SOAP BUBBLES, ONE INSIDE ANOTHER. 259 



a wire ring previously moistened with soap solution and mounted 

 in a corked bottle, so that the ring is vertical. A dexterous shake 

 will now free the bubble from the blowing tube, and will leave it 

 sticking to the ring. Now blow another bubble and bring it near 

 to the side of the first one ; you will find it extremely difficult, if 

 not impossible, to make the two unite in one large bubble ; 

 pressing the two together sideways will flatten them out some- 

 what, but they will not run together or coalesce into one. If, 

 however, a stick of warm dry sealing wax be briskly rubbed 

 with warm flannel, so as to " electrify " it, and be then brought 

 near, the two bubbles will often join into one, which sticks to 

 both of the supports. The reason for this is that the " surface 

 tension " prevents the two bubbles from actually coming into 

 contact under ordinary circumstances, the surface of the one 

 exerting a sort of repellant action on that of the other, and pre- 

 venting their coalescence ; somewhat as two drops of water on a 

 dusty plate or leaf will not run together, contact being prevented 

 by the film of dust sticking to each drop. The presence of an 

 electrified body, however, modifies the surface tension in such a 

 way as to render coalescence easy. 



Expt. 306. One Bubble inside Another. Blow a bubble and 

 mount it on a horizontal wire ring as in Expt. 300. Now dip the 

 blowing tube again in the soap 

 liquor, and by a quick movement 

 pass it through the top of the 

 bubble so that the end of the tube 

 is an inch or more inside ; you 

 will now be able to blow a second 

 bubble inside the first one (fig. 112.) 

 Whilst you are so doing you will 

 notice that the outer bubble in- 

 creases in size, the fact being that 

 the bulk of the air contained in 

 the space between the two bubbles 

 remains much the same, so that 

 as the innb ' bubble grows the 



volume of the outer one must Fi ^ n . 2 - ne B ^ )ble blown 



,. , . , inside another, 



simultaneously increase to about 



the same extent. By a careful shake you can dislodge the inner 

 bubble from the blowing tube, which can then be removed, leaving 

 the two bubbles one inside the other. As in Expt. 305 the two 

 bubbles cannot be made to touch one other absolutely, and con- 

 sequently will not coalesce ; but in this case an electrified body 

 will not cause them to unite in one when brought near. The 



