576 THE POPULAR SCIENCE MONTHLY. 



numerous questions which it cannot fail to suggest to us. "Why is it 

 so easy to blow bubbles with some liquids, and so hard to form them 

 with others? Why does a bubble when blown at the end of an open 

 tube gradually contract and disappear ? Why, when it bursts, does it 

 not still remain a liquid film, but is shattered to an almost impercep- 

 tible dust ? These and a hundred others remained unanswered, and, 

 as I have said, perhaps un-put, until after the genius of Newton had 

 attacked the far more difficult problem of the colors which bubbles 

 display. To night, however, I hope to be able to give you the an- 

 swers to some of these so long-delayed inquiries, as it is now perhaps 

 two hundred years since men of science began to turn their attention 

 to the phenomena of liquid bubbles, and of those properties of liquids 

 on which they depend, and their efforts have been rewarded with no 

 small measure of success, although it is certainly only within the time 

 of many of us here that they have been able to give anything like a 

 complete explanation of them all. In order, however, that we may 

 understand how best to study the laws and constitution of a soap- 

 bubble, it is necessary that we should in the first place clearly com- 

 prehend what it really is. We all know how soap-bubbles are ordi- 

 narily formed. A common tobacco-pipe is dipped into a mixture of 

 soap and water, and when it is withdrawn a thin liquid film stretches 

 across the mouth, which we can blow out into a bubble, and then 

 shake off and detach from the tube. I will now perform the experi- 

 ment of blowing a bubble before you ; only, instead of employing a 

 tobacco-pipe, I will use this glass funnel, and for the common soap 

 and water I will substitute a mixture of Castile soap, water, and gly- 

 cerine. [A large bubble was speedily blown, and it showed the usual 

 beautiful colors. This and the succeeding experiments were dexter- 

 ously and successfully performed, and were much applauded]. 



You now see how, by using a proper liquid, and by taking proper 

 precautions, we are able to obtain bubbles of enormous size. But I 

 wish you for a moment to confine your attention to the bubble, not in 

 its full-blown beauty, as you saw it just now, but rather in that stage 

 in which it was merely a thin film covering the mouth of the funnel. 

 Now, this film was originally the topmost layer of that portion of the 

 liquid inclosed by the funnel, which as I withdrew it skimmed off a thin 

 slice from the surface a slice so thin that had I allowed it to drain for 

 a while its thickness would not have exceeded four millionths of an inch. 

 But, although the total quantity of liquid contained in it was so small, 

 the surface of the film was no less than twice the area of the orifice 

 of this large funnel. Hence, both from the method of formation of 

 the film and from its constitution when formed, it is evident that, 

 if, in any respects, the surface of a liquid differs from the internal 

 mass, if there are laws which govern and forces which are at play 

 on the surface, the effects of which we do not recognize elsewhere, 

 these peculiar properties must be to us of primary importance, if we 



