514 
PROFESSORS A. W. REIROLD AND A. W, RtTCKER ON THE 
electrically measured thicknesses is to be accounted for by the gradual substitution 
of one black tint for the other. The line, as we have said, is difficult to see. It is 
quite probable that the white flecks which draw^ attention to it are more numerous 
on some occasions than on others, or may sometimes be altogether absent, and thus 
the fact that no line has been seen does not prove that the black film was not really 
divided into the two parts- of different thicknesses. It will be observed, however, 
that we have so framed the proposition under discussion that the possible agency of 
other causes in bringing about changes in the electrically measured thickness is not 
denied. 
Proposition VI .— Whereas the specific conductivity ofhlach films formed of solutions 
containing about 3 per cent, of KNO^ is approximately the same as that of the 
liquid in mass, the specific conductivity of similar films formed of solutions con¬ 
taining no KNO^, hut consisting of a .solution of oleate of soda or oleate of 
potash in water is very different from that of the liquid in mass. 
Idiis is merely a deduction from our previous work and from Propositions III. 
and IV. 
If the optical and electrical methods give the same result, it may be infeiTed that 
the specific resistance is not affected by the tenuity of the film. Hence, since this is 
the case for films which contain considerable proportions of salt, the first part of the 
proposition is true. If the results obtained by the optical and electrical methods are 
at variance, one of the assumptions made (viz., that the refractive index and the 
specific conductivity are independent of the thickness) is incorrect. We havm already 
shown that the change is too great to be accounted for by a variation in the refractive 
index. We are, therefore, driven to attribute the anomalous, or rather imjDossible, 
values of the thickness of unsalted films, obtained by the electrical method, to a change 
in the specific conductivity. 
Proposition VII .— The change in specific conductivity — 
(l.) Is independent of moderate changes of temperature; 
(2.) Is not due to the absorption or evaporation of ivater by the film; 
(3.) Is not due to a change in the composition of the liquid by electrolytic 
decomposition produced by the current used to measure the resistance of 
the film; 
(4.) Is not affected by a very large change in the quantity of carbonic acid in 
the air around the film. 
(1.) The first of these statements is proved by such facts as the following. The 
value of /3 (the apparent thickness of the black) was, on one occasion, 139 p.p. when 
