Black  Spot  in  Thin  Liquid  Films, 
751 
A  large  bottle,  containing  some  of  the  solution,  was  con- 
nected to  the  film-box  by  a  long  rubber  tube.  The  pressure 
was  altered  by  changing  the  elevation  of  this  bottle. 
As  shown  in  the  table,  the  films  were  allowed  to  thin  to 
the  first  black,  then  the  pressure  was  diminished  by  4*6  cms. 
of  mercury.  This  caused  the  films  to  thicken  from  18  fji/x  to 
30*6  fi/ju.  The  pressure  was  then  increased  by  9*2  cms.,  and  the 
films  were  quickly  thinned  to  the  second  black  at  a  mean 
thickness  of  8*1  fifju. 
Surface-tension  in  the  Black  Film. — In  this  connexion  it 
may  be  of  some  interest  to  describe  some  work  that  was 
undertaken  to  determine  the  surface-tension  in  the  black  film. 
The  method  of  Sondhaus  *  was  employed. 
A  ring,  15  centimetres  in  diameter,  was  made  of  glass- 
fibre.  This  was  attached  to  one  arm  of  a  sensitive  Oertling 
balance  and  lifted  from  the  surface  of  the  solution.  The 
film  formed,  although  a  catenoid,  was  quite  approximately 
a  cylinder. 
The  following  table  is  a  typical  record  of  the  observations 
made. 
Table  IV. 
Age  of 
Supporting 
Surface- 
Temp. 
film. 
lorce. 
tension. 
°0. 
rain. 
rags. 
mgs.percm. 
0 
2495 
26-5 
24-0 
No  colour. 
2 
2475 
26-35 
Coloured  quickly. 
8 
2465 
26-20 
5  per  cent,  first  black. 
18 
2455 
26-07 
All 
25 
2450 
2600 
10  per  cent,  second  black. 
37 
2445 
2595 
25 
All                „        ., 
44 
2440 
25-90 
,,        ,, 
56 
2435 
25-83 
26 
" 
With  these  results  plotted  in  a  curve,  it  is  quite  evident 
that  to  the  discontinuity  in  the  thickness,  there,  apparently, 
corresponds  no  discontinuity  in  the  tension.  Otherwise, 
however,  the  varying  rate  with  which  the  tension  falls  is  quite 
similar  to  the  manner  in  which  the  film  thins.  And  it  should  be 
observed  that  these  phenomena  point  to  a  variable  condition 
in  the  second  black  film,  both  with  respect  to  tension  and 
thickness. 
The  Five  Black  Films. — Viewing  the  films  with  a  miero- 
*  Sondhaus,  Pog'g.  Ann.  Ergbd.  viii.  p.  26G* 
