The  main  difficulty  in  connection  with  the  original  idea
that  absorption  bands  in  the  ultra-violet  exerted  by  such
compounds  as  acetylacetone  are  due  to  such  equilibria  as  are
expressed  by  the  reversible  equation —

CH3— C— CH2— C— CH  CH3— C  =  CH— C— CH3

II  II  ^  I  II

0  0  OH  O

that  is  to  say,  that  the  band  is  due  to  the  actual  oscillation  of
the  hydrogen  atom  between  the  carbon  and  oxygen  atoms,  is
that  it  has  been  found  by  recent  physical  measurements  that
such  migration  of  a  hydrogen  atom  is  very  slow  compared  with
the  oscillation-frequency  of  light.  Again,  it  has  been  shown
by  Lowry  and  Desch '  that  absorption  bands  are  sometimes
exhibited  by  substances  in  which  there  is  no  possibility  of  a
migrating  hydrogen  atom.  Similarly,  I  have  found  that  the
di-ethyl  and  the  di-methyl  derivatives  of  acetoacetic  ester

C2H5  CH3

I        ^O  I        ^O

CH3— C— C— Cf^  CH3— c-c-cf^

II       I         \0-C,H,  II       I         \0-CA

O     QH,  O      CH3

'  C/iem.  Soc.  Trans.,  9S.  807  (1909).
ABSORPTION  SPECTRA  487

both  show  absorption  bands  in  the  presence  of  strong  sodium
ethoxide.  It  is  quite  evident  in  these  two  cases  that  the
absorption  band  must  be  due  to  some  other  cause  than  the
migration  of  a  hydrogen  atom.  Clearly  therefore  the  original
idea  must  be  modified.
Perhaps  the  most  important  fact  that  must  be  taken  into
consideration  is  the  very  profound  influence  that  the  solvent
has  upon  the  absorption  of  various  substances.  A  moment's
consideration  of  this  influence  of  the  solvent  will  show  that
some  account  must  be  taken  of  the  residual  affinities  of  the
two,  solvent  and  substance  dissolved.
