Am.  Jour.  Pharm.  ) 
December,  1920.  ) 
Isotopy. 
851 
undergoes  spontaneously  slow  conversion  into  radium,  and  that  this 
conversion  is  accompanied  by  a  liberation  of  three  atoms  of  helium 
for  every  atom  of  radium  produced.  Radium  further  undergoes 
change  by  loss  of  one  atom  of  helium,  into  a  gas  termed  niton  or 
radium  emanation,  and  this  emanation  by  loss  of  four  atoms  of 
helium,  passes  into  a  form  of  lead,  which  differs  from  lead  extracted 
from  the  common  ores  by  having  a  distinctly  lower  atomic  weight. 
The  sequence  of  change  may  be  represented  thus 
u  — 
He3 
 ^  Ra 
238.5 
12(3  X  4) 
226.5 
Ra  — 
He 
 >■    Nt  (radium  emanation) 
226.5 
4 
222  .5 
Nt  — 
He4 
 >  Pb 
222  .5 
16(4  X  4) 
206.5 
Numerous,  carefully  made  determinations  of  the  atomic  weight 
of  lead  from  the  ordinary  ores  (which  are  not  radioactive)  gave 
207.2  as  the  average,  but  when  the  same  determinations  are  made  on 
lead  from  radioactive  minerals,  which  always  contain  some  lead,  the 
atomic  weight  is,  as  shown  above,  sensibly  lower. 
An  explanation  that  has  been  offered  to  account  for  phenomena 
of  this  type  is  that  many  of  the  common  elements,  if  not  all,  are 
really  made  up  of  two  or  more  closely  accordant  elements,  agreeing 
in  all  or  nearly  all  their  chemical  and  physical  properties,  except 
slight  differences  in  the  atomic  weights,  and  that  the  differing 
proportions  in  which  such  mixtures  occur  determine  the  slight 
variation  in  the  atomic  weights  of  the  elements  obtained  from 
different  sources.  One  difficulty  in  this  view  is  to  explain  why  in 
certain  occurrences  of  the  elements  the  proportions  are  always  the 
same,  since  from  such  sources,  the  atomic  weights  are  constant. 
It  must  be  remembered,  however,  that  the  most  striking  instance  of 
variability,  that  just  set  forth,  relates  to  an  element  occurring  under 
two  distinctly  different  conditions,  respectively  with  and  without 
radioactive  association.  It  must  be  borne  in  mind,  by  the  way, 
that  the  lead  obtained  from  radioactive  ores  is  not  itself  radioactive. 
The  emissive  power  ceases  with  the  production  of  that  element. 
In  consequence  of  these  capacities  of  certain  elements  to  change 
in  atomic  weight,  several  of  them  may  be  found  in  the  same  posi- 
tion in  the  periodic  system,  and  for  this  reason  they  have  been  called 
"isotopes,"  from  Greek  words  "equal"  and  "place." 
