THE ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 
449 
in fact was designed, to reduce the thickness of the film by evaporation, and would 
thus alter its constitution. 
Ludtge, anticipating the objection that the effect observed may have been due to 
the larger proportion of water absorbed by the older and thinner film, points to similar 
results obtained with a decoction of Quillaja. He appears to have overlooked the fact 
that if the experiments were performed in a dry atmosphere, the cold produced by 
evaporation might have been the true cause in both cases. 
The difficulty of estimating accurately what the magnitude of the effect of such 
a cause might be was much enhanced by the fact that no observations had been made 
on the amount of water gained or lost by a soap film in a given time. The retrogression 
of the colours, which, after indicating for some time a decrease, often show an increase 
in thickness, had indeed been observed by Plateau. It was, however, impossible to 
say how far this change was directly due to a bsorption. Considerable masses of liquid 
are, as we shall hereafter show, often drawn into a film from the reservoir furnished 
by the relatively thick portions in contact with its solid supports. An imperceptible 
current of this kind might, whatever its cause may be, increase the thickness of the 
film without altering the proportions of its constituents. On the other hand, films 
thinning under the influence of gravity might, and, as our observations prove, often 
do, absorb large quantities of vapour without any sign of retrogression in the colours. 
Under these circumstances, the results of previous observers being in conflict, and so 
little being known as to the magnitude and rapidity of the changes of constitution 
which a soap film could undergo without rupture, it appeared desirable to investigate 
the subject from a novel point of view. 
Sondhaus* had attempted to study the passage of an electric current through a soap 
film, but failed, as his apparatus was not sufficiently delicate. With this exception, 
no investigation had, so far as we are aware, been made previous to our own on the 
resistance of liquid films. Experiments of this kind nevertheless seemed likely to 
afford information as to the probability of gaining, from observations on liquid films, any 
accurate knowledge of the magnitude of the radius of molecular attraction. In the 
entire absence of experimental results it was doubtful whether the specific resistance 
of a film is the same as that of the liquid from which it is produced. If the magni¬ 
tude of the radius of molecular attraction is as great as is supposed by Ludtge or 
even by Quincke, some indication of the approach of a film to double that magnitude 
might be given by a change in its mean specific resistance. On the other hand, 
something might be learnt by properly contrived experiments as to the alterations in 
the constitution of a film wrought by variable hygrometric conditions. 
The object then of our investigation has been to trace the effect on the specific 
resistance of a soap film— 
(1.) Of change of thickness ; 
(2.) Of change in the hygrometric condition of the surrounding air. 
* Pogg. Ana. 1876, Bd. clvii., p. 95. 
