[     746     ] 
LXVIII.   The  Black  Spot  in  Thin  Liquid  Films. 
%  E.  S.  Johonnott,  Jun.,  Ph.D.* 
IN  some  previous  workf  on  this  subject  it  was  found  that, 
in  general,  the  black  film  consisted  of  two  portions. 
That  first  formed  varied  in  thickness  between  40  /Jifju  and  12  ja/j,. 
The  first  black  was  found  to  break  naturally  into  the  second 
when  the  atmosphere  was  incompletely  saturated.  With  a 
thoroughly  enclosed  film  it  was  necessary  to  heat  the  system 
in  order  to  form  the  second  Wack  film  ;  which  had,  apparently, 
a  constant  thickness  of  6  /ul/jl  in  all  cases. 
Effect  of  Change  in  Pressure. — Recently  it  was  discovered 
that  great  changes  in  the  character  of  these  films  could  bo 
produced  by  changing  the  pressure  on  the  atmosphere 
surrounding  the  films.  A  sudden  increase  in  the  pressure 
was  accompanied  by  a  rapid  thinning  of  the  film  ;  while,  on 
the  other  hand,  a  sudden  diminution  of  the  pressure  was 
accompanied  by  a  correspondingly  rapid  increase  in  the  thick- 
ness. On  subjecting  the  first  black  film  to  sudden  increase 
in  pressure  it  was  possible  to  convert  it  quickly  into  a  second 
black ;  if  then  the  pressure  was  suddenly  diminished,  the 
second  black  became  a  first  black.  In  many  cases  it  was 
possible  to  continue  this  thickening  until  the  film  showed  the 
yellow  of  the  first  order. 
The  evident  explanation  of  the?e  effects  is  that  the  heating 
due  to  adiabatic  compression  is  accompanied  by  an  increased 
vapour-pressure  and,  consequently,  evaporation  from  the 
surface  of  the  film.  While,  on  the  other  hand,  adiabatic 
expansion  causes  cooling  and  condensation  on  the  film. 
Fig.  1  represents  a  simple  arrangement  for  producing 
these  effects.  The  film  is  formed  on  a  rectangular  frame  of 
glass-fibre,  F,  mounted  on  a  rubber  stopper.  This  stopper 
fits  firmly  into  the  mouth  of  an  inverted  beaker  containing 
the  solution  J.  Blowing  or  sucking  in  a  rubber  tube,  attached 
at  P,  the  pressure  in  the  upper  vessel  is  varied  without 
contaminating  the  solution.  If  the  upper  vessel  is  carefully 
chosen  the  films  may  be  observed  quite  advantageously 
with  a  low-power  microscope.      A  simple  gas-jet  makes  an 
*  Communicated  by  the  Author. 
t  Phil.  Mag.vol.  xlvii.  p.  501  (June  1899).# 
X  The  solution  was  made  in  the  following  manner : — 7  grams  of 
oleic  acid  (refined)  and  2  grams  of  caustic  soda  are  thoroughly  boiled 
in  about  100  c.c.  of  distilled  water.  This  gives  about  7'5  grams  of  oleate 
of  soda  which  is  diluted  to  about  50  of  water  to  1  of  soda  and  again 
boiled.  An  excess  of  acid  is  not  harmful,  while  an  excess  of  soda  is 
decidedly  so. 
