On  the  absorption  of  inorganic  salts  in  solution  in  various
solvents  a  very  great  deal  of  work  has  been  done  recently,  but
it  must  be  remembered  that  in  these  cases  the  influence  of  the
solvent  itself  must  come  into  play.  In  this  branch  of  the
subject  we  must  include  all  the  metals  which  give  coloured
salts,  such  ag,  for  example,  copper,  cobalt,  nickel,  and  the  rare
earths,  and  also  the  coloured  salts  of  inorganic  acids,  including
permanganates,  chromates,  bichromates,  nitrites,  and  nitrates,
the  last  of  which  only  gives  absorption  in  the  ultra-violet.
