1885.] on Liquid Filmfi. 245 



that of the two surface-layers. If, lastly, tlic changes were irregular, 

 they might safely be ascribed to alterations in temperature or consti- 

 tution. 



To obtain the desired facts it was necessary (1) to devise a method 

 of forming the films in a closed chamber, (2) to measure their thick- 

 ness, and (3) to determine their electrical resistance. 



The films were formed in a glass box at the lower extremity of a pla- 

 tinum ring which communicated by means of a tube with the out- 

 side. In the earlier experiments a cup of the liquid was raised by 

 rackwork to the ring and then withdrawn, leaving a film behind it. 

 The latter was blown out by air which had been dried and passed 

 through tubes containing " liquide glycerique." When large enough 

 it adhered to a second platinum ring placed vertically below the first, 

 and on some of the air being withdrawn it assumed the cylindrical 

 form. 



The thickness was measured by means of the colours displayed, 

 two independent determinations being obtained by two beams of light 

 incident at different angles. Newton's Table of Colours was re- 

 vised, and it was found that the differences between the thicknesses 

 given by him and those determined by new experiment were far greater 

 than the error of experiment of a single observer. Hence, if accu- 

 rate measurements are required by means of Newton's scale, every 

 experimenter must reconstruct that scale for himself. 



At first the electrical resistance was determined by means of 

 Wheatstone's Bridge. The edges of the film where it is close to its 

 solid supports are often, however, the seat of j)henomena which might 

 afl"ect the results. Thin rings of white or black appear which alter 

 the resistance considerably, and which introduce errors for which it is 

 almost impossible to make any accurate allowance. This fact, com- 

 bined with the advantage of avoiding errors due to polarisation, and 

 of being able to select any particular part of the film for examination 

 instead of the whole, led us to adopt a different method. Gold wires 

 attached to a movable support were thrust into the film, and the 

 difference of potential between these when a current was passing 

 through the film was compared with that between the extremities of a 

 known resistance included in the same circuit. 



The result of these observations was to prove that the specific 

 resistance of the films altered in an irregular manner, varying between 

 200 and 137 ohms per cubic cm. A closer inspection showed that 

 abnormal results were always accomj)anied by abnormal variations in 

 the thermometer or hygrometer. When those films were selected which 

 had been observed when such variations were especially small, it was 

 found that the range of variation of the specific resistances was only 

 between 137 and 146, and that the mean value was 143, that of the 

 liquid in mass being 140*5 (at the same temperature). It was also 

 proved that between thicknesses varying from 1370 to 374 millionths 

 of a millimetre, no regular change in specific resistance could be 

 detected, the actual variations lying within 2*5 per cent. 



