Properties  of  the  <x  Rays  from  Radium.  171 
There  can  be  no  doubt  that  the  rays  from  radium  C  are  reduced 
in  velocity  by  their  passage  through  matter. 
Complexity  of  the  Rays. 
We  have  seen  that  M.  Becquerel  considers  the  rays  from 
radium  to  be  homogeneous,  i.  e.  that  they  consist  of  particles 
all  projected  at  equal  speeds.  This  conclusion  is  based  on 
the  observation  that  no  appreciable  broadening  of  the  trace 
of  the  ravs  was  observed  by  increasing  the  magnetic  field  from 
10,000  to  20,000  c.G.s.  units. 
After  the  experimental  results  given  by  Bragg  of  the  differ- 
ence in  the  range  of  ionization  of  the  rays  from  the  various 
radium  products,  the  evidence  of  the  homogeneity  of  the  rays 
requires  to  be  extremely  convincing  before  it  can  be  accepted. 
As  a  matter  of  fact,  it  is  not  difficult  to  show  that  the  a  rays 
from  a  thick  layer  of  radium,  far  from  being  homogeneous, 
undoubtedly  consist  of  rays  unequally  deflected  in  a  constant 
magnetic  field.  Since  the  width  of  the  beam  of  rays  is,  in 
most  cases,  comparable  with  the  amount  of  its  deviation  by  a 
strong  magnetic  field,  the  unequal  deflexion  of  the  rays  is 
most  clearly  shown  when  a  narrow  beam  of  rays  is  employed. 
This  is  clearly  brought  out  in  the  photograph  shown  in 
fig.  2.  One  milligram  of  pure  radium  chloride  (obtained  from 
the  Societe  Centrale  de  produits  chimiques,  ParisJ  was  spread 
in  a  narrow  groove.  After  passing  through  two  narrow  slits, 
the  rays  fell  on  a  photographic  plate  placed  4  cms.  above  the 
radium.  The  apparatus  was  exhausted  to  a  low  pressure,  and 
then  placed  between  the  poles  of  an  electromagnet  which 
was  first  excited  by  a  weak  current,  producing  sufficient  field 
to  bend  away  the  /3  rays  but  not  appreciably  to  deflect  the 
ol  rays. 
The  trace  of  the  rays  is  seen  in  the  narrow  line  A  in  the 
photograph.  After  two  hours,  a  strong  constant  current  was 
passed  for  another  two  hours,  and  the  trace  B  of  the  deflected 
beam  of  rays  is  shown  in  the  figure. 
The  current  was  kept  very  constant  during  the  experiment, 
and  the  strength  of  the  magnetic  field  certainly  did  not  vary 
at  any  time  more  than  \  per  cent. 
If  the  rays  are  homogeneous,  the  two  bands  should  be  equal 
in  width  and  their  edges  equally  defined.  The  band  B  is, 
however,  much  broader  than  A,  and  the  edges  lack  the  defi- 
nition of  the  latter.  A  close  examination  of  the  original 
negative  shows  that  while  the  edges  of  A  are  fairly  sharply 
defined,  the  band  B  is  dark  in  the  centre,  but  the  photo- 
graphic impression  falls  off  gradually  on  either  side,  and  it  is 
difficult  to  fix  with  certainty  the  extreme  edges  of  the  deflected 
