852 
Isotopy. 
i  Am.  Tour.  Pharin. 
1      December,  1920. 
An  interesting  feature  of  the  phenomenon  is  that  there  is  now 
exhibited  in  chemistry  a  disposition  to  return  to  the  "whole  number" 
theory  of  atomic  weights.  In  1815,  Prout  advanced  the  view  that 
all  elements  are  aggregations  of  the  hydrogen  atom,  which  is  the 
lightest  known,  and  hence  all  atomic  weights  should  be  whole 
numbers  if  referred  to  hydrogen  as  unity.  This  attractive  supposi- 
tion was  soon  rendered  unacceptable  by  the  researches  of  Dumas, 
Marignac  and  Stas,  who  showed  that  the  most  exact  determina- 
tions of  many  atomic  weights  did  not  allow  of  the  supposition  of 
their  being  w^hole  numbers.  If,  however,  the  duplex  or  multiple 
composition  of  the  common  forms  of  the  elements  is  assumed,  it 
may  be,  as  noted  above,  that  a  mixture  of  two  isotopes  in  certain 
proportions,  though  each  has  a  whole  number  atomic  ratio,  will  give 
a  fractional  ratio  as  compared  with  hydrogen.  Thus,  one  of  the 
most  striking  of  fractional  atomic  weights  is  that  of  chlorine,  the 
figure  for  which  stands  practically  midway  between  two  integers. 
Now  if  this  element  is  a  mixture  of  two  isotopes,  having  respectively 
the  atomic  weights  of  35  and  36,  it  is  easy  to  see  that  in  a  certain 
proportion  the  mixture  will  yield  a  fractional  ratio. 
Isotopes  are  so  far  not  separable  by  chemical  means.  In  this 
respect,  they  constitute  one  step  further  in  analogy  to  elements  long 
known  and  separable.  The  known  elements,  although  only  a  few 
score  in  number,  present  us  with  an  epitome  of  the  evolutionary 
relations  of  nature  at  large,  just  as  the  modern  theory  of  atomic 
structure  presents  us  with  an  epitome  of  the  solar  system,  the 
nucleus  with  its  more  or  less  loosely  held  circulating  electrons, 
resembling  the  sun  with  planets,  moons,  meteors,  comets  and  cosmic 
dust  tributary  to  it.  Such  elements  as  chlorine  and  potassium 
stand  at  the  extremes  of  the  series,  their  separation  being  almost 
automatic,  then  the  members  of  the  chlorine  and  potassium  groups, 
respectively,  agree  closely  among  themselves,  and  their  separation 
requires  special  care.  Closer  affiliations  are  noted  between,  for 
instance,  nickel  and  cobalt,  and  still  closer  between  the  cerium 
metals,  and,  finally,  the  most  recent  specific  separation  of  a  supposed 
element  into  its  constituents  by  chemical  means  is  the  decomposition 
of  didymium  into  two  distinct  contrasting  metals,  neodymium  and 
praseodymium,  the  former  producing  a  series  of  bright  red  salts 
and  the  latter  a  series  of  brilliant  green  ones.  The  next  problem  of 
separation  is  that  of  the  isotopes. 
