258 



SCIENTIFIC AMUSEMENTS. 



with attached Pan 

 and Weights. 



Expt. 303. Soap Film Spring Balance. Procure a small 

 ordinary drawing-slate and break away the slate and one end of 

 the wooden frame so as to leave the other end and the two sides 

 grooved where the slate fitted in (a a , fig. 110) ; then get a 

 straight knitting needle (6), and with the aid 

 of a file cut it to such a length that it can 

 slide up and down in the groove parallel to 

 the end of the frame without sticking or 

 passing out sideways. Attach a loop of 

 thread (c c), to the wire supporting a small 

 scale pan (d), or bucket ; and similarly, fix a 

 loop of string (e) to the wooden frame, so 

 that it can be hung up and supported by the 

 loop from a wire projecting out of the cork of 

 a bottle filled with water to make it heavy 

 and steady, or from a stand (such as fig. 12). 

 The frame and wire being dipped into the 

 g ' Film supportedb? W %. b ? S^y pulling the loop attached 

 Frame of Wood to the wire a rectangular film is formed ; by 

 and Cross Wire carefully putting weights (sand, shot, &c.), 

 into the scale pan, or removing them, it ,can 

 be shown that the wire descends or ascends 

 so as to increase or diminish the area of the film, according as 

 the weight is increased or diminished, precisely as the index of 

 a spring balance moves under similar conditions. In short, the 

 surface tension of the soap film causes it to behave precisely as 

 though it were an elastic membrane like a sheet of thin rubber. 



Expt. 304. Ring-shaped Soap Film. Make a flat circular soap 

 film, as in Expt. 300 ; carefully place on it a bit of thread, the 

 ends of which are tied together, and which is moistened with 

 soap liquor. Now touch the film inside the thread loop with a 

 hot wire or needle ; this will break the film inside the loop, but 

 will not affect that part of it which lies outside. The effect of 



the pulling force brought into 

 play by the tension of the film 

 will be to pull the thread loop 

 into a perfectly circular form 

 lying concentrically inside the 

 circular wire supporting the film, 

 as in fig. 111. 



Expt. 305. Two Soap Bubbles 

 will not touch each other 

 readily. Blow a soap bubble, but before shaking it free from the 

 pipe or glass tube, &c., used for blowing, bring it in contact with 



Fig. 111. Annular Soap Film. 



