652 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
Our observations show that the black, at least in a cylindrical film, does not become 
thinner by lapse of time or by increase of area. In illustration of this point we may 
refer to Film III., Jan. 8, and Film III., Feb. 8. 
Film III., Jan. 8.-—The measurements began when the length of the black portion 
was about 11 millims., and were continued until the whole area of the film was black. 
The first measurement was made 45 minutes after the film was formed. 
The resistance of the film between the needles 
50 minutes after the film was formed was 2‘63 megohms 
60 „ „ „ 2-69 
72 „ „ „ 2-67 
The mean of all the observations was 2'672. 
Film III., Feb. 8.—This film, after being formed, was left to itself for an hour. At 
the end of this time about 15 millims. were black, and observations were at once 
commenced and were continued at intervals during two hours, at the end of which 
the film burst. The extreme length of the black was 29 millims. 
After l h 20 ra the resistance was F644 
2 h 20 ra „ „ F607 
2 h 50 m „ „ 1-680 
The mean being 1"654. 
Although in any given film the thickness of the black appears to be fairly constant, 
it will be seen from the above table that the thicknesses vary a good deal in different 
films. The most serious deviations from the mean occurred on Jan. 8 in Films II. and 
III., the values deduced from these being 7"82 and 7T9. We have no reason however 
to think the experiments on this day less trustworthy than others, as none of the 
precautions usually taken to ensure accuracy were neglected. Defective insulation at 
the needles, involving a deviation of the circuit, would have resulted in an increased 
and not a diminished value of the thickness. The fact that Film I. of the same date 
yielded the value 12"53, which does not differ much from the mean of the others, 
precluded the possibility that a different liquid had by some mischance been used. 
The number 12"53, however, had been derived from a single observation, and hence 
might appear to be less trustworthy than the others. To set all doubt on this point 
at rest, the specific resistance of the liquid used was without delay redetermined, and 
was, as was expected, found to be normal. 
II. Optical experiments. 
The object of these experiments was, as has been already stated, the measurement 
of the mean thickness of a number of black soap films by an optical method. 
