with Soap-bubbles. 411 



of the inner one. The air of the outer bubble may then be 

 withdrawn until the space between the highest point of the 

 two bubbles is no more than two or three millimetres (fig 2, 

 PI. II.). _ ^ * ' 



The great pressure which the air-film will cany is well 

 shown by the next experiment, which, moreover, is more easily 

 carried out than the last. 



Exp. 3. — Proceed as in the last experiment, but instead of 

 making a large drop on the first bubble, hang on a moistened 

 ring of wire rather smaller than the fixed ring. This ring- 

 should be weighted until it pulls the bubble so much out of 

 shape that a tangent to the curve at the points where the film 

 meets the hanging ring makes an angle of 20° or 30° with the 

 plane of the ring (fig. 3, PI. II.). A bubble may then be 

 blown inside and allowed to drop, when it will be found to 

 rest on the conical seat provided by the outer bubble, while 

 the heavy drops of liquid are kept apart, and thus there is no 

 fear of contact (fig. 4) . These drops may now be both removed 

 with the end of the blowpipe ; then, if the lower ring is pulled 

 down slowly, it will be found that the inner bubble is being- 

 squeezed out of shape until it becomes a beautiful oval, while 

 the outer bubble shows the effect of the pressure by a corre- 

 sponding enlargement (fig. 5) . If the lower ring is pulled 

 down still further, the outer bubble is simply pulled in half, 

 and the inner one, often unbroken, gently floats away. This 

 shows that contact was not made, as in that case both would 

 be immediately broken. If, however, instead of pulling the 

 ring too far, it is held in the position shown in fig. 5, it will 

 be found that it is possible to swing the pair of bubbles round 

 and round, and yet in spite of this violent treatment the 

 bubbles refuse to touch one another. Or, if the lower ring is 

 cautiously inclined and pulled away, the outer bubble will peal 

 off it and remain attached to the upper one only. The two 

 bubbles will now be spherical again, but there will be no 

 heavy drop as in fig. 2. The air of the outer bubble may be 

 withdrawn as before, until the two bubbles are barely separate. 



This experiment, and many of those that follow, may be 

 made more beautiful by using for the inner bubble a solution 

 strongly coloured by fluorescine, or still better by uranine 

 (for the knowledge of which I am indebted to Mr. Madan) ; 

 then, if sunlight, electric or magnesium light is thrown on to 

 the bubbles, the inner one appears a brilliant green, while the 

 outer one remains clear as before. 



The power of the surface-tension to do work is demon- 

 strated by blowing a large bubble below the ring and 

 hanging on the weighted ring. If now a very small ring, a 



