^86 THE POPULAR SCIENCE MONTHLY. 



would press upon the plate ; but the plate would be able to resist the 

 pressure, and the bubble would remain a hemisphere with a flat base. 

 If, on the other hand, the bubble were formed on the surface of a 

 liquid, there would be precisely the same pressure on the bottom, only- 

 it would be acting on a medium which would give way to it ; the 

 liquid, therefore, would yield to the pressure of the air, and we 

 should have the bubble as it were a little buried in the liquid by 

 its own pressure. As the pressure increases with the smallness of 

 the bubble, we should expect a small bubble to be very deeply buried, 

 and a large bubble to be slightly buried. I will now pour into the 

 cell, the image of which you see, a small quantity of liquid, and blow 

 in it a very small bubble. You now see the images of two bubbles 

 which have risen to the surface, and that they are very much buried 

 in the liquid by virtue of their pressure. I will now blow a large 

 bubble. You see that within it the surface of the liquid is very much 

 less depressed. I will blow a still larger one. Now I have succeeded 

 in blowing a very large bubble, and the lower part of it is not appre- 

 ciably depressed. I will now blow a great number of bubbles in con- 

 tact, and will then point out one or two facts. You now see that odd 

 network which represents a great number of bubbles. There are two 

 points I wish you to notice. In the first place, when two bubbles 

 meet, the surface between them may be either plane or curved. It 

 is plane if both bubbles are of equal size, and therefore compress the 

 air within them with equal force ; but, if they are unequal, the smaller 

 bubble, compressing the air more strongly, indents the larger, and 

 the surface which divides them is curved. Notice also another very 

 curious point, namely, that in no case do more than three bubbles 

 meet in a point, excepting for au instant. This follows from the law 

 that a large number of bubbles, as well as each one, will assume the 

 smallest possible surface. I cannot go into the proof of this, but it 

 follows from the law I have already given you. As the bubbles form, 

 collapse, and disappear, you see that they always so arrange them- 

 selves that uo more than three shall ever meet in a point. 



Now, then, we have got our bubble on the surface of the liquid. 

 Let us consider what will happen to it after that. Evidently the 

 liquid of which it is composed will run down the sides by virtue ot 

 its own weight; but there will be a certain resistance to this motion, 

 greater or less as the viscosity of the surface is great or small. Hence, 

 there are two different dangers which may beset the bubble. The 

 first of these is, that when the surface-viscosity is small, then the liquid 

 runs down the sides of the bubble very easily ; the consequence is, 

 the bubble becomes very thin and bursts. There is, however, an 

 opposite danger which may imperil the bubble when the surface-vis- 

 cosity is great ; and that is, that the liquid does not flow down in a 

 straight line or regular curve, but in irregular masses, which every 

 now and then tear away from each other. Now, these ruptures make 





