances.  The  central  electrode  was  insulated  by 
an  ebonite  cork  and  connected  to  an  electrometer.  Below 
the  testing-vessel  there  was  another  brass  cylinder,  S,'  having 
an  internal  diameter  of  0'6  cm.  The  opening  between  this 
and  the  testing-vessel  was  closed  with  mica  or  aluminium. 
The  active  wire  could  be  easily  placed  in  position  through  an 
opening  in  the  side.  The  distance  between  the  mica  and 
active  wire  was  4*7  cms.,  which  made  the  maximum  angle  of 
the  cone  of  rays  less  than  8°.  A  water-pump  was  used  to 
exhaust  the  chamber  S.  The  pressure  was  easily  measured 
on  a  gauge,  and  the  equivalent  thickness  of  air  at  76  cms. 
pressure  was  calculated.  This  method  of  varying  the  amount 
of  material  through  which  the  a  particle  had  to  pass  before 
entering  the  testing- vessel,  was  found  to  bo  very  simple  and 
exceedingly  satisfactory.  In  this  way  it  was  possible  to  take 
an  entire  set  of  observations  without  disturbing  the  apparatus 
or  even  going  near  it.  In  order  to  be  able  to  measure  very 
small  amounts  of  ionization  due  to  the  a  rays,  the  /3  rays 
were  bent  away  by  placing  the  cylinder  S  between  the  poles 
of  a  strong  electromagnet.  The  ionization  w^as  measured  by 
an  electrometer  using  the  constant  deflexion  method>  and 
the  values  obtained  were  corrected  for  the  decay  of  radium  C. 
The  mica  was  covered  with  a  leaf  of  Dutch  metal  to 
prevent  electrostatic  action,  and  the  thickness  of  the  two  was 
found  to  correspond  to  2*22  cms.  of  air  at  a  pressure  of 
76  cms.  As  this  and  the  4'7  cms.  of  air  between  the  mica 
and  the  active  wire  was  not  sufficient  to  entirely  absorb  the 
a  particles,  the  lower  part  of  the  curve,  fig.  2,  wsls  obtained 
with  an  additional  covering  of  aluminium  equivalent  to 
1*22  cms.  of  air. 
The  results  of  this  experiment  are  shown  in  the  curve  of 
fig.  2.  The  abscissas  represent  the  total  number  of  cms.  of 
air,  or  its  equivalent,  through  wrhich  the  a  particles  have 
passed  before  entering  the  testing-vessel  T.  The  ordinates 
represent  the  ionization  produced,  expressed  as  percentages 
of  the  total  ionization  caused  by  the  a  particles  over  their 
entire  path.  The  percentages  are  not  exact,  for  the  maximum 
value  was  obtained  by  extrapolation  from  the  curve.  The 
conclusions  to  be  drawn  from  this  curve  are  substantially 
the  same  as  those  of  Bragg  and  Kleeman,  and  of  McClung. 
The  number  of  ions  per  cm.  produced  by  the  a  particle 
increases  gradually  over  the  first  6*4  cms.  of  its  path  and 
then  decreases  very  rapidly,  falling  to  less  than  0*5  per  cent, 
of  the  total  at  7"1  cms. 
Two  experimental  conditions  might  account  for  the  ap- 
parent rapid   decrease  in    the  ionization   per  cm.   of  the  a 
