THICKNESS AND ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 529 
soap film is entirely new. We hope that the observations which we have made on it 
will be of interest. 
It is difficult to assign a reason why the addition of salt to the liquid should 
produce so great a change in the results. In part, the better conducting salt 
probably masks effects which when soap alone is used become predominant, but we 
think it likely that in part at all events it actually prevents the changes to which 
the change of conductivity is due. 
However this may be, wm venture to think that there can be no doubt as to the facts 
that whereas the conductivity of a thin unsalted soap film is considerably greater 
than that of the liquid in bulk, the difference is so reduced in the case of a salted 
film with or without the addition of glycerine that the mean thickness of a number 
of black films whether measured electrically or optically is the same to within the 
limits of the error of experiment. 
MDCCCXCIII.—A, 
8 Y 
