624  Prof.  W.  H.  Bragg  on  the  Ionization  of 
Determination  of  Stopping-power,  anal  of  B,I  in  Pentane. 
Electrodes  3  mm.  apart  (nearly).     Volts  applied  =  300. 
Temperature  o£  apparatus  =  3  5°  C. 
Apparatus  charged  with  vapour  from  standard  pentane. 
Distance  from  Ra 
^r  to  middle  of 
Leak  in 
Pressure  inside 
ionization  chamber. 
ten  seconds. 
apparatus. 
2-8 
1982 
2-9 
1431 
41-15  cm, 
3-0 
1192 
3-1 
1171 
3-2 
1193 
3-3 
1227 
41-15  cm. 
Thin  Cu  foil  over  Ra  108 
These  being  plotted,  it  is  found  that  K  =  2'95,  1  =  1044, 
the  copper  leak  having  been  deducted. 
Thus  11  =  2*95  in  this  mixture  of  pentane  and  air,  at  a 
pressure  of  41*15  cm.,  and  a  temperature  (observed)  of  308° 
(absolute).  But  at  a  pressure  760  cm.,  and  293°  absolute, 
Kin  air  is  4-83  ^     7600    30g 
Hence  the  mixture  stops  — -  .  „  .  — -  =3*14  times  as 
much  as  air.  ^'j5     41i0     "ld6 
A  special  set  of  readings  at  3'2  cm.  is  now  taken,  three 
for  10  seconds  and  three  for  20  seconds.  The  means  are 
1196  and  2325  respectively.  Comparing  these,  it  is  found 
that  the  10-second  reading  should  be  multiplied  by  1*03  in 
order  to  allow  for  leakage  by  the  insulators. 
Again,  a  set  is  taken  with  600  volts  between  the  plates, 
and  it  is  found  that  the  mean  reading  when  the  copper  leak 
is  deducted  is  1134.  At  the  same  time  the  reading  for  300 
volts,  copper  leak  being  deducted,  is  1088.  Thus  saturation 
is  nearly  complete. 
A  quantity  of  the  gas  is  now  drawn  over  into  an  exhausted 
bulb,  whose  temperature  (that  of  the  oven)  is  311  (absolute) : 
the  pressure  is  observed  to  be  34*5.  The  weight  of  this  gas 
is  *2536  gr.  It  is  then  calculated  from  a  knowledge  of  the 
capacity  of  the  bulb  that  the  mixture  weighs  2' 22  times  as 
much  as  air.  From  this  it  is  found  that  to  every  molecule 
of  pentane  there  are  *23  molecules  of  air,  assuming  the 
pentane  molecule  to  weigh  2*5  times  as  much  as  air.  If 
s=  stopping-power  of  pentane,  we  have  therefore 
•2S  +  *     a-14; 
1-23 
5  =  3-59. 
