510 
PROFESSORS A. W. REINOLD ARD A. W. RUCKER ON THE 
and by many made subsequently (see Tables I. and VI.); yet, without the salt, the 
refractive index of the film should be as much as 13 per cent, less than that of 
the liquid in mass. 
To this d priori argument the following may be added. During the progress of our 
work a paper has appeared by Herr Drude (‘Wied. Ann.,’ vol. 43, 1891, p. 169), in 
which the author, using an experimental method totally different from that employed 
by us, compares the refractive indices of black and coloured soap films, of which the 
latter may unquestionably be taken as nearly the same as if not absolutely identical 
with that of tlie liquid in mass. Using solutions both with and without the addition 
of a salt (viz., Na^SOJ, the author finds that the refractive indices of the black and 
coloured films do not differ by more than about 1 part in 140. Such a variation, if 
satisfactorily established, would only affect the apparent thickness of the films as 
measured by the optical method by 2‘6 per cent., whereas, according to the measure¬ 
ments given above, the presence or absence of the salt alters the apparent thickness 
by 100 per cent. 
On the whole, then, we certainly think that the differences of thickness indicated 
by the optical method of measurement may be relied upon as being not merely 
apparent but real, and we shall hereafter treat this point as established. 
Having thus studied the effect of the composition of the films on their thickness, 
we proceed to describe the results of observations in which the apparent thickness 
was measured by the electrical method. 
Proposition IV.—If the p)roportion of potassium nitrate mingled ivith the solution he 
diminished, the thichness of a hlach film, as measured hy the eleetrical method, 
increases, hut in a far larger ratio than ivould he inferred from the optical 
method. If the prop)ortion of salt he diminished to zero the tliichiesses thus cal¬ 
culated are greater than the greatest thichness at ivhich a film can appear hlach. 
Pi such cases, therefore, the electrical method does not give the true thichness of 
the film, and the hypothesis that the specific conductivities of the film and of the 
liquid in mass are identical is untenable. 
This statement is proved by the following table. A very large number of observa¬ 
tions have been made with unsalted solutions, but we give the results of the latest 
experiments only which were made with all the precautions which experience has 
shown to be necessary. 
