218 Professor A. W. Biiclcer [Marcli 20, 



changes in the coilstitution of the liquid of which the films were 

 formed, it is very improbable that the specific resistance of individual 

 films would not have shown progressive changes. As has been stated, 

 none such were observed. The mean thickness of the five films made 

 of "liquide glycerique " which w^ere observed was 11-9 X 10 ""^ mm., 

 while that of 13 films made of soap solution without any glycerine 

 was 11-74 X 10"^ mm. 



The assumption made in these calculations that the specific 

 resistance of a film, the thickness of which is ten or twelve millionths 

 of a millimetre, is the same as that of the liquid in mass, is not justi- 

 fied by the previous experiments, which had proved it to hold good 

 only to the much greater thickness of 370 X 10 ~*^ mm. It was there- 

 fore desirable to check the results by an independent method. For 

 this purpose 50 or 60 plane films were formed side by side in a glass 

 tube which was placed in the path of one of the interfering beams in 

 a Jamin's Interferential Kefractometer. The compensator was ad- 

 justed so that it had to be moved through a large angle to cause one 

 interference band to occupy the position previously held by its neigh- 

 bour, i. e. to alter the difference of the paths of the interfering rays 

 by one wave length. This angle was determined for the red light of 

 known wave length transmitted by glass coloured with copper oxide. 

 When the films had thinned to the black they were broken by means 

 of a needle which had been included in the tube along with them, and 

 which was moved, without touching the tube, by a magnet. The 

 rupture of the films produced a movement of the interference fringes 

 which was measured by the compensator, and from which, in accordance 

 with well-known principles, the thickness of the films could be 

 deduced. 



The mean thickness given by seven experiments on films made of 

 " liquide glycerique" was 10"7 X 10~^ mm., that obtained from nine 

 experiments on films made of soap solution was 12-1 x 10 ~^ mm. 

 The mean of these, or 11*4 X 10 ""^ mm., differed only by 

 0'4 X 10 ~^ mm. from the mean thickness deduced from the electrical 

 experiments. 



The last point to which reference is necessary is one which lies 

 outside the main line of the enquiries above described, but w^hich is 

 nevertheless not without interest. In the course of the observations it 

 was noticed that the rate of thinning of a film seemed to be affected 

 by the passage of the electric current through it. Some experiments 

 made on this point last year proved the fact beyond the possibility of 

 doubt. The current appears to carry the matter of the film with it, 

 so that it thins more rapidly if the current runs down, and less 

 rapidly if the current runs up than if no current is passing. This 

 may be shown as a lecture experiment. 



A vertical rod which can be moved up and down by rackwork is 

 passed through the centre of the cover of a glass film-box. To the 

 lower extremity is attachefl a horizontal platinum wire, from which 

 another similar horizontal wire is suspended by two silk fibres. A 



