S O'AP-B UBBLES. 5 79 



which evaporates very rapidly, and therefore above the surface of 

 the liquid there is a small atmosphere of alcohol formed by evapora- 

 tion from the wine. This evaporation goes on in the drops as well as 

 in the main body of the wine, but more rapidly at the upper surface 

 of a drop than at the lower ; the reason being that the lower part 

 is more completely immersed in the little cloud of alcohol which 

 hangs over the wine itself. A drop is thus formed composed in the 

 upper part of water, with comparatively little alcohol, and in the 

 lower part of water with a larger proportion of alcohol. The experi- 

 ment I have just shown proves that the tension in the upper part 

 must be greater than that in the more alcoholic cr lower portion of 

 the drop. Hence you may often see drops of wine actually running 

 up the side of a glass, in obedience to a force exerted in an upward 

 direction, by the greater surface-tension of the portion containing the 

 larger percentage of w T ater. A very important deduction may be 

 made from the fact that the surface of a liquid is in a state of ten- 

 sion. You saw in the first experiment that, as soon as the film on 

 the one side of the thread was broken, that on the other side con- 

 tracted very rapidly, and took up a form with as small a surface as 

 was possible under the circumstances. If Ave were to generalize from 

 this particular instance, we should be led to the conclusion that, be- 

 cause the surface of a liquid is in a state of tension, and thus each 

 part of it is tending to contract, therefore it will always assume 

 a shape which will have the smallest possible surface. I will now 

 show an experiment to illustrate this fact. I have here a tube formed 

 of four plane pieces of glass, through which I shall be able to send 

 light, and so show you the image of its contents on the screen. It 

 forms, in fact, a little box of glass, the ends of which are open, one 

 of them being considerably narrower than the other. I now put the 

 larger end of this tube into a mixture of soap and water, and I with- 

 draw it with a film adhering to it. This film has a tendency to as- 

 sume that shape which has the smallest possible surface ; and evi- 

 dently, by moving up the tube toward the narrow end, its surface 

 can be made smaller than it is at present. You now see on the 

 screen an image of the tube. I move it for a moment in order to 

 form the film. You now see the image of the film, and I think you 

 will observe that it is slowly moving up the tube, and therefore that 

 its surface is becoming smaller and smaller. The experiment might 

 be prolonged until the tilni burst ; but, at all events, you have there 

 sufficient proof that it moves into a position in which its surface is 

 diminished. And, further, inasmuch as the image of the tube is turned 

 upside down on the sci-een, what appeared, to you to be a motion 

 from above to below was, in fact, a motion from below to above ; the 

 film was in reality moving upward, although to you it appeared to be 

 moving down. It was really raising its own weight instead of being 

 pulled down by it. We have thus now established this quality of 



