ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
647 
rate to the film to prevent its thinning. It is convenient to describe this operation, to 
which we shall have frequently to refer, as flooding the film. As it was important that 
the total quantity of liquid on the floor of the box should not increase, as this would 
have involved a slow change in the forms of the films, the liquid used in flooding was 
always taken from the box itself. Two holes It, It' (fig. 1), were provided at the back 
of the cover for the insertion of a small pipette, and liquid was drawn up to a fixed 
mark on the tube so as to supply on each occasion the same amount of liquid to the 
film. By this means also we ensured, as far as possible, that the temperature of the 
liquid used in flooding should be the same as that of the film itself. 
The second method which was used to prevent or retard thinning consisted in 
passing an electric current up the film. The binding screws for attaching the leads 
are seen at b, V, c, c (fig. 2). The electromotive force generally used was about 45 
volts, and a reflecting galvanometer in the circuit indicated the strength of the 
current. An account of the effect of an electric current in retarding or accelerating 
the thinning of a liquid film has been already published by us.* 
The wires M, M', pass through perforations in two discs of sheet india-rubber, and 
can be moved in any direction. If the pointed ends are wetted with the solution, 
they may be used to detach and carry away small bubbles which sometimes cling to 
the supports, and which might, if allowed to remain, cause irregular thinning. 
A glass partition N, about 3 inches broad, is attached to the cover of the box, and 
extends to within half an inch of the liquid at the bottom. It was designed to prevent 
the spray caused by the bursting of one of the films from injuring the other. 
Two thermometers T, T', were fitted by corks into holes in the lid near the back of 
the box, one at each side, and a hair hygrometer P served to indicate any change 
which might occur in the hygrometric state of the air. 
To ensure constancy of temperature, the film box was enclosed in a glass tank filled 
with water, the dimensions of which were such that there were 3 inches of water on 
every side of the box except at the top and bottom. 
The experiments described below were all made under such conditions that the 
variations of temperature and hygrometric state were within the limits which previous 
experiment had proved to be necessary to maintain the films in a constant state. We 
have not therefore thought it necessary to give the thermometer and hygrometer 
readings. 
The lengths of the films were measured by a cathetometer placed at a distance of 
11 feet from the box. Their diameters were measured by a small telescope moving 
along an optical bench of Prof. Clifton’s pattern. The position of the telescope could 
be read by a vernier to one or two hundredths of a millimetre. The optical bench 
rested on a solid stone table, and was about 8 feet from the film box. Usually a narrow 
strip of millboard was placed behind each film, and the edge of the film was thus viewed 
by reflected light. When the film was thick enough to show colours, the readings 
* ‘ Pbys. Soc. Proc.,’ vol. 6, p. 357. 
