582 THE POPULAR SCIENCE MONTHLY. 



drop, and every soap-bubble, is in itself almost a mathematically 

 accurate sphere. It is very possible, of course, to make a liquid as- 

 sume any number of other shapes you please, but I wish now to draw 

 your attention to the fact that we are able to give it another very 

 simple form, namely, that of a cylinder : and I will show you upon 

 the screen the conversion of a spherical soap-bubble into a cylinder. 

 You now see the image of a glass funnel. I take another of precisely 

 the same dimensions, and blow upon it a small bubble, which I make 

 adhere to the first, and then I draw it out into a very accurate cylin- 

 der. This proves that the form of a quantity of liquid may, under 

 proper conditions, be cylindrical ; but if we make the cylinder of such 

 dimensions that the length is very considerable in proportion to the 

 breadth, then the liquid will only retain the cylindrical form for a 

 very short time indeed. The slightest jar or disturbance of any kind 

 will of course make it deviate from its shape, and that deviation 

 when once begun is continued, as it were, by the liquid of its own 

 accord. The series of transformations through which the cylinder 

 will go, I have represented for you in the diagram. At the top is the 

 long cylinder, which represents the liquid in its first state. Assum- 

 ing that it is slightly disturbed, you see that it swells out in some 

 places and contracts in others ; and the elevations and depressions 

 grow greater and greater, until the mass of the liquid becomes, as in 

 the lower figures, little more than a series of balls tied together by 

 very fine liquid threads. The transformation does not end here ; the 

 threads are soon broken, and thus what was originally a continuous 

 cylinder is transformed into a series of alternately large and small 

 spheres. I shall have to make use of this particular transformation 

 of the cylinder later in my lecture ; but I wish for the moment to 

 call attention to the fact that one very interesting instance of it is 

 observed whenever water flows out from the bottom of a vessel 

 through a small circular hole. In such a case the form of the column 

 of water would be approximately that of a long cylinder. But, as I 

 have already pointed out, this is a state of what is called unstable 

 equilibrium of equilibrium which may exist for an instant, but not 

 for a longer time. Hence the above series of transformations are 

 gone through. We have alternately contraction and elevation ; these 

 go on until at length the falling column of water is broken up into a 

 falling column of drops. 



We must now, however, pass on to another property of the sur- 

 faces of liquids, namely, that they press on the liquid, or air which 

 they contain, in much the same way as a blown-out bladder presses 

 on the air within it. I will show you an experiment illustrating this 

 in the following way: If a bubble presses on the air within, then it is 

 evident that, if we made a hole in its side, the tendency of the com- 

 pressed air would be immediately to escape through the hole, and we 

 should have a current of air flowing out of the bubble, which would 



