1893.] Thickness and Electrical Resistance of Liquid Films. 397 



the unsalted hard soap solution is confirmed by experiments on a 

 solution of unsalted soft soap, which gave a mean appearent thickness 

 of 162 /jLfi. 



In different films the measured thicknesses of the black differ widely 

 from each other, the limits being roughly 80 jll/ll and 230 This 

 large variation is due in some cases, at all events, to a real variation 

 in the thickness. Two different shades of black are (in cases where 

 the solution contains little or no salt) frequently seen in a film. They 

 are separated from each other by a line of discontinuity which is 

 irregular in shape. Comparative measurements on the two shades of 

 black are difficult to make, as the regions they occupy are rarely 

 sufficiently extended or separated by a line sufficiently approximating 

 to a horizontal circle for the application of the method of measure- 

 ment which the authors employ. Measurements, however, have been 

 made, and the results indicate that the electrical thicknesses of the 

 two kinds of black are approximately as 2 : 1. 



Details are given in the paper of numerous experiments carried out 

 with the object of determining the cause of the great increase in elec- 

 trical conductivity in black films made from unsalted soap solutions. 



The results have" shown that the increase of specific conductivity in 

 question — 



1. Is independent of moderate changes of temperature. 



2. Is not due to the absorption or evaporation of water by the film. 



3. Is not due to change in the composition of the liquid by 

 electrolytic decomposition produced by the current used to measure 

 the electrical resistance of the film. 



4. Is not affected by a very large change in the quantity of C0 2 

 in the air around the film. 



5. Is practically unaltered if the films are formed in an atmo- 

 sphere of oxygen. 



The next question to be answered was whether the large changes 

 in specific conductivity affect black films only, or whether similar 

 phenomena can be detected in the case of thicker films. 



The conclusions arrived at were (1) that the specific conductivity 

 of a film increases as the thickness decreases and (2) that this in- 

 crease is less in the case of a film to which a salt has been added and 

 is nil when the proportion of salt is as much as 3 per cent. The 

 following figures illustrate the first of these conclusions : — 



Hard Soap 1/60. 



Optical, i.e., true, thickness of 



641 



296 



97 



27-7 



Ratio of electrical to optical 



1-66 



1-98 



4-47 



5-8 



