Slack  Spot  in  Thin  Liquid  Films.  753 
saturated  vapour  and  superannuated  liquid  states,  says  : — 
"  We  cannot  expect  experimental  evidence  on  this  part  of  the 
curve  unless  this  state  of  things  may  exist  in  some  part  of 
the  thin  superficial  stratum  of  transition  from  the  liquid  to 
its  own  gas,  in  which  the  phenomena  of  capillarity  take 
place/'' 
Suppose  that  we  imagine  a  liquid  film  in  equilibrium  with 
its  surrounding  saturated  atmosphere.  Let  heat  be  applied 
to  the  system.  Tnen  may  it  not  be  that  the  film  will  become 
coated  with  a  thin  stratum  of  superattenuated  liquid  which 
will  continue  to  thicken  until,  possibly,  its  outer  portion 
passes  over  into  the  unstable  condition,  causing  the  attenuated 
layer  quickly  to  evaporate  ?  If,  on  the  other  hand,  heat  taken 
from  the  system  should  produce  a  corresponding  layer 
of  supersaturated  vapour  next  to  the  film,  it  should  be 
expected  that  a  thickening  of  the  film  would  result  from 
condensation. 
Observations  made  with  the  apparatus  shown  in  fig.  1 
accord  with  this  theory.  For  example,  suppose  that  we 
obtain  a  first  black  film  near  its  limiting  thickness,  so  that  a 
slight  increase  in  the  pressure  will  cause  the  second  black 
film  to  appear.  This  second  black  at  first  appears  as  small, 
perfectly  round,  black  spots  in  the  first  black  film,  which 
expand  to  replace  the  first  black  as  the  pressure  is  increased. 
If  then  the  pressure  be  slightly  lessened,  perfectly  round 
spots  of  the  first  black  will  appear  in  the  field  of  the  second 
black  film.  This  latter  effect  is  more  difficult  to  obtain  than 
the  former,  indicating  that  the  supersaturated  vapour  con- 
dition is  more  difficult  to  realize  than  the  superattenuated 
liquid.  In  fact,  the  thickening  process  ordinarily  is  so  rapid 
that  a  second  black  film  will  appear  to  pass,  as  has  already 
been  remarked,  continuously,  to  even  the  yellow  of  the 
first  order. 
Of  course  this  is  merely  speculation.  Possibly  the  best 
excuse  for  making  such  is  contained  in  the  following  words 
of  Maxwell  : — "  The  surface  which  forms  the  boundary  be- 
tween a  liquid  and  its  vapour  is  the  seat  of  phenomena,  on 
the  careful  study  of  which  depends  much  of  our  future  pro- 
gress in  the  knowledge  of  the  constitution  of  matter." 
Rose  Polytechnic  Institute, 
Terre  Haute,  Indiana,  Feb.  14,  1906. 
