Black  Spot  in  Thin  Liquid  Films. 
Table  1. 
749 
Age  of 
Fringes 
No. 
Mean 
Mean. 
Remarks. 
Film. 
Deflect. 
Films. 
Thickness. 
Temp. 
hr.  rain. 
fifi. 
°0. 
1     18 
4-0 
221 
160 
255 
All  first  black. 
4    33 
4-1 
221 
16-4 
25-0 
}f             ,, 
7     58 
60 
221 
24-0 
23-5 
3>                           >> 
21    33 
3-8 
221 
15-2 
253 
J)                           )> 
26    23 
30 
217 
122 
26-5 
Two  second  black. 
26    51 
25 
217 
1015 
28-0 
27      3 
1-5 
215 
616 
28-7 
All  second  black. 
In  the  light  o£  the  results  recorded  in  the  former  paper, 
this  table  requires  little  comment.  The  increase  in  the 
thickness  with  falling  temperature  ;  the  limiting  thickness 
of  the  first  black  film  at  about  12  /jl/ll  ;  and  the  mean  thick- 
ness of  the  second  black  at  about  6  fifi,  were  therein  illustrated 
a  number  of  times. 
In  the  above  table,  however,  since  the  number  of  films 
was  over  four  times  as  great,  the  number  of  fringes  displace- 
ment was  correspondingly  greater. 
In  the  former  paper  it  was  remarked  that,  "  a  number  of 
observations  indicated  that  the  thickness  of  the  second  black 
film  was  not  constant. "  With  the  number  of  films  there 
employed  the  deflexion  was  only  about  *35  fringes,  when  all 
of  the  frames  held  second  black  films.  In  the  present  case 
the  corresponding  deflexion  was  1"5  fringes.  With  this 
greater  sensibility  it  was  at  once  apparent  that  the  second 
black  film  was  of  variable  thickness.  While  the  limiting 
thickness  of  the  second  black  was  approximately  6  fi/i,  the 
upper  limit,  in  many  cases,  was  at  least  50  per  cent,  greater. 
In  fact,  in  behciviour,  the  black  films  were  found  to  be 
quite  similar;  i.e.,  with  a  sealed  atmosphere  and  a  rising- 
temperature,  a  rapid  thinning  immediately  after  birth  was 
characteristic  of  both  types.  This  rate  of  thinning  became 
smaller  and  smaller  as  the  limiting  thickness  was  approached. 
The  rate  of  thinning  of  the  second  black  was  so  great,  at 
first,  that  usually  the  limiting  thickness  was  reached  before 
observations  could  be  made.  The  persistence  of  the  black 
films  at  the  limiting  thickness,  together  with  the  tendency  to 
reach  this  condition  quickly,  seems  to  indicate  that  the  films 
were  of  quite  uniform  thickness,  at  the  limit. 
The  data  in  Table  II.  were  taken  from  a  number  of 
tables  similar  to  Table  I.  The  thickness  is  given  in  micro- 
millimetres  (/>tyLt). 
