418 Experiments ivith Soap-bubbles. 



do. Or if the two rings are connected with a key and a 

 single bichromate-cell so that when the key is not pressed the 

 rings are connected together, but when depressed they form 

 the terminals of the cell, then at the moment of making the 

 contact the bubbles unite because the electrostatic attraction 

 between surfaces so very close together is able to squeeze out 

 the air, which mere pressure had hitherto failed to do. 



Exp. 17. — Bearing in mind how exceedingly delicate this 

 is as a test of difference of potential, the following experiment 

 seems the more decisive. The cover of the electrophorns may 

 be brought so close to the side of the bubbles, shown in fig. 4, 

 as to pull them completely out of shape, and yet the outer film 

 so completely screens the inner from the electrical action, 

 even though the inner one is to all appearance in contact 

 with the Suter one, that there is no difference of potential 

 between them, and so the film of air is not destroyed. I do 

 not know any experiment which so clearly shows as this that 

 electrical force is confined to the absolute surface of a con- 

 ductor, and is not felt at any depth within it however small. 



Plateau has mentioned, p. 168, that a hemispherical film 

 blown on a plate will screen a smaller hemisphere blown 

 within it, and also resting on the plate, from electrical dis- 

 turbance ; but in this case the two films are widely separate, 

 and there is not the same delicate test as in the case of two 

 bubbles apparently coincident, which instantly join when the 

 smallest electrical stress exists between them. 



Exp. 18. — One more experiment, which is a combination 

 of these two, is worth performing. If one of the bubbles of 

 fig. 16 is replaced by the combination shown in fig. 4 while 

 the other remains as before, and if the cover of the electro- 

 phorus is raised anywhere in the neighbourhood, immediately 

 the two outer films join and become one, while the inner 

 bubble undamaged and the heavy ring slide down to the 

 bottom of the now enlarged single bubble, and give rise to 

 the form shown in fig. 18. 



I am perfectly sensible of the fact that these experiments 

 lie very closely on that ill-defined border-line which separates 

 scientific work from scientific play, but I trust that the 

 beautiful way in which they illustrate the action of certain 

 forces may be sufficient excuse for my showing members of 

 the Physical Society what cannot fail to remind some of them 

 of their nursery days. 



The following particulars may be of service to those who 

 wish to repeat any of these experiments. 



