the surface tension of soap films 293 
loaded with lead so that the equilibrium of the can is stable when 
the can is floating with its axis vertical. The can is connected to | 
the joint A by a piece of flexible rubber tube. Since the walls 
of the can are thin, the pressure excess diminishes only very 
slowly as the can sinks in the cistern. 
The internal radii of the flow tubes must not be too small. 
With tubes of small radii, the flow of air is so small that a con- 
siderable time elapses before the film reaches its full height, and 
there is a danger that the film may break before the necessary 
measurements can be made. 
§8. The bubble holder. The details of the arrangement on 
which the spherical film is formed and measured are shown in 
Fig. 7. A brass plate 8°5 cm. in diameter is carried by a tripod ; 
Fig. 7. 
a central opening in the plate communicates with the tube 
by which a connexion is made with the joint BC (Fig. 6). <A ring 
rests upon this plate. The upper end is bevelled so that the edge 
lies on the internal cylindrical surface of the ring; this secures a 
definite base for the spherical film. The joint between the ring 
and the plate is made air tight by a little of the soap solution. 
This arrangement allows a number of rings of different radii to be 
used and also allows the rimg to be adjusted on the table. A rod 
rising from the tripod base is furnished with an adjustable hori- 
zontal arm which carries a clamp holding a glass scale divided to 
millimetres. It is convenient to adjust the scale and ring so that, 
when the spherical film is formed, the lower edge of the scale may 
be as near as possible to the highest point of the film. There is 
then little error due to parallax if the film and scale are observed 
through a telescope set at the same level as the top of the film, 
and the film and the scale will be both in focus at the same time. 
A flame placed behind the film at a sufficient distance to avoid the 
