94 Professors Reinold and Riicker on the Influence of an 
will be of interest to see, when the hard-steel experiments are 
complete, how far the resulting inference of low molecular 
permeability in that case is borne out. 
As to the question whether the tension of the lines of 
force is proportional to % or to %b», it is a question for ex- 
periment, and I shall take an early opportunity of endeavouring 
to decideit. But in the mean time it appears quite clear that 
if lines of force mean anything, it is that their number is 
proportional to the force. Consider a magnet-pole. We may 
represent the distribution of force about it either by the law 
of the inverse square of the distance, or by a system of lines 
of force radiating from the pole. Ifthe two are equivalent, 
it involves the consequence that the force at any point is 
proportional to the number of lines of force. 
XII. The Influence of an Electric Current in Modifying the 
Rate of Thinning of a Liquid Film. By Prof. A. W. 
REINOLD, F.R.S., and Prof. A. W. Ricxrr, #.R.S.* 
1% 1877 the results of some experiments made by us on 
liquid films were published in the Proceedings of the 
Royal Society (Proc. Roy. Soc., No. 182,1877). One object 
of the investigation was the determination of the electrical 
resistance of the films. The method of Wheatstone’s Bridge 
was employed for this purpose, and the current was passed 
through a film only at the moment when an observation was 
required. Under these circumstances the films generally 
thinned until they became black; and we succeeded in obtaining 
several measures of their resistance when this colour was dis- 
played. Subsequently this method was abandoned for another, 
in which continuous currents were passed through the film, 
and the difference of potential between two fine wires thrust 
into it was measured by an electrometer, and compared with 
that between two other points in the same circuit separated 
by a known resistancet. 
Although in many respects a great improvement, this method 
was in one point inferior to that previously employed. The 
behaviour of the films was very irregular when compared 
with that of those previously examined by the galvanometer, 
although they were formed of a liquid having the same com- 
position. Sometimes they thinned rapidly, sometimes slowly; 
* Communicated by the Physical Society. Read December 13, 1884. 
t “On the Electrical Resistance of Thin Liquid Films, with a Revision 
of Newton’s Table of Colours,” Phil. Trans, 1881. 
