1885.] on Liquid Films. 247 



times indicated by the abscissae. After a while a piece of blotting- 

 paper which had been hung up inside the case was moistened with 

 water. While this was being done the observations were interrupted. 

 On their renewal it was found that although the film thinned as steadily 

 as before, the j)roduct of the resistance and thickness diminished in- 

 stead of increasing. Curve III. shows the steady absorption of water 

 which follow^ed the moistening of the air. These experiments proved 

 that it is possible for a film to undergo great changes in composition 

 without any indication of the fact being afforded by the colours it dis- 

 plays. They show^ that if the composition of the " liquide glycerique " 

 is to be kept constant, all change in the temperature and hygrometric 

 state of the air must be as far as possible prevented. In later experi- 

 ments this condition has been secured by placing the film box in the 

 centre of a water tank, and by keeping an endless band of linen hung 

 up within the case, and which dips into the liquid, continually 

 moistened. Observations made with this apparatus show that these 

 precautions wliich are certainly necessary are also sufficient. 



The second point to which special attention has hitherto been 

 given by Prof. Reinold and myself is the measurement of the thick- 

 ness of very thin films. If the thickness is less than a certain magni- 

 tude, the films appear black, and thus their colour gives only a limit 

 to and not a measure of their thickness. Black films display many 

 remarkable properties. In general there is a sudden change in thick- 

 ness at the edge of the black indicated by the omission of several 

 colours, or sometimes of one or two orders of colours. It is only 

 under rare conditions that a gradual change in thickness can be 

 observed from the white to the black of the first order. 



To determine the thickness of the black its resistance was 

 measured, and the thickness calculated on the assum/ption that the 

 specific resistance loas the same as that of the liquid in mass. 



The observations were made in several different ways and proved 

 that the thickness of the black portion remains constant in any given 

 film, liowever much its area may alter. Thus, in the case of a group 

 of films measured by Wheatstone's bridge, the average resistance of a 

 black ring 1 mm. in breadth was 1'761 megohms when the total 

 breadth was 2 mm., and 1-760 megohms when the total breadth lay 

 between 10 and 12 mm. 



Again, the resistance of the part of the film between the needles 

 used in the electrometer method was practically the same when 

 the black had extended over the whole film (40 mm. long) as it had 

 been when only the upper 11 mm. were black. The final measure- 

 ment differed from the mean by only • 1 per cent. Again, in another 

 film the resistance of the black per millimetre remained the same to 

 within 2 • 5 per cent, for an hour and a half. 



On the other hand the experiments also proved that the thickness 

 of the black was different in difierent films. The values found varied 

 between 7*2 x 10 ""^ and 14-2 x 10"*^ mm. These differences are 

 quite outside the possible error of experiment. If they were due to 



