678 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
Experiment XX. 
t 
C, 
c. 
c 
Increase. 
m. 
21 
{5b, 0(1,3)} 
{7b, W (1, 7)} 
-0-25 
24 
{106, W(l, 7)} 
{206, W(l, 5)} 
-0-60 
+ 0*35 
Experiment XXI. 
t 
Or 
Q 
C 
Increase. 
m. 
16 
{2-56, R (1, 0)} 
{26, R (1, 0)} 
+ 0-12 
20 
All black 
{136, T (1, 0)} 
+ 0-65 
+ 0-53 
Experiment XXII. 
t 
O r 
Ci 
c 
Increase. 
m. 
20 
{156, Y (1, 3)} 
{46, B (2, 0)} 
-0-27 
24 
All black 
{56, Y(l,8)| 
+ 0-48 
+ 0Y5 
Experiment XXIII. 
t 
c. 
Ci 
S 
Increase. 
m. 
22 
{36, W (1, 5)} 
{0-56, 0(1,5)} 
+ 0'40 
25 
{236, 0 (1,5;} 
{36, W (1, 5)} 
+ 1-01 
+ 0-61 
In all these cases there was a sudden increase in the diameter of the film which 
thinned most rapidly in the peculiar way just described. We do not wish to lay too 
much stress on experiments which can only be produced by accidental circumstances, 
but they certainly suggest the inquiry whether one of those circumstances may not 
be something abnormal in the thickness of the black. If its surface tension were 
really lower than usual, the rapid formation of the black surface and the convulsions 
by which it is accompanied would be intelligible. 
We therefore think it best to state the general result of our inquiry as follows :— 
When the black part of a soap film forms in a normal way, spreading slowly over 
the surface, no evidence of any change in surface tension dependent on the thickness of 
the film is furnished by a direct comparison of the tensions of thin and thick films 
over a- range of thickness extending from 1350 to 12 millionths of a millimetre. 
