Now,  if  the  chemical  properties  of  substances  be  considered
it  will  be  quite  evident  that  every  atom  or  group  of  atoms  in  a
compound  must  possess  residual  affinity  to  a  greater  or  a  less
extent,  if  under  residual  affinity  we  include  every  type  of
unsaturation  whether  as  regards  the  primary  or  the  secondary
valencies.'  Generally  speaking,  in  chemistry  we  say  that  a
compound  is  unsaturated  when  one  or  more  of  the  principal
or  ordinary  valencies  is  unsatisfied,  as,  for  example,  in  those
cases  when  we  write  a  double  bond.  Such  compounds  as
these  have  the  property  of  forming  addition  compounds  with
other  substances  and  so  they  must  possess  residual  affinity.
In  addition  to  this  special  type  of  residual  affinity  we  have  also
the  cases  of  compounds  which,  though  apparently  perfectly
saturated  as  regards  their  principal  valencies,  yet  indeed  form
double  arid  complex  compounds  with  other  substances.  It  is
generally  accepted  that  these  properties  are  due  to  the  so-called
contra-valencies.  Every  atom,  possesses  these  contfa-valencies
developed  to  a  certain  extent,  and  so  if  under  the  term  residual
affinity  be  grouped  every  type  of  unsaturation,  it  will  follow
that  every  atom  or  group  of  atoms  must  possess  this  residual
affinity  to  a  greater  or  less  amount.
Since  the  possession  of  residual  affinity  tends  to  endow  a
compound  with  the  power  of  forming  addition  compounds
with  other  substances  it  may  be  said  that  the  existence  of
residual  affinity  is  accompanied  by  the  existence  of  force
lines  in  the  surrounding  ether.  Every  atom  or  group  of
'  Baly,  Zatsc/ir.  Elektrochem. ,  17,  211  (1911).
488  SPECTROSCOPY

atoms  must  be  the  centre  of  a  field  of  force  in  the  ether,  the
strength  of  which  depends  upon  the  amount  of  the  residual
affinity  present  in  each  case.
