DEMONSTRATIONS IN PHYSICS 225 



best shown by projecting flat soap-films in circular rings, 1 the 

 arrangement of the apparatus being shown in detail at p. 827, 

 where the bands of colour are being projected. At present, 

 however, we are not dealing with colour, but simply with the 

 contractile power of the film. Having lifted a film from the 

 saucer, lay across it a loose thread of fine sewing-silk dipped 

 in the solution ; this thread will also be projected, and it will 

 be seen that it may be moved about freely. Prick on either 

 side of the thread with the end of a small screw of blotting- 

 paper, and the tensile force on the other side at once draws 

 the thread into a curve as fig. 114. In the same way a loose 

 closed loop, pricked in the centre, is at once drawn out into 



FIG. 114 



a circle by the tension of the film. If the ring is carefully 

 flattened, and a straight thin wire wetted with the solution 

 laid across it near one edge, it will adhere ; and if the film be 

 pricked in the small space between the wire and the edge of 

 the ring, the tension of the film will pull the wire more or 

 less towards the other side. 



That the contractile force is greater the smaller the 

 bubble, is projected by blowing two bubbles, one of an inch 

 and the other of two inches diameter, at the open ends of a 

 twice-bent tube, furnished with a stop-cock between them, 

 and with two others which allow communication with separate 



i The rings should be of wire about 1 mm. diameter, and 2 to 8 inches 

 diameter, and should be coated with paraffin, rubbed on them whilst hot, 



Q 



