580 THE POPULAR SCIENCE MONTHLY. 



liquids, namely, that their surfaces tend to become as small as possi- 

 ble, and we may extend the law to another and very interesting case. 

 Let me suppose for a moment that I take a mass of clay : it is 

 evident that I could mould it into an infinite number of different 

 forms ; each of these forms might have precisely the same volume, 

 might occupy exactly the same space, but they might all have very 

 different surfaces. For instance, if I took a rolling-pin and rolled the 

 clay out into a thin disk, and then compressed it into a round ball, it 

 is evident that, although the volume might be precisely the same in 

 the two cases, the area of the surface would be much greater in the 

 disk than in the ball. Now, in the experiment I showed you last, the 

 film moved up the tube, because it had a tendency to diminish its sur- 

 face as far as possible ; but, if I had continued the experiment longer 

 if I had allowed the film to move up to the narrowest part of the 

 tube, it would, even then, only in part have satisfied this tendency, 

 and not have done so completely it would have attained the smallest 

 surface possible under the circumstances, though not the smallest 

 possible surface. The reason why it would not have done so is this : 

 that forces were acting upon it other than that which tended to make 

 it contract, for it was also affected by the force of adhesion to the 

 sides of the glass tube ; and, as a matter of fact, liquids are ordina- 

 rily subjected to the action of no less than three distinct sets of forces. 

 The first of these is the attractive influence of the earth, or the weight 

 of the liquid ; the second is the adhesion of the liquid to the sides of 

 any solid vessel in which it may be contained ; and the third class 

 comprises those forces which are at play in the liquid itself. It is 

 evident, then, that the form which a liquid takes will not be due to 

 any one of these, but to all three. The form which it would assume 

 if left to the action of its molecular forces will be modified in the first 

 place by its weight, and in the next by the adhesion to the sides of 

 the solid vessel. Hence the question arises, if we take a liquid free 

 from both these disturbing forces free from the attractive influence 

 of the earth, or practically so, and free also from the force of adhe- 

 sion to the side of the solid vessel which of all the possible shapes 

 into which I might mould my mass of clay would the liquid assume 

 so as to have the smallest possible surface ? This question we are 

 able to answer very easily by means of experiment, and the method 

 by which we do so depends upon the application of an extremely sim- 

 ple principle. When we place a stone in a mass of water we have, in 

 order to immerse it entirely, to push aside, to remove to the right and 

 left, a certain quantity of water, the volume of which is precisely equal 

 to the volume of the stone ; and the stone sinks to the bottom, be- 

 cause its own weight is greater than the weight of the water which it 

 has so displaced. A piece of cork, on the other hand, would rise to 

 the surface, because its weight is less than the weight of the water 

 equal in volume to itself. 



