618  Prof.  W.  H.  Bragg  on  the  Ionization  of 
application  of  some  factor  to  all  their  ordinates  or  all  their 
abscissae.  Thus  the  product  of  the  coordinates  of  some 
standard  point  is  proportional  to  the  area  of  the  whole  curve, 
and  may  be  taken  as  a  relative  measure  of  the  total 
ionization. 
In  all  the  experiments  to  which  I  am  about  to  refer,  the 
a  particles  cross  at  right  angles  a  shallow  ionization  chamber 
whose  upper  electrode  is  a  brass  plate  and  the  lower  a  brass 
gauze  ;  the  distance  between  the  electrodes  is  about  3  mm. 
An  electromotive  force  of  300  volts  is  usually  applied  to  the 
gauze,  giving  an  electric  force  of  1000  volts  per  cm.  This 
is  generally  sufficient  to  ensure  saturation ;  that  is  to  say,  to 
avoid  all  errors  due  to  diffusion,  general  recombination,  and 
initial  recombination.  When  it  is  insufficient,  the  proper 
correction  is  found  and  applied.  The  ionization  chamber  is 
enclosed  within  a  vessel  which  is  satisfactorily  air-tight 
except  at  higher  temperatures,  and  this  again  within  an 
electric  oven.  The  gas  under  observation  can  thus  be 
subjected  to  various  pressures  and  temperatures.  In  the 
case  of  such  substances  as  benzene  and  carbon  tetrachloride, 
a  temperature  of  from  60°  to  90°  is  necessary  to  ensure  a 
convenient  gas- density.  There  is,  however,  this  drawback  to 
the  use  of  high  temperatures,  that  the  insulators  begin  to  lose 
their  efficiency,  and  the  joints  cease  to  be  air-tight.  I  find 
it  necessary  to  use  glass  as  an  insulator  instead  of  sulphur, 
for  the  latter  cracks  under  the  unequal  expansions  due  to 
alteration  of  temperature.  In  the  case  of  vapours,  a  certain 
quantity  of  air  usually  finds  its  way  into  the  apparatus,  for, 
as  just  mentioned,  the  joints  leak  somewhat  at  the  higher 
temperatures.  The  amount  so  entering  is  sometimes  deter- 
mined by  opening  a  communication  between  the  vessel  and 
an  evacuated  bulb,  and  weighing  the  quantity  of  mixture 
drawn  off.  The  bulb  and  connexions  are  placed  within  the 
oven,  and  communication  is  made  by  opening  a  pinch-cock 
worked  by  a  key  projecting  outside  the  oven.  In  this  way, 
condensation  in  cold  tubes  is  avoided.  This  method  is  not 
always  employed,  for  as  soon  as  the  stopping-power  of 
the  gas  is  sufficiently  well  known,  the  proportion  of  the 
mixture  is  much  more  easily,  and  I  think  at  Jeast  as  accu- 
rately, determined  by  observation  of  the  range  of  the 
a  particles  therein. 
The  insulation  leak  is  determined  by  measuring  the 
deflexion  of  the  electrometer  first  for  ten  seconds,  and  then 
for  twenty.  With  no  leak  the  latter  should  be  double  the 
former  ;  this  is  never  quite  the  case,  and  the  correction 
factor  can  be  obtained  from  a  comparison  of  the  two  values. 
