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PROFESSORS A. W. REINOLD AND A. W. RUCKER 
In order, therefore, to investigate further the properties of very thin films, it was 
necessary that the temperature and hygrometric state of the air in contact with the 
films should be more completely under control. An apparatus by which this end is 
obtained has been devised, and we have with it repeated our observations on the 
electrical resistance of black soap films, with all the advantages gained by the use of 
the electrometer instead of the galvanometer which was previously employed (see 
Phil. Trans., 1881, p. 457). These experiments were sufficient to test the constancy 
of the thickness of black films, but would not alone afford a trustworthy measure of 
its absolute value. If Newton’s value of the thickness corresponding to the beginning 
of the black be accepted as correct, a black film must be at least ten times thinner 
than the thinnest for which we have directly proved that the specific resistance is 
the same as that of the liquid in mass. It was therefore uncertain whether the 
physical properties of films of such different thicknesses were the same, and it was 
necessary to check by some independent method the absolute thicknesses deduced 
from the electrical experiments. The thickness of a single black film is indeed so 
small that it is probably impossible to measure it by any direct optical method. We 
have, however, succeeded in determining optically the average thickness of a number 
of such films, and have thus obtained the required independent confirmation of the 
results of the electrical observations. 
We propose therefore to describe (1) the electrical, (2) the optical experiments, and 
finally to compare the results with each other, and with those already referred to 
which were obtained some years ago. 
I. Electrical experiments. 
The liquid employed in the experiments of which we have already published a 
description, was invariably Plateau’s liquide glycerique , to which was added 3, 5, or 
7 per cent, of potassium nitrate. This liquid is admirable for many purposes. Films 
made with it are very persistent, thus allowing long continued observations to be 
made upon them, but they cannot always be depended upon to thin so far as to 
exhibit the black of the first order. Sometimes the black appears, and extends to a 
distance of two or three millimetres, or even more; at other times no trace of black is 
seen after several hours. We have not on any subsequent occasion been able to 
secure with a liquide glycerique a formation of black at all comparable in extent with 
that obtained in several of the experiments described in our first Paper above referred 
to. It was necessary for our purpose to discover a liquid from which films could be 
made possessing the two-fold property of persisting and of becoming black with 
tolerable regularity. Moreover, the black must extend to a distance of at least 11 
millims. from the top, to admit of observations of any value being made upon it by the 
electrometer method. Experiments extending over many months were carried out 
with the object of discovering such a liquid, and a number of different solutions were 
i 
