EXPERIMENTS WITH SOAP BUBBLES.1

By C. V. Boys.

[With 1 plate.]

I had a certain feeling of hesitation in suggesting that you would perhaps be interested in seeing some experiments which I have devised with soap bubbles. I feared that such a subject would have but slight scientific interest for a learned body like yours. However, your accomplished secretary has assured me that my experiments will be well received, and so I trust that you will be indulgent.

To me a soap bubble is a beautiful thing. It appeals to several senses and to many kinds of minds; it is a source of delight to children, and we who know somewhat of the mysteries of molecular physics which it helps to reveal look at it with admiration. With its aid we are enabled to make clear the action of forces relating to other branches of physical science with greater facility and delicacy oftentimes than by any other means. I have observed that the soap bubble even arouses the curiosity of monkeys, and especially of those whose intellectual development is furthest advanced, viz, the chimpanzee and the orang-outang. In a word, among the objects with which we all are familiar and which excite in us a genuine scientific interest the soap bubble takes precedence of all others of the same weight.

Before I show you any of my experiments, it would seem to be incumbent on me to pay the tribute of my admiration to Plateau, that man of genius who, after being stricken with blindness, obliged to make use of the eyes and hands of his daughter-in-law, contrived and developed experiments and theories relating to the science of capillarity which have compelled the admiration of the scientific world, and whose great work, "Statique des Liquides," is a fit monument to the author's genius. When I reflect upon the wealth of

Lecture before the Société française de Physique, April 12, 1912. Translated by permission from Journal de Physique, series 5, vol. 2 (August, 1912). See Soap bubbles: Their colours and the forces which mould them, by C. V. Boys, F. R. S., 10th thousand, Society for Promoting Christian Knowledge, Northumberland Avenue, London; E. S. Gorham, New York; and Boys (C.V.), member of the Royal Society: Bulles de Savon, Four lectures upon Capillarity, before a Juvenile Audience. Translated from English by Mr. C. Ed. Guillaume, Sc. D., with new notes by the author and the translator, 18 mo., 60 figures and 1 plate. Paris, Gauthier Villars, 1912.

knowledge which Plateau has bestowed upon us, it seems to me that I have but picked up a few crumbs which fell from the rich man's table. The formation and existence of a soap bubble depend upon the weak superficial tension of the solution of soap and upon the remarkable property, studied by Willard Gibbs, by virtue of which the superficial tension varies, according to the needs of the moment, between elastic limits, in the true sense of the term. I was surprised to find by experiment that an increase amounting nearly to 20 per cent of the normal tension could be produced. Unfortunately, I can try no experiment here which would permit me to demonstrate to you this fact adequately.

The tension at the upper parts of a large bubble must be greater than that at the lower parts, for it must balance both the weight of the bubble and the tension of the lower parts. That is why there must be a superior limit to the possible size of a soap bubble. A bubble whose color is the bright apple green, weighing five onethousandths grain per square inch, can not exceed 100 inches in diameter, for in that case the additional force which the upper part has to withstand is one-half grain per linear inch, which is one-fifth of the normal tension of the film, i. e., 24 grains per linear inch. Similarly a bubble white of the first order, or weighing one onethousandth grain per square inch, might extend to five times this diameter before this cause of failure would operate. But it is not possible in practice to blow such large bubbles. One great difficulty is, if mechanical means be not employed, to send air in sufficient quantity. With regard to this, Prof. Wood, of Baltimore, told me that he found the principle of injection very advantageous. Indeed, when we reflect, it exactly meets the necessities of the case. Internal pressure diminishing as the bubble increases in size, a small quantity of air blown into the tube will carry with it a large amount of air at a small but sufficient pressure.

I have tried several forms of injectors, but the simplest and hitherto the best is made of a bent pipe such as is employed in the testing of illuminating gas for sulphur. I blow into the narrow end by means of a mouthpiece, while the wide end is surrounded by a cambric band with a serrated edge which feeds the bubble with liquid as it increases. With this device I have not only blown a bubble of 80 centimeters diameter, but am convinced that this is by no means the practical limit. It may be worth mentioning that I make use of Plateau's liquid consisting of a solution in water of oleate of soda with glycerine added. The proportions are as follows: Oleate, 1 part; water, 40 parts; one-third of its volume of glycerine is then added. I have increased the proportion of oleate, especially when I have wished to blow large bubbles.

These weights can be read off directly from the colored plate in my book.

If the pipe is warmed a little, the bubble which will then contain slightly warmed air will be a genuine Montgolfier's balloon, and will rise by its ascensional force. If the bubble have a diameter of 1 foot, it is surprising how long it will remain sufficiently warm to float in the air. When it begins to descend, it can be stopped by means of a current of air directed upward either by blowing with the pipe or with a pair of bellows. In the latter case, by accommodating the movements of the bellows to the soap bubble's natural period of oscillation, we may at once keep it in the air and cause it to execute vibrations of great amplitude. We may form it again and blow into the interior a very little illuminating gas, and it will then float of itself. The very heat of our breath suffices to make a bubble ascend if it be sufficiently large, e. g., 6 inches in diameter. The tin funnel of the old-fashioned gazogene is all that is needed for this experiment. But with a heated pipe, we may make even very small bubbles ascend for a few moments. The most convenient method of warming, however, is to allow the warm gases above a candle flame to be drawn in by the injector for a few seconds. Bubbles so blown will rise above buildings and float away out of sight.

It is clear that a cold bubble blown with air will float upon carbonic acid. I once entered the Grotto del Cane, near Naples, and blew numerous bubbles which floated all about me in the heavy gas, to the great delight of the custodian. Vapor of ether is more easily prepared and is still heavier than carbonic-acid gas. It is easier to use it for the purpose of supporting a bubble filled with air, but the vapor soon condenses upon the bubble, evaporates in the interior, and at the end of a short time the latter sinks into the vapor. If it be caught on a ring and brought near a lighted candle, the bubble will burst into flame, thus showing that the vapor has penetrated the interior. We may again blow a bubble by means of the tin funnel and hold it in the vapor. If we then bring a lighted candle near the open end of the funnel, a long flame like that of a Bunsen burner is formed by the issuing vapor. It is interesting to observe that if benzene (CH) be substituted for ether, the bubble will float as well but the penetration of the vapor will be less rapid. However, it finally enters and the bubble then burns with a brilliant flame. With pentane (C,H12), on the contrary, the bubble floats without the penetration of vapor, this substance being practically insoluble. A bubble of oxygen floating upon ether or benzene explodes violently like a bomb, when ignited.

The vapor of ether and a few other liquids somewhat diminish the superficial tension of a soap bubble, while the greater number of organic vapors increase it. This is especially noticeable with ammonia, which undoubtedly acts chemically upon the free molecules of oleic acid, combines with them and still further neutralizes the 85360°- -SM 1912-15

special influence of the dissociated molecules, which influence Willard Gibbs showed and which tends to diminish the superficial tension. This action of ammonia may be very simply shown by resting a bubble upon a ring of wire of somewhat less diameter. If we hold above the bubble the stopper from a bottle of diluted ammonia, the bubble withdraws toward the lower side of the ring. If, on the other hand, a glass containing ammonia be held beneath the bubble, the action will be still more rapid, the bubble rising in opposition to gravity and squeezing itself through the ring. The motion occurs in each case as if the bubble were inconvenienced by the smell of the ammonia. If the bubble be too large to rise through the ring, a tear is formed, indicating its distress. Naturally, the cause of these actions is the increase of tension of the liquid sheet on the side of the ring to which the ammonia is applied, and if the bubble is too large, this increase of tension attracts a little of the liquid from the rest of the bubble and from the wire. This it is which forms the tear.

Dupré proved long ago that the speed with which soap bubbles burst is determined by the equality of energy in the movement of the little drops discharged at the speed of the retreating edge to that which is necessary to draw out the liquid sheet in opposition to its own tension. This may be expressed in Newton's manner in the following way: If the tension of a sheet of soap water be sufficient to support the weight of a certain number of feet in a sheet of a certain thickness or color, the speed with which a sheet of this thickness or color will break is the same as the speed acquired by a stone which has fallen freely this same number of feet under the influence of gravity. This is manifestly based on the supposition that the liquid is perfectly mobile. When the rigidity and viscosity increase, the speed is reduced. For example, a solution of saponin has a surface tension 50 per cent greater than that of a solution of soap. So a saponin bubble should burst more quickly than a soap bubble if surface tension only were of importance. For the benefit of those who are not familiar with saponin bubbles I will show you one as a curiosity (pl. 1, fig. 1). I shall next make a froth of saponin and glycerin in a cell in the lantern which you see projected on the screen. I may continue the operation until I obtain the ordinary appearance of froth. But the cells which are formed soon begin to burst. You may see the free edge retreat with a slow, irregular movement, which is due to the fact that the liquid is far from being a perfect fluid. Hence I conclude that a soap bubble will burst rather less quickly than the calculation would indicate. Mr. Bull could easily show you this by the aid of his very powerful micro-cinematograph.

In marked contrast with the slow bursting of this particular bubble, I can show you that the speed of a true soap bubble's bursting, which may be as great as that of an express train, may be rendered visible