1865 New Experiments with Soap Bubbles
John Broughton, B.SC. The Intellectual Observer. August, 1865
Review of Natural History, Microscopic Research and RECREATIVE SCIENCE
NEW EXPERIMENTS WITH SOAP BUBBLES.
BY JOHN BROUGHTON, B.sc.
OUR subject is not of very difficult or complex manufacture. It is very easy to take a little soap, and shake it up with some soft water in order to form a lather, to take a tobacco pipe and dip it in the solution, and then the foundation of the bubble is made. For across the bowl of the pipe the soap, dissolved by the action of a cause of whose nature we know but little, has enabled the water to form a film, which, liquid though it be, is braced with a drum-like tension.
This strain, whose existence can be readily demonstrated, would, if the soap were not present in sufficient quantity, cause the film to break immediately by its own contractility. Now blow through the pipe, and the liquid membrane will be swelled out, its capacity for expansion far exceeding caoutchouc, and the thing of beauty grows rapidly to its maturity, and glows with all the magnificence of its perfections of colour and form, changing every second in the former, until the limit of its strength being reached, fragile as lovely, it vanishes into invisible spray, leaving its name as a byword for beauty without substantiality. To make evident this contractile force of the film, which is at once the cause of its existence and destruction, it is only necessary to blow a bubble with a moderately wide glass tube instead of a tobacco-pipe, and when it has attained to considerable dimensions, to present the end of the tube to the flame of a lighted candle, when the contractility of the film will expel the contained air with such force as to extinguish the candle. If, instead of expanding the bubble to its limit of strain it be dexterously jerked off its parent pipe, by virtue of the same cohesive force it instantaneously closes the rent made in its side and floats a short time, a sphere of nearly mathematical perfection ; but the contact of a foreign body destroys at once the uniformity of tension on which its existence depends, producing undue strain at one particular part, and so the unmanageable beauty commits suicide at a touch.
If our plaything escape the profaning touch of anything less fairy-like than itself (and I may here remark that they have a great dislike to touching one another, brought into actual contact with difficulty), it contains within 1ts own constitution elements 'of destruction. For the film of which it is made, thin though it be, is liquid, and must, therefore, obey the liquid laws; hence the solution gradually runs down to the lowest point, till the upper surface becomes so thin that it can support the strain to which it is exposed, and the bubble by bursting resolves itself into the simpler form of spray. Besides this, as will subsequently be shown, the film is of excessive tenuity, and the rapid evaporation of liquid from its comparatively enormous surface, soon reduces its thickness to the bursting point. In fact, a soap bubble depends its being on the accurate adjustment of the equilibrium of many component forces, each of which acting alone would be its destruction, and this equilibrium, being but unstable, is destroyed by the slightest cause. But, on the other hand, if these conditions of its existence be not interfered with, a soap bubble will bear an amount of rough treatment that is surprising in a being otherwise so ephemeral.
Thus our bubble, though a charming thing while it lasts, has such a fragile nature that it can with only the greatest difficulty be employed for any long-continued observation; and for this reason many attempts have been made to discover some means of prolonging its existence. For though gene- rally only considered as a toy, it has already been found to have its uses. Newton spent much time in endeavoring to discover the cause that produced its glorious tints, and how well he succeeded is known to most of my readers. Faraday made soap bubbles the vessels for containing the gases between the poles of the electro-magnet in his great experiments on the magnetism of gases. AB a means of prolonging the existence of the bubble, Brewster* recommends the addition of some sugar to the solution, which, by somewhat diminishing the fluidity, has some effect, and makes it slightly more available for optical purposes. But the successful method is that em- ployed by M. Plateau, the Belgian physicist, who, finding a liquid film necessary to his researches on the deportment of liquids freed from the influence of gravity, invented a solution which forms bubbles of a beauty and permanence almost incredible. The recipe for this solution in its most perfect form is as follows : -
Dissolve one part of pure oleate of soda in fifty parts of distilled water, and to every three volumes of the clear solution thus formed, mix two volumes of pure glycerine.
This mixture, used instead of soap and water, with a common tobacco-pipe, gives bubbles whose tints are truly gorgeous, and which are best observed in the following manner :-Take a piece of iron wire, about the thickness of a darning needle, and after cleaning off any rust with sand-paper, bend it in the form here given, the diameter of the ring being about 1.75 inches. This is readily managed by wrapping it round any convenient cylinder, such as a broom-handle, leaving the perpendicular portion about four inches long, to serve to affix it to a piece of wood as a. foot, the wire ring being a support for the bubble.
To place a bubble on the ring it is only necessary to blow one whose diameter is about one and a-halftimes that of the ring, and then by mea.nR of the pipe allow the bubble to rest lightly on the ring at one point, after which, by continuing to blow, the whole of the circumference of the ring can be brought gently in contact with the bubble without injury to the latter. When this has been accomplished, the pipe can be withdrawn by slightly slanting the bowl of the pipe with one edge towards the bubble, and thus removing it, when the latter remains comfortably resting on the ring. The above is easier to perform than describe, since the bubble has now a comparatively robust constitution, and bears a deal of handling. If any difficulty be experienced after one or two trials, it will be found to disappear if the ring be first well wetted with the solution to remove any traces of grease.
When the bubble is thus established, there is no present fear of its bursting, for if it be made with well-prepared materials, it will possess wonderful durability. When shielded from draughts by being covered with a glass shade, it forms an ornament for a drawing-room. Their duration in a pure atmosphere varies generally from an hour and a half to four hours, but occasionally they last much longer. On one occa- sion a bubble of fine dimensions remained for twenty-seven hours before bursting. Several bubbles on rings placed in a bright light form quite a ble.ze of beauty : their colours are best seen by arrangmg them on a black ground, in order that they may be visible by reflected light only, by which the tints, for optical reasons, are seen in much greater brilliancy.
The extraordinary permanence of our now improved play- thing depends greatly on the purity of the oleate of soda. This substance, which is merely the soda soap of a peculiar fatty acid, requires, for its preparation in a state of purity, some chemical skill, and the employment of a process which may be found in the manuals of organic chemistry. The following process, however, gives it in sufficient purity for most purposes, and is easily carried out. 'l'ake some good sweet almond oil, such as is used by clockmakers and gunsmiths, put it into a convenient vessel,_such as a porcelain basin or a clean iron saucepan, and mix it with about one-and-a-half times its bulk of a. strong solution of caustic soda.; then heat it to boiling for some time, keeping it well stirred. The oil will thus gradually, by the action of the alkali, be converted into a soap. The heating must be continued till all the oil has been decomposed. This may be ascertained by dissolving a. few grains of the mixture in water, when, if any unconverted oil be present, it will be seen to float. If after some time the oil still separates, more soda. must be a.dded, and the boiling continued till the desired end is attained, some excess of soda. being always necessary. The mixture should then receive the addition of an equal bulk of water, so 88 to bring it into complete solution, and the whole allowed to coo], when a. quantity of white soap will separate from a clear liquid. This soap should be strained and squeezed on calico, till no more liquid can be pressed out. Then it must again be dissolved in hot water, and a little more soda. added, and allowed to cool. It will now again separate, and must be pressed as before, till the cake of soap is quite hard, when it will be pure enough for making the solution.
Should the foregoing process (which is less troublesome in than its description appears to indicate) be found too cult, there is yet another plan which, though not quite so successful, is far more easy. It is thus :-Shred finely 150 grains of Castile soap (which can be procured of any druggist), and shake it up in a bottle with half a pint of distilled or pure rain water until it is at length dissolved, then allow the turbid solution to settle, and filter through blotting-paper. The clear solution can then be used to mix with glycerine instead of that of the pure oleate.
I will now suppose a bubble made with our solution, and successfully placed on its ring. It will after a few minutes be glowing with gorgeous colours, which vary almost every minute from the richest violet to the most brilliant. Now darken the room. Then take some common spirit (brandy will do) that has previously been Aha.ken up with some common salt, and with it some cotton wool, which inflame. Now look at the bubble illuminated by the yellow light thus produced. Instead of the former lovely tinted sphere we see a yellowish thing, and on a. closer inspection, we find that the/arts formerly the brightest are now marked by streaks a.n smears of a dead black, which are shifting about continually. They are not beautiful certainly, but are curious when we remember that they mark the lines and surfaces of equal thickness where the yellow light, by reflection from the two surfaces of the film, is made to extinguish itself, and produce the darkness. If instead of a salted spirit flame, a bull's.eye lantern with a piece of red glass be used, so as to view the bubble by red light, a similar effect will be seen. Those parts which are dark by red light are green by daylight, for then the red rays are blotted out of the composite white light, leaving the green ones.
When our bubble's life has lasted some considerable time, it commonly happens, especially if shielded from draughts by a glass shade, that a small purple spot makes its appearance at the uppermost portion, owing to the liquid draining away to the antipodal extremity. This spot when it first appears is scarcely larger than the point of a pin ; but small though it be, it indicates the eventual decay of the beautiful sphere ; for by very slow degrees the spot enlarges, and bright yellow points, like minute' flecks of gold, appear within its circumference. During the enlargement of the purple spot, a series of Newton's rings, of every brilliant rainbow tint, forming con- centric circles for about 25° round it as centre, gradually make their appearance, while, less definite in position and extent, the colours of the remainder of the bubble are constantly varying from one bright shade to another. In fact, nothing can be more beautiful or interesting than to watch the slow consumptive changes which end in the dissolution of our play- thing. It is, from these wonderful optical effects alone, a source of unwearying interest. Slowly the spot increases, the velvety purple becomes very nearly black, and, in addition to the bright golden spots above-mentioned, others appear of intense black- ness, which resemble minute boles. Frequently the central spot attains a diameter of three-sixteenths of an inch, with edges looking like a mosaic of gold and gems; but while we are gazing on the changes which occur almost every second, it vanishes, leaving no trace, save a film spread over the wire ring. I t frequently happens that these effects of the central spot and are not at first obtained, but a few trials, and perhaps a slight addition either of the oleate solution, or of glycerine, will ensure success, which will well repay the expenditure of trouble. These changes, from the first appearance of the spot to the destruction of the bubble, commonly occupy about twenty minutes.
Another of the many interesting diversions which the bubbles furnish is that of filling them with a gaseous mixture, which will just neutralize the influence of their gravity. For this purpose a bladder or gas holder, provided with a stop-cock, should be filled with a mixture of about one volume of common coal-gas and eight volumes of air. By means of an attached tobacco-pipe bubbles of about three inches diameter can then be blown with this mixture; and after detaching, by touching with a wet finger the drop of liquid clinging to their under surface, they can be detached from the pipe and started floating. If the balance be well-adjusted, which is readily accomplished by making the balloon a little smaller or larger, according as the gas employed be or weak, it will hang, like Mohammed's coffin, self-suspended m the air, if the latter be quite still; but being a very sensitive indicator of currents, it will generally move in some particular direction. Thus it will slowly creep along the walls of the room, mount to the ceiling, and descend by the other branch of the current that carried it up, or it will go so near the ceiling that its destruction appears inevitable. But not so; there being always a. cushion of motionless air on the surface of large objects, it will float out of danger in a most surprising manner, unless some asperity of surface should attack it. If it escape the latter danger, it will remain floating and creeping about till it gradually loses some of the coal-gas by solution in its liquid sides, when, becoming heavier, it sinks and bursts.
Notwithstanding the sensitiveness of our bubble to unkind usage, it possesses in some respects a very remarkable invulnerability. It is well known that ghosts and other apparitions do not sustain the least injury either when they are shot at, chopped at with battle-axes, or pierced with sword thrusts- in fact, this peculiarity is invariably demonstrated in perfect ghost stories. This peculiarity is shared by the fairy-like subject of the present article. Pierced with needles, thrust at with knitting-pins, they remain without a wound, nor does a scar remain to testify the gash. Drops of water and small shot may be sent through them without introducing any perceptible symptom of inconvenience ; indeed , they may be pierced with a pen-knife (in a manner that would reduce a less eerie thing into slices), with scarcely any disturbance of its unsubstantial being. But like the silver bullet which infallibly destroys a ghost, so does a particle of grease on any of the above offensive weapons destroy our bubble, for then it re- quires but a single touch, and all is over--it vanishes; its liquid sides can no longer adhere, as though its stainless nature could not endure the impure contact.
This property of invulnerability suggests, however, to the unpoetical mind some curiosity as to the thickness of this liquid film that forms our bubble. It certainly seems at first almost impossible to effect a measurement of so intangible a magnitude, and of a substance which cannot, from its nature, be put (as in the case of gold leaf) into the balance and weighed. It is true that there is furnished by the light which produces such splendid tints on reflection from its two surfaces, an indirect means by which the distance that separates them can be ascertained, supposing the angle of vision at which a definite colour is visible, and the refractive index of the solution be known. But this method does not readily apply to our present inquiry, and would only be fairly applicable to thicknesses producing the circles of colour produced at the upper part of the bubble, which are described above, since the same colour may be produced by various thicknesses of film. '
Again, a slight examination of the bubble shows that the colours vary considerably even in spaces only a line square, especially when it is well expanded. This circumstance renders the determination of the mean thickness of the bubble by the optical method practically useless. It is, therefore, necessary to have recourse to some other indirect means of weighing and measuring. It is true we cannot put our bubble into the balance, but still there is a device by which we can make a determination so readily that any reader can make a direct experiment on the thickness of the film of any particular bubble. The method I refer to is to blow a bubble with a gas of known specific gravity, which is less than that of air, and of such a. size that it will neither ascend or sink in the air, whose specific gravity is known. The size of the balloon being known, the weight that its contained gas will support is readily calculated, and this gives the weight of the balloon. This being known, the thickness of a. spherical shell which such a weight of liquid would form of the diameter of the balloon is readily determined by a calculation, and this is equal to the mean thickness of the balloon. From such experiments the writer has found that bubbles have a thickness varying from n-.too to -rr.ho of an inch*, the latter number representing that of a thin bubble. Wonderfully thin as is their dimension, it is greatly exceeded by that of gold leaf, which has been obtained of a thickness not exceeding one two-millionth of an inch, while gold leaf itself is exceeded slightly by the thinness of the film of the black spot I have described as being formed on the upper pole of an undisturbed bubble, which has probably a thickness even less than three eight- millionths of an inch. When we consider that this film yet contains only about a fiftieth of its weight of soap, a substance of highly complex atomic constitution, we obtain some notion of the almost infinite divisibility of matter.
Thin as is the film that forms our bubbles, yet its equality of tension is such, that by dexterous management they may be blown to a. giant size. For this purpose the tobacco-pipe must be discarded, as it neither gives a large enough film to commence with nor supplies air with sufficient rapidity. A small glass funnel, of about lf in. in diameter, cemented by some marine glue, with its mouth downwards, into a glass tube connected with a. double-action bellows, must be employed. The connection between the tube and bellows should be made with a caoutchouc tube, and supplied with a stop-cock or clamp for regulating the supply of air. A small flat basin, containing some of the bubble solution, should be employed to form a film across the mouth of the funnel, which is then expanded by careful supplies of air from the bellows. When the bubble has attained a considerable size it can be supplied with more material by carefully moistening the edges of the funnel with a small brush dipped in the solution. By these means the size of the bubbles can be increased to a wonderful extent (on one occasion &·diameter of 1 ft. 7 in. was attained), and an object produced, of whose splendour and no description can give an adequate idea. Its great globular surface has spaces of large extent blazing with one particular tint, to which the colour of another part vies in contrast, and the whole pro- duces an effect which is well worth some pains to procure. This experiment should be performed in as still an atmosphere as possible, since the larger the surface of the bubble, or rather globe, the more sensitive it becomes to injury from currents.
There are so many other interesting experiments t.o be obtained from our toy, that it would be easy to fill many more pages with their description and discussion; but I will now only describe one more, which is the most fascinating of all. This consists in harnessing the bubble, and thus making it au actual balloon. This is a matter so easy of performance, that the only wonder is that it has not been done long ago. The following is the method :-
Take some foreign post paper, not too highly glazed, and cut out a circle of the size then by means of sealing-wax attach a piece of fine thread to the centre, using as little wax as is consistent with a secure attachment. Allow the paper disc, thus prepared, to steep for some time in the bubble solution, until thoroughly moistened, and it is then fit to attach to the bubble. For this purpose, blow a small bubble with the tobacco pipe, and bringing the surface of the moistened paper disc gently against the bubble by means of the thread, in the manner of a boy's sucker, it will immediately adhere, without at all affecting the health of the bubble, which can be blown to its usual size, and gently detached from the pipe. It will then remain attached to the thread, and can be swung about at pleasure, When this process has been managed with care, there is no difficulty in blowing a bubble by means of a coal supply (using a caoutchouc tube for connection), and attaching the disc as before, detaching it from the pipe as soon as it has attained a considerable ascending power.
Under these circumstances it can be carried about at will, floating in the air as high as the thread will allow, forming the most delightful plaything that can be imagined. It is very amusing to balance it by means of a considerable length of thread, preserving a slight preponderance of the latter, so that it slowly sinks. It may then be thrown towards the floor, to which it descends until the excess of string rests upon the ground, when the balloon remains gracefully suspended in the air, waving about at each breath, without at all appearing to suffer from its shackled condition. The unusual appearance of such a balloon has a singular effect, which is increased if a fibre of unspun silk be employed in the place of the visible thread. In the same manner a car of silver foil can be attached to the paper disc, and a miniature balloon ascent performed. A balloon suspended in the above manner is a very sensitive indicator of air currents. On this property a most interesting experiment can be performed by placing in the middle of the room a burning lamp, and bringing a balloon, nearly counter- poised with excess of thread, within its influence, when it will be slowly drawn towards the lamp, and will then immediately ascend with the current of convection, as though it would be dashed against the ceiling J but not so, it will be carried round by the current, after an apparently hair-breadth escape, and will descend in the stream of colder air, only to be again dragged towards the lamp to perform the same round.
These are only a. few of the illustrations of natural laws that may be obtained by means of our plaything ; but the above descriptions will enable the reader, if he care to repeat them in a. practical form, to devise many more, and to share in the pleasure and instruction these experiments have afforded the writer.*
The solutions whose preparation is described above may also be obtained of Mr. Ladd, optician, Beak Street.