192  Views  on  Constitution  of  the  Atom.   { Am;4riir'i9i5arm' 
Professor  W.  H.  Bragg  and  his  son  have  developed  this  method 
and  obtained  information  as  to  the  distribution  of  atoms  in  various 
crystals,  the  distance  between  the  atoms,  and  the  wave-lengths  of  the 
Rontgen  rays.  He  has  himself  summarized  this  work  in  an  article 
published  recently  in  Science  (December  4,  1914). 
Crystal  Reflection  of  Gamma  Rays. — The  gamma  rays  have  been 
reflected  from  the  regular  layers  of  atonjs  in  a  crystal  in  a  manner 
quite  similar  to  that  employed  with  Rontgen  rays,  a  radiant  taking 
the  place  of  the  X-ray  tube.  In  this  way  Rutherford  has  found  that 
the  wave-lengths  of  the  most  penetrating  gamma  rays,  glancing  from 
the  crystal  face  at  42',  is  about  0.7  x  10  ~9  cm.  This  is  the  smallest 
wave-length  known,  but  the  phenomenon  may  as  justly  be  attributed 
to  an  electromagnetic  disturbance  in  the  ether  as  a  radiotelegraph 
wave  with  a  length  of  several  kilometres. 
It  is  interesting  to  note,  in  some  papers  of  Rutherford's,  how  he 
appears  to  use  language  and  ideas  which  are  at  present  totally  irrec- 
oncilable. I  am  not  venturing  to  criticise  this,  for  the  statements 
do  indeed  represent  the  facts.  At  one  time  Rutherford  is  speaking 
of  "  a  gamma  ray,"  thus  suggesting  linear  propagation  of  energy, 
and  at  another  time  he  is  discussing  the  wave-length  of  a  gamma 
ray  with  a  meaning  applicable  to  ordinary  light  radiation  and  there- 
fore involving  an  expanding  wave  front.  We  are  reminded  of 
Newton's  troubles  in  explaining  diffraction  by  a  corpuscular  theory, 
and  concluding  that  light  has  an  "  eel-like  "  motion. 
Atomic  Number. — Since  the  positive  charge  on  the  nucleus  of  an 
atom  is  about  half  the  atomic  weight,  and  since  that  charge  is  neces- 
sarily a  whole  number  when  expressed  as  a  multiple  of  the  magnitude 
of  the  charge  of  an  electron,  it  has  been  suggested  by  van  den  Broek 
that  there  is  a  fundamental  and  important  number  connected  with 
every  element,  known  as  the  atomic  number.  This  number  is  pos- 
sibly the  place  of  the  element  in  the  periodic  table  (Hi,  He  2,  Li  3, 
Be  4,  Bo  5,  C  6,  etc.).  This  theory  has  had  strong  confirmation  from 
a  series  of  experiments  by  Moseley.  He  exposed  most  of  the  ele- 
ments to  the  bombardment  of  electrons  in  a  vacuum  tube,  these 
elements  forming,  in  turn,  the  anticathodes  of  a  Rontgen  tube.  The 
natural  or  characteristic  radiations  of  the  elements  were  reflected 
from  a  crystal  surface,  and  the  frequencies  of  the  vibrations  were 
thus  determined.  After  a  correction  for  the  outer  electrons,  it  was 
found  that  in  every  case  the  frequency  was  proportional  to  the  square 
of  an  integer  which  was  identified  with  the  atomic  number.  Accord- 
