488 
PROFESSORS A. W. REINOLD AND A. W. RUCKER ON 
All experiments on films which are absorbing or losing water are, of course, subject 
to some uncertainty as to their temperature. These observations however seem to 
show that the temperature of the film does not differ much from that of the air. 
While the film was losing water the temperature would, be lower than that of the air, 
and the error thus introduced would exaggerate the loss, while the heating of the film 
by the absorption of water would exaggerate the gain. We should therefore expect 
the change of constitution to appear very rapid about the time when the water was 
introduced, as at that time the temperature of the film would be changing most rapidly. 
The low value of p found at 10 '7 minutes (see Table XI.) makes it difficult to draw any 
absolutely certain conclusions. If however we suppose that at the moment the blotting- 
paper was moistened the value of p was as high as 21, the sudden fall to 17'8 would be 
explained by a change of temperature not much exceeding 1° C. There is therefore 
no evidence of any considerable change in temperature when a film suddenly ceases to 
lose and begins to absorb water. 
Turning next to the bearing of these observations on the experiments of 
MM. Plateau and Lctbtge, it is evident that arguments based upon a substance 
subject to such rapid and considerable changes of composition as are indicated by our 
observations are open to grave suspicion. 
It is indeed true that large changes in the constitution of a soap solution are 
attended with but trifling alterations in its surface tension. We have for instance 
proved by experiment that the loss of 25 parts of water alters the tension of a 5 per 
cent, standard solution by less than 3 per cent. But on the other hand it is evident 
that, under other circumstances than those detailed above, much larger quantities of 
water might be gained or lost. In our observations the period of the ultimate slacken¬ 
ing of the rate of change of constitution was never reached unless the films wnre formed 
in air filled with moisture of the right tension by means of the precautions we have 
described. 
The introduction of a small leaden dish of strong sulphuric acid into the case produced 
the most violent commotion in a film. Streams of liquid circulated round it with 
great rapidity, it became spotted with white flecks, and generally broke in less than 
five minutes. The irregular colouring and the short duration alike prevented accurate 
measurement in these cases, but it is probable the rate of change was far greater than 
in those above detailed. The bubble observed by M. Plateau lasted for three days, and 
was placed in a beaker containing caustic potash. The inner surface was, on the other 
hand, in contact with air which was itself in contact with the water in the manometer. 
Under these circumstances water may have been either lost or absorbed, and the 
phenomenon sought for, viz. : a change in surface tension, might have been either 
caused or masked by the changing constitution of the film. 
On the whole, then, we think that the study of the electrical resistance of soap films 
offers better opportunities of investigating their nature than any other method which 
has been hitherto proposed. It is far more sensitive to slight changes in constitution, 
