with Soap-bubbles. 415 



with a gas-bubble blown on the top of it will rise till the gas- 

 bubble breaks against the ceiling, -when the air-bubble falls 

 again, and may be sent up as often as desired by the addition 

 of a new gas-bubble ; or three bubbles, the lower one of air 

 the upper one of gas, so proportioned that the combination just 

 floats, will remain until the middle bubble is touched with 

 the finger, when the other two immediately go opposite ways. 

 There is no occasion to say more about experiments of this 



Exp. 12. — An experiment which is easily performed shows 

 in a striking way how the air-film resists being broken. If 

 a pair of bubbles are blown as shown in fig. 4, and the vi- 

 brating prongs of a large tuning-fork are brought quite close 

 to the line where one bubble rests upon the other, both films 

 will take up the movement of the fork, and a point of light 

 reflected by the two films is seen spread out into a pair of 

 rings, so violent is the motion, yet the films do not touch. 

 It is hardly possible to suppose that the two films remain as 

 close together where the movement occurs as in other parts 

 of the line of support; if they tend to separate they form an 

 exception to the general rule that a vibrating body attracts 

 an object in the immediate neighbourhood. In this case the 

 inner bubble is heavier than the air in the outer one, both 

 because of the weight of the film and the compression of the 

 air within due to its tension. But if the same experiment 

 is tried when the inner bubble is lighter than the air in the 

 outer one, as it may be by holding one of the prongs close to 

 the highest point of the bubbles shown in fig. 6, or when 

 either bubble is heavier or lighter than air, the same result 

 will be found — the bubbles will refuse to touch one another. 



Plateau has described (Statique des liquides) a number of 

 very beautiful experiments in which wire frames representing 

 the edges of geometrical solids are dipped in soap-solution, 

 after which they are found to carry combinations of films, 

 plane or curved according to the character of the frame. 

 Thus within a triangular prism, when it is removed from 

 the solution, is found a combination of nine plane films which 

 form three troughs meeting along the axis of the prism and 

 a triangular pit at each end. 



Exp. 13. — A spherical bubble may be dropped into one of 

 these troughs and rolled from end to end, it may be taken 

 out of one trough and dropped into another, or the frame 

 may be held with its axis vertical, when the bubble may be 

 dropped into the triangular pit, where, however, it will not 

 remain long. 



The characteristic feature of all the laminar figures is that 



