SOAP FILMS. 



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This property of the bubble is due to the different attractive action 

 of the particles of the liquid forming the outsides of the walls of 

 the bubble on one another as compared with their action on the 

 adjacent air, thus leading to a kind of strain in the soap liquid 

 termed surface tension, the effect of which in causing the film to 

 collapse is similar to the tension or strain in the -elastic rubber ball 

 when expanded. Like the rubber, if the soap bubble be expanded 

 to too large dimensions by continually enlarging it by blowing, 

 the strain on its walls becomes greater than its cohesion can 

 support, and the ball bursts. 



Expt. 300. Analogy between a flat Soap Film and a stretched 

 Membrane, such as a Drumhead. Prepare a flat ring with a 

 handle by bending a piece of copper wire, as indicated in fig. 106, 

 and dip it into the soap water; a thin 

 sheet of soap water like a fine skin or 

 membrane will then adhere to the ring, 

 and can be watched for a long time if the 

 ring be held steadily by sticking the 

 handle end into a cork fixed in the mouth 

 of a bottle, or in a candlestick, so as to 

 form a firm support, or by holding it by 

 a clamp such as that shown in fig. 12. 

 Such a ring, moistened with soap water, 

 may also be used to catch and support a 

 bubble blown by the pipe, and so prevent 



it from being carried away by currents of Fig. 106. Wire Ring held 

 air; the bubble is blown over the ring, 

 holding the pipe bowl downwards, until 

 the bubble touches the ring ; by carefully 

 jerking the pipe the bubble can be removed from the bowl without 

 separating from the ring, and is thus left sticking to the ring and 

 supported by it. Or the pipe may be held bowl upwards and the 

 ring held above the bubble so as to catch it at the top. 



Just as a soap bubble is comparable with a hollow elastic ball, 

 swelling when blown into and contracting and expelling air again 

 when allowed to take its own course, so is a flat soap film analogous 

 to a stretched drum head or tambourine. If you press upon the 

 centre of a tambourine, or on the tightly stretched paper cover of 

 an unopened jam pot, the membrane will yield more or less to the 

 pressure and cease to be quite flat, becoming somewhat hollowed or 

 concave; or if you pass a knotted string upwards through a minute 

 hole in the centre of the tambourine so that the knot catches the 

 membrane, and then pull the string, the membrane will again yield 

 to the pulling force, and become convex, or projecting in the 



by corked Bottle, and 

 supporting a Film of 

 Soap. 



