with Soap-bubbles. 417 



If, instead of a spherical bubble, one of cylindrical form, with 

 its length about three times its diameter, is used, the dis- 

 tortion produced by a small disturbing force is so greatly- 

 magnified that, using an electromagnet actuated by five 

 Grove's cells only, not only is the change of form manifest 

 when oxygen is the gas in the bubble, but it is even possible, 

 by making the length such that the form is very nearly un- 

 stable, to cause the bubble to divide the moment the current 

 is made to pass round the electromagnet. With the same 

 means I have not been able to detect any change of form in 

 a spherical bubble. Fig. 15 shows a convenient apparatus for 

 producing, as often as may be desired, a cylindrical bubble of 

 any degree of stability. 



The short tube a is in connexion with a supply of oxygen 

 which is employed to blow the spherical bubble shown by the 

 dotted circle. According to the position of the screw d this 

 bubble will be larger or smaller before it comes in contact 

 with the ring b, which is held down by the loose weight iv. 

 The gas-tap is then immediately turned off, and the ring b 

 raised by the action of the weight c, until the screw e brings 

 the movement to a stop, Thus the length of the cylinder 

 depends on the screw e, while its volume is determined by 

 the screw d, and so whatever degree of stability is found 

 suitable can be reproduced as often as may be required. 

 The poles of the magnet should be placed at about the level 

 of the line pp. 



There is one other property of a pair of soap-films resting 

 against one another, but not in contact, to which I have not 

 referred. In a lecture at the Royal Institution a few years 

 ago Lord Ealeigh showed that two water -jets if perfectly 

 clean will, if directed so as to meet one another at a small 

 angle, be reflected again and fall as two separate jets, never 

 really coming into contact at all. If the water is not per- 

 fectly clean, the experiment will not succeed. He showed 

 that such a pair of mutually reflected jets form a very delicate 

 electroscope, so that if a piece of excited sealing-wax is held 

 even at a considerable distance they instantly coalesce. As 

 the two jets in his experiments and the two bubbles in those 

 which I am about to describe are in each case kept apart by 

 a thin film of air, I expected to find a pair of bubbles attached 

 to two rings in the same way act as a delicate electroscope. 



Exp. 16. — If a pair of bubbles are blown on rings, which 

 must be insulated from one another, as shown in fig. 16, and 

 the cover of a small electrophorus is raised even at some 

 yards distance, instantly the two bubbles coalesce as seen in 

 fig. 17, but do not burst as they have hitherto been found to 



