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 imperceptible 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 answers 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 comprehend what it really is. We all know how soap-bubbles are ordinarily 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 A common tobacco-pipe is dipped into a mixture of soap and water, and when it is withdrawn a thin liquid film stretches across the mouth, which we can blow out into a bubble, and then shake off and detach from the tube.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 experiment 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 glycerine. [A large bubble was speedily blown, and it showed the usual beautiful colors. This and the succeeding experiments were dexterously 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
