680 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
on different occasions. We have seen black at the top of a film of which the rest was 
colourless, and therefore probably 250 times thicker than the black part. 
3. '" The thickness of the black in different films varies between narrow limits only, 
viz., from 7 to 14 millionths of a millimetre. t 
4. # In the same film the thickness of the black remains constant, at all events from 
a short time after its first formation. 
5. No certain difference of surface tension can be detected by direct experiment 
between a persistent black film and a film more than one hundred times thicker. 
6. J If an electric current of sufficient intensity is passed through a film which is 
partly black, the boundary becomes ill-defined, and there is a gradual transition from 
the thickness of the coloured to that of the black part of the film. 
7. When the current is broken the grey colour which has bridged over the gap 
between the black and the rest of the film disappears, and the definite boundary, 
indicating a discontinuity in thickness, is re-established. This operation occupies only 
a few seconds. 
The existence of the discontinuity and statements (6) and (7) can best be explained 
if we suppose that for certain thicknesses intermediate between the black and the 
white of the first order the film is in a state of instability. It may be maintained in 
this state by the application of an external force, but spontaneously abandons it as 
soon as the constraint is removed. 
The observations recorded in this paper prove that, under ordinary circumstances, 
the black and coloured films have the same surface tension to within (P5 per cent. 
[The re-establishment of the discontinuity between the black and coloured parts of 
the films, referred to in statement (7) above, is not instantaneous, but takes place in 
from ten to fifteen seconds. If, then, the force which produces this result is a differ¬ 
ence of surface tension, it must be very small. We are not, therefore, compelled to 
suppose that the viscosity of the black is great, as would have been necessary, had a 
measurable difference of surface tension been detected.—Nov. 16, 1886.] On the 
other hand, there is evidence that a difference of surface tension, so small that it 
would probably be impossible to measure it, may produce movements in the surface 
of a moderately thick bubble. 
Thus Felix Plateau § found that the slight elevation of temperature caused by 
bringing the finger near a film produced movement, and every one is acquainted with 
the violent currents seen in a newly-formed bubble. 
If this be admitted, the discontinuity at the edge of the black could be explained it 
we suppose that the surface tension has a critical value in the grey part of the film, 
* ‘ Phil. Trans.,’ vol. 174 (Pt. 2, 1883), p. 645. 
f In fifteen out of eighteen measurements made on single films the results lay between 10'7 and lo'4 
millionths of a millimetre. 
X ‘ Phil. Mag.,’ vol. 19, 1885, p. 94. 
§ ‘ Statique des Liquides,’ vol. 1, p. 294. 
