1881.] Molecular Electro-Magnetic Induction. 



525" 



gold wires which were in connexion with the opposite pairs of quad- 

 rants of a Thomson's electrometer. The resistance of the film between 

 the needles was calculated by comparing the deflection caused by the 

 difference of potential of the two wires when a current was passing 

 through the film, with that produced by the difference of potential 

 above and below a known resistance placed in the same circuit. 



A novel method, the same in principle with the above, was also used 

 to determine the specific resistance of the liquids from which the films 

 were formed. This was deduced from the difference of potential of 

 two platinum wires cemented into a glass tube in which the liquid was 

 contained. As these were at some distance from the electrodes, errors 

 due to polarisation were got rid of. The results of some test experi- 

 ments made on solutions of sulphuric acid agreed with those of 

 Kohlrausch to within 0'7 per cent. 



The authors conclude that their experiments show that the specific 

 resistance of a soap film thicker than 3*74 X 10 -5 centims. (the least 

 thickness at which trustworthy observations were made) is inde- 

 pendent of the thickness, and is equal to that of the liquid from which 

 it is formed. 



They have, therefore, detected no indication of an approach to a 

 thickness equal to the diameter of molecular attraction, and this leads 

 to the deduction that its magnitude must either be less than is supposed 

 by Quincke (0*5 X 10~ 5 centims.), or that the mean specific resistance of 

 the surface layer, the thickness of which is equal to that magnitude, 

 does not differ by 17 per cent, from that of the liquid in mass. 



They have further found that soap films, even in an enclosed space, 

 may, if the precautions above referred to are not. taken, readily lose 23 

 out of the 5 7' 7 volumes of water contained in every 100 volumes of 

 solution, and their experiments show that this quantity may probably 

 be largely exceeded. They think, therefore, that in all accurate observa- 

 tions on soap films these profound modifications of constitution must 

 either be prevented or measured by a method similar to their own. 

 They criticise from this point of view the observations of Plateau* 

 and Liidtge,t and conclude by pointing out the extreme sensitiveness 

 of the electrical method of investigation. 



IT. " Molecular Electro-Magnetic Induction." By Professor D. 

 E. Hughes, F.R.S. Received March 7, 1881. 



The induction currents balance which I had the honour of bringing 

 before the notice of the Royal Society^ showed how extremely sen- 



* "Statique des Liquides," (1873), vol. i, p. 210. 

 f " Pogg. Ann.," (1870), vol. cxxxvii, p. 620. 

 X " Proc. Boy. Soc," vol. 29, p. 56. 



