194  Views  on  Constitution  of  the  Atom.    { Am  ™Jrm- 
particle,  thus  reducing  the  charge  on  the  nucleus  by  two  electronic 
units,  there  is  a  corresponding  change  of  valency,  say  from  tetrav- 
alent  to  divalent,  so  that  the  new  product  is  two  columns  to  the  left 
in  the  periodic  table,  while  the  decrease  of  atomic  weight  is  four. 
When  a  beta  particle  is  expelled  there  is  no  effective  change  of 
atomic  weight,  but  the  nucleus  charge  or  atomic  number  has  in- 
creased by  one,  since  the  loss  of  a  negative  is  equivalent  to  the  gain 
of  a  positive  electronic  unit  charge.  The  subsequent  product  is  now 
found  one  column  to  the  right  in  the  table,  relative  to  the  parent 
element.  Two  elements  with  different  atomic  weight,  but  with  the 
same  nucleus  charge,  may  find  themselves  in  the  same  place  in  the 
periodic  table,  and  are  known  as  isotopes.  Such  elements  have 
chemical  and  physical  properties  so  similar  that  they  are  at  present 
deemed  inseparables  or  non-separables.  As  an  example,  it  is 
possible  to  obtain  radium  D  as  it  forms  from  the  disintegration  of 
radium  C.  If  this  radium  D  is  mixed  with  lead,  separation 
becomes  impossible,  because  their  chemical  properties  are  identical. 
This  theory  has  already  achieved  a  notable  success.  In  the  early 
days  of  radioactivity  Sir  William  Crookes  obtained  uranium  X  by 
chemical  separation  from  uranium,  and  it  was  shown  that  it  was  a 
linear  descendant.  The  uranium  was  found  to  eject  two  sets  of  alpha 
particles  of  different  ranges,  so  that  it  was  presumably  two  radiants 
and  not  one.  The  law  above  described  suggested  both  the  presence 
of  an  undiscovered  radiant  and  also  its  chemical  properties.  This 
substance,  uranium  X2,  or  brevium,  for  it  is  short-lived,  was  quickly 
discovered. 
The  conclusion  arrived  at  is  set  forth  in  the  following  typical 
scheme,  where  uranium  1  and  uranium  2  are  inseparables  or  isotopes, 
with  atomic  weights  differing  by  4,  but  with  atomic  numbers  and 
valencies  identical. 
IV.  V.  VI.  At.wts. 
Ur  Xi  ^ 
Ur  X2 
Ur 
2 
234-5 
ft  90 
a, 
92 
Ur 
1 
238.5 
a, 
92 
