G60 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
III. Summary of results. 
We are now able to sum up the results of our experiments. Observations have 
been made upon three liquids, the properties of which are given in the following- 
table. 
Date of 
observation. 
Nature of liquid. 
Percentage of 
KNO 3 in water of 
solution. 
Refractive index. 
Specific resistance ' 
at 15°. 
1877 
Liquide glycerique. 
3 
1-395 
214 
1883 
•>1 . 
5 
1-397 
166 
)» 
Soap solution without glycerine . 
2-88 
1-337 
41 
The first and third liquids were examined electrically; the former by the galvano¬ 
meter, the latter by the electrometer method. The second and third liquids were 
examined by the optical method. 
In no case was there any evidence, when the liquid films were cylindrical, of a 
change in the thickness of the black portion. In the case of the plane films formed 
in the tube, tire optical observations indicated an increase in the thickness of the 
black near its lower extremity. The evidence on this head is however doubtful. 
Whenever the area of the black portion of the film became somewhat extended the 
phenomenon, which may indicate a difference between the thicknesses of its various 
parts, disappeared. There seems, therefore, no doubt that in the case of films formed 
as in our experiments, the black portions assume a particular thickness either at, or 
soon after, their first formation, and that this remains unaltered either by lapse ot 
time or by alterations in the dimensions of the black area. 
Although, however, our observations prove that this thickness is practically con¬ 
stant for any one film, they indicate considerable variations in its magnitude for 
different films. The differences between the numbers given by the optical method 
are perhaps not much in excess of the probable error of experiment, but in the case 
of the electrical observations they far exceed it. They may be partly due to slight 
changes in constitution; but the following reasons negative the supposition that this 
is the only, or indeed an important, cause. 
In the first place, the constitution of a liquide glycerique is more difficult to main¬ 
tain unaltered than that of a soap solution. It is, however, in films formed with 
the latter that the greatest apparent variations in thickness occur. This, on the 
other hand, is in accord with the fact that films formed without glycerine are, as is 
proved by the colour phenomena they display, more uncertain and irregular in then- 
behaviour than those made of the standard solution. Again, if a change of con¬ 
stitution took place, we should probably have detected it by progressive changes 
in the calculated thickness, which would in reality have been due to alterations 
j( 
