39(5 



Profs. A. W. Eeinold and A. W. Riicker. [May 4, 



The change in the mean thickness of a black film due to variation 

 in the percentage of dissolved soap is shown in the following table : — 



Hard Soap. No dissolved Salt. 



Proportion of water to soap. . . . 



1/30 



1/40 



1/60 



1/80 



Mean thickness of black film 



21-6 



22'1 



27-7 



29*3 



When the solution contains 3 per cent, of KN0 3 variation in the 

 proportion of dissolved soap has little influence on the thickness of a 

 black film, as is evident from the following numbers : — 



Hard Soap. 3 per cent. KN0 3 . 



Proportion of soap to water. . . . 



1/40 



1/50 



1/60 



1/70 



Mean thickness of black film. . . 



13 



12 1 



11-55 



12 1 



II. Electrical Method. 



It has been stated that for a soap solution containing 3 per cent, of 

 KjST0 3 the thickness of a black film as measured electrically is 

 practically the same as that measured optically. If, however, the 

 proportion of KN0 3 be diminished, the thickness (measured electri- 

 cally) increased in a far larger ratio than would be inferred from the 

 optical method. If the proportion of salt be diminished to zero, the 

 thicknesses thus calculated are much greater than the greatest thick- 

 ness at which a film can appear black. In such cases, therefore, the 

 electrical method does not give the true thickness of the black, and 

 the hypothesis that the specific conductivity of the film and of the 

 liquid in mass are identical is untenable. 



The following table shows the change in apparent thickness due to 

 diminution in the quantity of dissolved salt : — 



Hard Soap. 





3 



2 



1 



5 







Mean apparent thickness of 

 black film (measured elec- 



10'6 



127 



24-4 



26-5 



154 



The large value obtained for the apparent thickness in the case of 



