524 
PROFESSORS A. W. REINOLD AND A. W. RUCKER ON THE 
and 11'9 [x.fx., respectively (‘Phil. Trans.,’ 1883, Part 2, p. 661). Both of these 
conclusions have been verified by more recent experiments. In the following table 
we give the ratios of the electrical to the optical thicknesses found in the case of two 
films observed on Jan. 9, 1892, which are in entire agreement with our 1881 observa¬ 
tions on thick films. 
Table XYIil. 
Optical thickness in /<./<. 
Ratio of electrical to optical thickness. 
Film 1. 
Film II. 
789 
0-996 
735 
1-012 
732 
1-064 
711 
1-022 
708 
0-989 
672 
1-012 
661 
1-045 
631 
1-052 
627 
1-014 
618 
1-023 
611 
1-005 
585 
0-995 
Taking the means of these values and making the proper corrections for tempe¬ 
rature we get 
Specific resistance of liquid in mass.142’G ohms, per cub. centim. 
,, ,, from Film 1.142’0 ,, ,, 
,, ,, from Film II.138’0 ,, ,, 
On referring to page 479 of our paper of 1881 it will be seen that the agreement 
here obtained is as good as that on which our former conclusions are based. 
The thickness of black films formed of salted liquide glycerique has also been 
measured again and again, with the result that the accuracy of our 1883 measure¬ 
ments has in every case been confirmed. 
Preliminary Discussion of the Results. 
Up to this point we have as far as possible confined our attention to experiment. 
We now propose to give our reasons for postponing any full discussion of the possible 
causes of the phenomena we have observed. 
The most probable directions in which to look for an explanation are in the 
formation of a pellicle of dlfierent conductivity from the interior, or in a change 
in the conductivity due to the direct action of the surface forces as the film becomes 
thin, or in a combination of both. 
If we assume that the film consists of two surfirce layers and an interior, which can 
