THICKNESS AND ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 521 
time it thinned to the white of the first order, was thickened by sending a current up 
it, and was again allowed to become thin. The following table gives the optical 
thickness and the ratio of the electrical to the optical thickness. It will be observed 
that the latter differs considerably from unity, and increases as the film becomes 
thinner. 
Table XIV. 
Optical thickness in /<./<. 
Ratio of electrical to optical, 
i.e., to true thickness. 
641 
1-66 
484 
1-69 
388 
1-60 
332 
1-81 
320 
1-78 
314 
2-07 
1-90 
307 
1-84 
296 
1-98 
The electrical thickness was also measured on each occasion when the film displayed 
the white of the first order. Another film was then formed, and after its thickness 
had been very rapidly reduced by means of a current, the electrical thickness of the 
white was again determined. The numbers thus obtained were 407, 430, and 465 fx.fx. 
respectively, the mean being 434 fi.fji. As the true thickness corresponding to this 
colour is 97 ja.p.., it will be seen that the ratio is 4'47. 
Summing up we have the results given in the following Table. 
Table XV.—Solution of oleate of soda containing 1 part of oleate in 60 of water, 
but no metallic salt. 
Colour. 
Optical, i.e.-, true thick¬ 
ness, in 
Ratio of electrical to 
optical thickness. 
Green of fourth order G (4, I) . 
641 
1-66 
Green of second order G (2, 1) . 
296 
1-98 
White of first order. 
97 
4'47 
Black. 
27-7 
5-8 
The optical thickness assumed for the black is the mean of those obtained from 
films of similar constitution (see Tables IV. and VII.), the electrical thickness, viz., 
160 /x.p,., is deduced from the value obtained for films containing 1 part of soap to 
40 and 70 of water respectively. 
It is important to remember that after the white had been exhibited, the film was 
thickened and then allowed to thin to the white again, and that the ratio of the 
electrical and optical thicknesses changed from a large to a relatively small, and then 
again to a large value. This is shown below. 
MDCCCXCITI.—A. 3 X 
