Electric Current on the Thinning of a Liquid Film. 97 
had changed to black, the extent of which had become 19 
millim., 2. e. nearly half the length of the film. 
The change from white to black in an experiment such as 
this is very striking: the black does not begin at the top and 
spread downwards as might have been expected, but the whole 
area of the white passes by insensible gradations into black, 
and it is difficult to say at times whether itis black or white. 
This intermediate condition between black and white can 
generally be obtained by accelerating the thinning of a film 
by a dewnward current until white has been formed, and 
then breaking the circuit and leaving the film to itself. 
This and similar observations show that a downward current 
has the remarkable effect of rapidly thinning that part of the 
film which is thicker than the black; but does not necessarily 
affect the latter. The film is, however, put into such a con- 
dition that, on the cessation of the current, the development of 
the black proceeds at a rapid rate. ; 
(3) A film 40 millim. long had a ring of black 2°5 millim. 
in breadth at the top. A downward current of 6°2 m.a. was 
applied. In one minute the black entirely disappeared, being 
replaced by white. The current rose to 6°5 m.a. 
The conditions in this case appear to be nearly the same as 
in the last. In each there are 2°5 millim. black, and the 
strength of current applied is about the same. Never- 
theless in the latter the effect is to destroy the black and 
increase the conductivity of the film. 
This is to be explained as follows :—When a film is formed 
between metal supports, the thin portion of the film is not 
immediately in contact with the metal. There is between 
them a thick ring of liquid of variable length, and from’ this 
the film proper hangs. As the film thins this massive ring of 
liquid is maintained more or less unaltered. Experiments 
were made with supports of various materials, shapes, and 
thicknesses, with the object of determining whether these had 
any marked effect on the mode or rate of thinning of the films. 
Glass, iron, and platinum have been used, and the edges of the 
supports made thick and rounded or finely bevelled. Although 
a thick and rounded edge would support a larger quantity of 
liquid than a sharp edge, the thinning of the films seemed 
little influenced by these modifications. When therefore a 
downward current is passed through a film having a certain 
area of black immediately depending from a ring of thick 
liquid, the current, carrying matter with it, soon floods the 
black with liquid and causes it to appear first grey and then 
white, its thickness and therefore its conductivity increasing. 
In the same way may be explained the fact that when a down- 
