THICKNESS AND ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 513 
films we employ is almost always a horizontal circle. This is by no means the case 
with the boundary between the two black tints. Sometimes one completely encloses 
a patch of the other, sometimes the line of separation is sinuous or stands higher at 
one part of the film than at another. In some cases when no difference of hue was 
observed, the passage of an electrical current through the film either created the 
difference or enabled us to notice a boundary line which we had not before perceived. 
It is a very curious fact that under such circumstances tiny white flecks appear in 
the black along the boundary line between the two tints. Sometimes this line has 
been observed before the passage of the current, sometimes it has not been noticed 
until the flecks have drawn attention to its existence. 
These eye observations are confirmed by electrical measurements. It is indeed only 
on one fortunate occasion that we have been able to measure electrically the apparent 
thicknesses corresponding to the two black tints when exhibited by the same film. 
Both hues were then seen and the line of demarcation was above the upper needle. 
The thickness (measured electrically) of the part between the needles was 200 /r./r. 
When the line of separation was about half-way between the two needles, the thick¬ 
ness (assumed uniform throughout, and, therefore, really the mean thickness of the 
two parts) was 157 p./r. When the line had reached the lower needle, so that both 
needles had passed through the upper black region, the thickness was 116 /r./r. In this 
film the curve of separation was approximately a horizontal circle, so that when the 
needles were in the upper or lower black region the whole of the corresponding 
horizontal ring on the cylinder was of the same hue. In a few other cases we have 
seen the line of demarcation pass the upper needle, and as it sank, the apparent 
thickness of the film, as measured electrically, invariably diminished. In one case we 
obtained a measurement when the two needles were in the apparent thickness 
was 110 /x./r., which, as will be seen by comparing the results for unsalted solutions 
in Table VII. and IX. is relatively small, but agrees with the value 116 /x./x. obtained 
on the occasion already described. Unfortunately on this occasion no measure could 
be made on 
The value of the confirmation—given by the variation of the electrical measures 
made upon the same film—of the fact disclosed by eye observation that, in an 
uusalted film there are discontinuous thicknesses, both of which are black, is 
enhanced by comparison with the extreme constancy of electrical measures which 
we have previously obtained with salted solutions, provided only that they were 
made upon the same film. 
When experimenting with salted solutions we have failed (“ Limiting Thick¬ 
ness, &c.,” loc. cit., p. 652) to detect any change in the thickness of the black part of 
a film which lasted for more than two hours. 
These observations certainly suggest that even in cases where we have not noticed 
a line of separation, which is the only optical indication of the discontinuity of the 
black part of the film which our methods allow, the extreme variability in the 
MDCCCXCIII.— A . 3 u 
