412 Mr. C. V. Boys' s Experiments 



centimetre or more in diameter, is placed on that part of the 

 bubble which is stretched across either ring, and then the 

 part within the small ring is made to burst, the air will escape 

 through the small hole and the heavy ring will be lifted until 

 it comes in contact with the upper one. If the film over the 

 whole of the heavy ring is burst instead, the ring is pulled up 

 so suddenly that it is difficult to follow it with the eye, and it 

 strikes the upper ring with such violence that the noise is 

 loud enough to be heard across a large room. 



A suspended ring affords a simple and accurate means of 

 measuring the surface-tension of the soap-film. A plane 

 film is formed across a fixed horizontal ring and a light 

 smaller ring is attached to the plane film, which is then broken 

 within the smaller ring so as to leave an annular film only. 

 Weights are then hung on to the suspended ring until the 

 angle between the film and the plane of this ring approaches 

 90°. At this point equilibrium becomes unstable, and the 

 lower ring falls away, but now both rings will be found to 

 carry plane films, though the moment before neither did. On 

 repeating the experiment a few times it will often be found 

 possible to use such a weight that the ring will hang for some 

 time, but will gradually sink, while the angle referred to 

 above will approach more and more nearly to 90° as the 

 surface-tension of the film diminishes ; and thus the exact 

 surface-tension at the particular moment of separation may 

 be found by dividing weight of the ring and attached 

 moisture by twice its circumference. 



Exp. 4. — Bubbles blown with coal-gas are lighter than air 

 and rise. If therefore an inner bubble is blown with such a 

 mixture of air and gas as to rise, it will rest against the upper 

 side of the outer bubble, where there are no heavy drops but 

 where the films are thinnest and cleanest (fig. 6). A pair 

 of bubbles blown in this way will sometimes last an hour 

 when exposed to the air of the room. The inner bubble may 

 be gradually enlarged by blowing in gas until the outer one 

 can barely withstand the pull. The forms assumed under these 

 circumstances are extremely graceful, and their beauty is 

 increased by the play of colours on the two bubbles which 

 the multiple reflexions seem to intensify. If, when the inner 

 bubble is not too large, as in fig. 6, a little gas is gently let 

 into the outer bubble, it is possible to so adjust the mixture of 

 gas and air that the inner will float either near the top or 

 near the bottom of the outer bubble, or about the middle, as 

 may be desired (fig. 8). If under these conditions the bubbles 

 are left undisturbed, the richer gas above the inner bubble 

 will diffuse into the poorer and heavier gas below, and the 



