1916] on Problems in Capillarity SIX 



vapour from the breath, but when large volumes of air have to be 

 used it must come from a gas holder or water pump or compressed 

 air cyHnder. After the impurities of the vessel have been removed., 

 dipping B into the solution in A, a small bubble can be blown on 

 the liquid, and this can be expanded until it passes up the wet sides 

 of the cylinder as a flat circular film. The film becomes thin enough 

 to show brilliant rings of colour in less than a minute, these being 

 made brightly visible by holding an illuminated white sheet of card- 

 board behind and just above the film, w^hile also arranging a black 

 background below the white card. The plane of the film is at right 

 angles to the cylindrical sides, and will thus be level when the cylinder 

 is vertical. In this condition the centre of the film becomes thin 

 enough in a few minutes to show a black circle which steadily extends 

 in diameter until, after some hours, the whole film is black. Such 

 black films under these circumstances may be kept for a year. 



Around the black circle as first seen there is a ring of silvery 

 white very sharply marked off from the black. The silvery region 

 then grades very smoothly into pale and deep golden, becoming^ 

 orange and deep amber, then a brownish madder and full purple,, 

 clearing into a bright steel blue. This grades into greenish blue and 

 fades into a light gamboge, developing into a more greenish yellow^ 

 and then to a brilliant green. After becoming again yellow, a crim- 

 son tint develops and intensifies to a full purple, followed by an 

 extremely intense grass green. This ends the most striking part of 

 the colouring, the increasing thickness being then denoted by alter- 

 nate red jiurple or pink and green bands of diminishing depth of 

 colour until they fade into the transparent colourless thick film. 



These flat films have a position of equilibrium relative to the 

 supporting vessel, tending to set at right angles to the plane of the 

 walls at the place of contact ; thus the film in the cylindrical glass 

 shade remains flat, because that is the plane perpendicular to the 

 supporting walls. If a film be similarly blown across the centre^ of a 

 spherical flask, then at any inclination this condition will be fulfilled, 

 and a uniform film should be able to remain in any position if it has 

 the full diameter of the sphere. When there is the minimum of 

 liquid and the film is well drained, and consequently nearly uniform- 

 in thickness, it is seen to remain practically quiet in all positions. 

 AVhen, however, a few^ c.c. of soap solution are left in the flask, the 

 film is much thicker in the lower part, and, if, further, it is well 

 moistened where it comes in contact w^ith the flask, is more mobile and 

 tends to assume a vertical plane, because the thicker part will sink to- 

 the lowest position, w^hence it will quickly revert w^hen tilted. The 

 appearance when projected on the screen shows that, however the 

 flask be rotated, the film remains very nearly vertical. It will, how- 

 ever, remain fairly steady in any position if it is slightly less than 

 the full diameter of the spherical flask, and is blown up clear of the 

 excess of liquid. When this modification is employed a striking 



