NOTES  ON  FROTH. 
549 
making  use  of  mixtures  of  salts,  I  have  sometimes  distinguished 
those  containing  an  acetate  by  this  property.  The  acetate  of 
iron  is  pre-eminent ;  but  the  acetates  of  copper,  lead,  and  other 
metals  share  this  property  to  a  considerable  extent.  Yet  acetic 
acid  itself  shows  no  disposition  to  froth ;  and  the  bubbles  made 
when  alcohol  or  ether  are  shaken  instantly  disappear.  Acetate 
of  iron  gives  off  much  dissolved  air  when  it  is  exhausted  by  the 
pump,  but  it  froths  equally  afterwards.  The  citrate  of  iron  is 
analogous  to  the  acetate. 
This  property  of  frothing  is  quite  independent  of  specific 
gravity.  A  heavy  solution  of  sulph-indigotic  acid  froths,  but  a 
solution  of  chloride  of  ammonium  of  high  density  produces  no 
persistent  bubbles  when  shaken ;  while,  on  the  other  hand,  a 
weak  solution  of  soap,  that  differs  little  from  distilled  water, 
will  produce,  as  every  one  knows,  a  very  permanent  froth. 
The  froth  on  the  surface  of  a  colored  liquid  is  always  of  a 
lighter  tint  than  the  liquid  itself.  This  is  what  might  be  antici- 
pated, when  it  is  remembered  that,  in  looking  at  froth,  we  see 
the  light  which  has  traversed  only  very  thin  films  of  the  liquid, 
and  hence  has  been  subjected  to  but  little  absorption.  Some- 
times, however,  the  color  of  froth  is  totally  different  from  that 
of  the  liquid  on  which  it  floats ;  cochineal,  for  instance,  gives  a 
deep-red  aqueous  solution,  which  froths  considerably  when  agi- 
tated, but  the  bubbles  appear  of  a  pale  bluish-purple.  This  is 
due  to  the  dichromatic  character  of  the  liquids  in  question ;  they 
admit  many  rays  of  the  spectrum  for  a  certain  distance,  which 
are  afterwards  absorbed,  and  that  in  such  a  way  that  the  sum  of 
the  rays  transmitted  by  the  thin  stratum  conveys  to  the  observer 
a  different  impression  of  color  to  that  which  is  conveyed  by  those 
rays  which  can  penetrate  a  thicker  stratum.  Thus  the  thin  film 
of  cochineal  solution  which  constitutes  a  bubble,  transmits  the 
red  ray,  a  yellowish-green  ray,  a  bluish-green  ray,  and  nearly 
the  whole  of  the  blue  and  violet  of  the  spectrum ;  the  resulting 
color  is  accordingly  bluish-purple,  mixed  with  much  white  light ; 
a  thicker  stratum  of  the  same  cuts  off  both  the  green  rays,  and 
a  still  thicker  one  transmits  only  the  red. 
Of  a  similar  nature  is  an  appearance  observed  in  port  wine. 
The  new  wine,  when  shaken,  forms  a  bubble  which  is  faintly  red, 
while  old  port  forms  one  that  is  colorless.  This,  indeed,  is  some- 
