658 
PROFESSORS A. TV. REINOLD AND A. TV. RUCKER 
acid, and if fresh, liquid which had not been exposed to the action of the gas was used 
to flood the thick film it bulged instead of contracting. 
Oxygen, on the other hand, appeared to increase the phenomenon ordinarily 
observed, or, at all events, to accelerate it. 
Thus, when the film box was filled with air, the difference of the diameters 
increased by 2‘07 mm. in 24 ra , when the left film was flooded at 3 minute intervals, 
the right film being allowed to thin. 
The films were then blown with purified oxygen, and a current was passed through 
the box. 
In two successive experiments which were then made under the same conditions as 
before, except that oxygen was used, the change in the diameter was 206 mm. and 
2‘80 mm. in 9 minutes. 
These experiments cannot be considered conclusive, but they indicate that the 
phenomenon under discussion is affected by the nature of the gas with which the 
films are in contact. To have put the matter to a thoroughly satisfactory test would 
have required a complete remodelling- of our apparatus. We did not carry the 
inquiry further, partly on this account, and partly because we had collected evidence 
which proved that the greater tension of the thicker film was not due merely to its 
thickness. The cause of the difference was therefore outside the immediate scope of 
our investigation. 
One reason for this conclusion is that the results obtained with the same liquid 
are very irregular. In three consecutive experiments, performed on the same day, 
in each of which the thin film displayed the colours of the third order, the values 
of 8 were 3'37, 2'50, and 1*73. On the previous day, when a longer interval elapsed 
between the floodings, two consecutive experiments gave for similar thicknesses 
8=1-05 and 1*56. Again, different solutions behaved very differently. The above 
results were obtained with a solution of potash soap which had been recently made. 
An exactly similar solution a year old gave 8 = 0'48 when displaying the green of the 
third order. 
In the next place, so much of the change takes place before the thinner film is thin 
enough to display any colours that if it is really due to the thinness of the film the 
radius of molecular attraction must be at least = 10“ 3 mm. Many of our experiments 
have shown that between one half and one quarter of the total change takes place 
while the thin film is still colourless. 
On the whole, then, we are inclined to the view that this phenomenon is merely 
a striking instance of the difficulty which many observers have found in preserving a 
liquid surface pure. Oberbeck found that the superficial viscosity of water exposed 
for 24 hours to the air became immeasurably large, but regained its former value 
when the surface was cleansed by a clean strip of platinum. He concludes, “ A ir 
mtissen daher schliessen, entweder, class der freien Wasseroberflache ein recht bedeu- 
tender Oberfiachenwiderstand zukonunt, oder dass eine reine Wasseroberflache in 
