SOAP-BUBBLES. 585 



most convincing proof that the surface-viscosity of this solution is 

 very much greater than the viscosity of the interior of liquid ; and 

 that the resistance offered to motion by the surface is many times 

 larger than that experienced by moving bodies in the interior. 



Another experiment will illustrate this enormous surface-viscosity 

 of the mixture of saponine and water in a still more striking way. 

 I have already explained to you that it" we blow a bubble at the end 

 of an open tube, the bubble will gradually contract until all the air is 

 expelled. What, however, will occur if, instead of simply allowing 

 the bubble to drive the air out, I suck the end of the tube and draw 

 it out more quickly ? I will first perform the experiment on some of 

 the soap and water I have used before, and you will see that, although 

 the bubble will contract more rapidly than before, it will retain 

 throughout the whole of the experiment its spherical form. I will now 

 repeat the same experiment with saponine and water. In this case, 

 on account of the great viscosity of this thin film it will be unable to 

 follow the retreating air as quickly as it must do to retain its spherical 

 form ; the consequence is, it will be unable to retain that form, and it 

 will therefore collapse and wrinkle up into a purse-shaped bag. 



I hope I have succeeded in proving to you that these three prop- 

 erties of liquid surfaces exist. I must now go on to explain how 

 they can be applied to the theory of soap-bubbles. Let us suppose, 

 in the first place, that a bubble is rising in a vessel of water. It will 

 tend to assume a spherical form ; but as it rises to the surface it will 

 be flattened in the direction in which it is moving, and, instead of be- 

 ing a perfect sphere, it will be longer in one direction than the other. 

 Evidently, as it moves, it has to displace the water in front of it, 

 which flows away to the right and left out of the way of the bubble. 

 But, as I have explained, all liquids offer a certain amount of resist- 

 ance to the motion of one part upon another; and, although the re- 

 sistance offered by water is extremely small, it must be taken into 

 consideration. The liquid, therefore, has to flow out of the way of 

 the advancing bubble, and to overcome the resistance offered to its 

 motion ; but as the bubble rises nearer to the surface it moves faster 

 and faster, and therefore the water must be removed from its path 

 more and more quickly. But the resistance offered to its motion be- 

 comes greater the faster it moves ; hence you have the bubble rising 

 more quickly, the water being obliged to get more quickly out of the 

 way, and finding more and more difficulty in doing so, and having, 

 when the bubble gets very near the surface, less space between the 

 bubble and the surface to flow away in. The result is, that the water 

 cannot get out of the way, and therefore the bubble carries it up with 

 it and forms a thin liquid film, which we see as foam upon the surface, 

 through which the bubbles of air are rising. Supposing the bubble 

 thus formed were placed \ipon a solid plate, it would have the form of 

 half a sphere ; and, as the bubble compresses the air in it, the air 



