ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
683 
This supposition is also less satisfactory than the last, inasmuch as the equilibrium 
would be unstable. An accidental thinning below Q' would produce rupture. The 
fact that this does not occur at once might be explained by supposing that the surface 
viscosity would play a more important part as the film became thinner. 
We need hardly point out that the best way of attempting to test such theories 
would be to measure the surface tension of a grey film. Unfortunately, however, 
such a film can only be obtained when the electric current is passing, and the very 
fact that the grey displays itself is a proof that changes in the surface are going on 
which may complicate the conclusions to be drawn from the experiment. 
We have frequently tried to secure satisfactory observations of this kind, but have 
not succeeded in obtaining any in which we could with certainty discriminate between 
the possible effects of the renewal of the surface and a real change in surface tension 
due to a change in the thickness. Experiments XX. to XXIII. are the only observa¬ 
tions we have made in which an apparent change of surface tension could not be 
accounted for by the change of surface. In these observations, however, the latter 
cause would have produced a contraction instead of a bulging of the black film, and 
they cannot therefore be set down as particular cases of the phenomenon studied in 
the earlier part of the paper. 
On one occasion also, when we thickened a black film by passing a current through 
it till it became grey, no appreciable change of the diameters took place. This is, 
perhaps, on the whole in favour of the first of the above hypotheses, as the renewal 
of the surface and the decrease of the “ specific ” tension would act in opposite 
directions. 
Our observations do not, therefore, throw much light on this intricate question. 
Experiments which might do so would have to be made with far more complicated 
apparatus, and with very uncertain prospect of success. Nothing that we have 
noticed, however, negatives the view that the discontinuity at the edge of the black 
is connected with a critical value of the surface tension, caused by an alternation from 
attraction to repulsion in the inter-molecular forces, if the variations in the magnitude 
of the surface tension for thicknesses > 12 X 10 -6 mm. are small. On the other hand, 
4 s 2 
