AmAj?iir*i9i5arm' }    Views  on  Constitution  of  the  Atom.  189 
surrounding  air.  Remember  also  that  it  is  capable  of  continuing  to 
do  so  for  a  period  exceeding  one  thousand  million  years. 
On  the  Nucleus  of  Atoms. — Sir  Joseph  Thomson  had  suggested 
an  atomic  structure  having  a  number  of  electrons  revolving  in  con- 
centric circles  within  a  vague  entity  consisting  of  positive  electricity. 
Sir  Ernest  Rutherford,  working  with  Geiger,  found  that  when  alpha 
particles  bombarded  thin  gold  leaf  the  great  majority  of  the  particles 
passed  through,  but  about  one  in  every  eight  thousand  returned  on 
the  side  of  entry.  He  came  to  the  conclusion  that  the  returning  alpha 
particle  had  encountered  a  highly-charged  nucleus.  Working  out 
this  idea  on  the  basis  of  Newtonian  mechanics,  he  concluded  that  the 
nucleus  was  small  in  its  effective  dimensions,  but  that  it  had  a  charge 
equal  to  about  one-half  of  the  atomic  weight  of  the  atom.  This  idea 
was  amply  verified  by  a  long  series  of  experiments,  and  it  fitted  well 
with  results  obtained  by  Barkla,  who,  stimulating  matter  with 
Rontgen  rays,  obtained  evidence  that  an  atom  contained  electrons 
to  a  number  about  half  that  of  the  atomic  weight. 
Putting  these  results  together,  we  obtain  something  resembling 
a  miniature  solar  system,  with  the  minute  central  body  consisting  of 
a  large  positive  charge,  and  in  concentric  surrounding  rings  are 
electrons  revolving  like  planets,4  but  perhaps  with  several  in  each 
ring,  and  so  adjusted  that  their  total  charge,  which  is  negative, 
balances  the  positive  charge  on  the  nucleus. 
The  Alpha  Particle  and  Hydrogen  Nucleus. — In  one  of  Wilson's 
photographs  of  the  track  of  an  alpha  particle  (Fig.  3)  there  appears, 
near  the  end  of  the  path,  a  small  projecting  spur,  which  he  explained 
as  due  to  an  atom  of  an  air  molecule,  struck  by  an  alpha  particle, 
receiving  sufficient  velocity  to  ionize  the  air  for  a  short  distance.  It 
immediately  occurred  to  Rutherford  that  if  an  alpha  particle  passed 
through  hydrogen,  then  the  light  hydrogen  atom,  or  rather  its  nucleus, 
would  be  projected,  not  merely  to  a  greater  distance  than  the  nucleus 
of  the  oxygen  or  nitrogen  atom,  but  to  almost  four  times  the  distance 
of  the  alpha  particle  itself.  This  forecast  was  verified  by  Marsden, 
using  a  zinc  sulphide  screen,  when  the  scintillations  due  to  the 
hydrogen  nuclei  were  seen  at  a  range  nearly  quadruple  that  of  the 
alpha  particles.  This  important  verification  has  placed  the  theory 
of  the  nucleus  on  a  foundation  which  excludes  doubt  as  to  its  reality. 
4  There  is,  however,  no  direct  experimental  evidence  that  the  electrons 
revolve.   They  may  vibrate  about  stable  positions. 
