Soap Bubbles. 21 



trated by hanging a bubble to a fixed wire ring, and hanging to 

 the bubble a second ring with a small weight attached. On 

 allowing the air to escape by breaking the film inside the fixed 

 ring, the shrinking up of the film lifts the weight up to the 

 upper ring. The colours of the bubble were for a long time a 

 great puzzle to the early investigators, but since the discovery 

 of the undulatory theory of light a satisfactory explanation of 

 them has been found. They belong to a class of phenomena 

 known as " colours of thin plates," and are caused by the 

 "interference" of the light waves, reflected from the outer 

 surface of the film with those reflected from the inner surface 

 through the thickness of the film. The effect is to destroy 

 some one colour in the light, leaving its complementary colour 

 unbalanced, and therefore visibly tinting the otherwise white 

 light. The matter is very clearly explained in Tyndall's 

 charming book, " Lectures on Light." The colours are very 

 well seen on a flat soap film attached to a ring of wire, or in a 

 bubble partly filled with smoke, as was observed by Leidenfrost 

 in the last century. He compares the resulting effect to a 

 brilliant star, and then he moralises quaintly, " But all this 

 glory vanishes the moment the bubble bursts ; the fetid smoke 

 which escapes shows with what filth it was filled, and so offers 

 us a striking picture of the gilded miseries of humanity." The 

 well-known fact that two bubbles may be pressed together 

 without coalescing is well illustrated by blowing one bubble 

 inside another one hanging on a wire ring. The outer one 

 must have either a drop of liquid or a wire ring hung to its 

 under part, so as the thick under part of the inner one does not 

 touch it — otherwise they will coalesce ; or if the inner one is 

 filled with coal gas, it rises to the top of the outer one, which 

 answers the same purpose. When the ring on which the outer 

 one is fixed is very light and small, the whole combination 

 rises in the air, the inner bubble representing the bag of gas, the 

 outer one the net, and the ring the car of the miniature balloon. 

 As an experimental tool, the bubble may be used to illustrate 

 the diffusion of gases, a phenomenon examined and investigated 



