AmAp?n^9T[5arin')    Views  on  Constitution  of  the  Atom.  187 
there  is  an  increase  in  the  kinetic  energy  of  the  projected  electrons. 
Indeed,  we  may  write  approximately  that 
I  mv2  =  hn 
where  the  left  member  i$  the  kinetic  energy  of  the  electron,  n  is  the 
frequency  of  the  X-rays,  and  h  is  Planck's  number  equal  to  6.6  x  icr27 
erg  second. 
This  interesting  relation  represents  a  fact,  but  explains  nothing 
and  itself  demands  as  much  explanation  as  the  initial  difficulty. 
In  conversation  W.  H.  Bragg  once  gave  to  me  a  striking  analogy. 
Let  the  moon  represent  the  source  of  X-rays,  and  the  moonlight, 
shining  on  the  earth,  represent  the  Rontgen  rays  falling  on  a  body. 
Just  as  the  Rontgen  rays  eject  here  and  there  a  high-speed  electron, 
so  the  moonlight  would  have  to  cause  something  about  the  size  of  a 
pea  to  leap  out  with  great  velocity,  first  one,  say,  from  Siberia,  then 
one  from  South  Africa,  next  from  New  Guinea,  and  so  forth. 
Violent  and  very  local  action  produced  by  a  uniform  and  homo- 
geneous radiation ! 
The  Counting  of  the  Alpha  Particles. — Before  C.  T.  R.  Wilson 
had  taken  the  photographs  already  ^escribed.  Sir  Ernest  Rutherford 
had  devised  an  apparatus  which  enabled  him  to  detect  the  alpha 
particles  entering,  one  by  one,  into  a  testing  chamber  in  such  a 
manner  that  their  number  could  be  counted.  In  this  way  he  was 
able  to  deduce  that  the  alpha  particle  had  a  charge  twice  as  great,  but 
opposite  in  sign,  to  the  charge  of  an  electron  ;  that  a  gramme  of  the 
metal  radium  expelled  3.57  x  10  10  alpha  particles  per  second,  and  he 
found  accurate  values  for  other  fundamental  constants. 
For  the  detection  of  the  individual  alpha  particle  this  eminent 
physicist  was,  with  a  pretty  humor,  justly  awarded  the  Nobel  Prize 
for  chemistry. 
The  Detection  of  the  Individual  Electron. — The  alpha-ray  de- 
tector was  altered  and  improved,  and  a  few  years  later  Geiger  was 
able  to  report  that  he  had  succeeded  in  detecting  single  electrons 
entering  a  suitable  orifice  in  his  new  testing  chamber.  As  I  had, 
owing  to  the  kind  help  of  the  secretary,  Dr.  Owens,  the  pleasure  of 
performing  experiments  with  this  type  of  apparatus  at  The  Franklin 
Institute,  it  may  be  well  to  give  a  fairly  full  description  of  it. 
When  an  electron,  say  a  beta  particle  from  a  radiant,  enters  the 
small  hole  in  the  testing  vessel  B,  it  approaches  a  point  N  which  is 
almost  sparking  to  the  high-potential  chamber  (Fig.  2).    The  ions 
