1893.] Thickness and Electrical Resistance of Liquid Films. 395 



of varying composition. Two methods of experiment were employed : 

 (1) an optical method,' in which the mean thickness of about 50 plane 

 black films contained in a tube was deduced from observations of in- 

 terference phenomena;, and (2) an electrical method, in which the 

 thickness of a cylindrical black film was derived from a measurement 

 of its electrical resistance. The optical method involves the assump- 

 tion that the refractive index of a thin film of liquid is the same as 

 that of a large quantity of the same liquid. 



Reasons are given for the belief that the refractive indices in 

 question, if not identical, differ only slightly, and hence that the 

 thickness of a film as determined by the optical method is the true 

 thickness. 



In the electrical method the assumption is made that the specific 

 conductivity of a liquid does not alter when the liquid is drawn out 

 into a thin film. 



If the results obtained by the two methods agree, the conclusion is 

 that the specific resistance of a film is not affected by its tenuity ; if 

 they differ widely from each other, a change in the specific conduc- 

 tivity of the liquid must have taken place. 



The authors showed, in 1883, that for a solution of hard soap con- 

 taining 3 per cent, of KN0 3 , with or without the admixture of 

 glycerine, the meau thicknesses of black films, as measured by each 

 of the two methods, were in close agreement. For such solutions, 

 then, the specific conductivity is the same whether the liquid be 

 examined in considerable bulk or in the form of a film 12 jlljll in thick- 

 ness. The accuracy of this result has been confirmed by a large 

 number of observations made during the last three years. 



If the proportion of KN0 3 added to the solution be diminished, the 

 thickness of a black film, whether measured optically or electrically, 

 is found to undergo a change. 



I. Optical Method. 



The following table shows the change in the (true) thickness of a 

 black film due to a change in the quantity of dissolved salt. 



1 part of Hard Soap in 40 of Water. 





3 



1 



0-5 







Mean (true) thickness of black 



12-4 



13-5 



14-5 



22-1 



Experiments made with soft soap and with solutions containing 

 glycerine confirm these results. 



2 f 2 



/ 



