Popular Science Monthly/Volume 51/September 1897/The Forces in an Air Bubble

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THE FORCES IN AN AIR BUBBLE.
By M. G. VAN DER MENSBRUGGHE.[1]

IN 1880 I had the honor of lecturing before the Class of Science on the metamorphoses undergone by a drop of water, when I described the several phases of the grand cycle which the drop passes through from the moment it forms part of the great ocean mass till the time when after long journeys and numerous transformations it again joins its companions in the sea. A few months ago I in a similar way told the history of a grain of dust, dwelling especially on the universality and abundance of solid particles in the atmosphere, pervading everywhere on the surface of the earth.

I now proceed to describe the career of another minute body hundreds of times lighter than a drop of water or a solid corpuscle, confining myself to the consideration of its relations with liquids and solids, and we shall find that it in no wise falls behind its rivals in activity and prowess. This marvelous little being is a simple particle of air.

Although this particle and its companions wholly escape our vision, they are diffused everywhere around us, and even penetrate our organism to such an extent that without a multitude of them playing a definite part within our body we could not breathe or live for an instant. We can not isolate these particles of air, and could not see them if we did, but we can isolate masses of them by various methods and distinguish them very clearly. Thus let us take a watch glass and a capsule of water, and turning the concavity of the watch glass down, incline it slightly and plunge it into the liquid. Immediately we see a bright line that appears to define the limit of the moistened part of the concave surface of the watch glass. The rest of this face of the glass is kept from being wetted like the whole of the convex surface by the intervention of a mass of particles of air, which, somewhat compressed during the immersion, group themselves into a gaseous globule. Before it was isolated by our maneuver the globule had constituted part of one of the thousands of concentric layers of our atmosphere, each of which weighing upon the one beneath it and communicating to it besides the weight of the layers above it, they all together determine a total even pressure at the level of the sea of fifteen pounds to the square inch. Our globule of air is likewise subject to this pressure, which is transmitted through the water, and added to it is the weight of the liquid that lies above it.

This globule really betrays its presence only by the bright liquid layer around it. When we inquire for the force by which the regular shape of the globule is controlled, we find it, according to the researches of Plateau, in a thin liquid portion surrounding the volume of air, which is not more than one twenty thousandth of a millimetre thick, and which is endowed with a contractile force always impelling it to occupy the least possible space on the body it covers; and by virtue of its curvature it exercises upon the air imprisoned by the liquid a pressure greater in proportion as the globule is smaller. If these dimensions are extremely small, the gaseous globules are always spherical—as, for instance, the bubbles of carbonic acid that rise through a frothy liquor.

Our globule of air imprisoned in the watch glass, acted upon by the pressure of the atmosphere and by that of the liquid above it, and further by the capillary pressure of the bright film encompassing it, possesses, to resist these three pressures, an elastic or repulsive force which is more marked as it is more closely compressed, and by virtue of which the globule occupies a larger space the instant the external pressure is removed or diminished. Perfect quiet seems to rule in the film enveloping our particles, but this calm is only relative; and if we supposed the ultimate particles immensely magnified, we should find the conditions very stormy indeed. We have already mentioned the elastic force within the globule which gives it a tendency to expand. The liquid enveloping it is also subject to a law which is illustrated when wet objects dry, and when a cup of water placed on the scale of a balance is found to be losing weight from day to day. The superficial particles of water have a constant tendency to separate from the rest of the mass and go off as invisible particles of vapor so light that they rise in the ambient atmosphere. This passage from the liquid to the vaporous condition goes on gradually, so that the distance between the molecules becomes greater the nearer we approach the free surface. While this takes place in the radial direction, the movement gives rise in the tangential direction to contractile forces that act to give the liquid surface the smallest possible extent.

We may now suppose ourselves witnessing a struggle between rival particles, some of which are continually trying to escape into the globule of air, while others—our gaseous particles—are all the time striving to penetrate into the water. The spherules escaping into the air have at the same time an extremely pronounced tendency to resolve themselves into molecules incomparably more tenuous still, and to produce vapor even lighter than the air. As water is a medium of perfect mobility, each detached spherule gives rise to vibratory movements, and these are communicated to the whole liquid mass. If we turn our attention to the particles of air, we find them making incessant efforts to lodge themselves in the open parts of the line of battle. As soon as one of them has penetrated between two liquid molecules in vibration, these, obedient to their mutual attraction, make it advance still further; and so on till it reaches the midst of the mass. Thus many particles of air one after another penetrate to the deepest parts of the water, where they are strongly compressed and acquire greater cohesion, while the mean cohesion of the water continues to diminish; and as the particles of vapor passing into the air finally saturate it, so no more particles of air can go into the water after it is saturated with gas.

It follows that the lower the temperature, and, consequently, the stronger the cohesion of the water, the more considerable may be the quantity of dissolved air; and for this reason, doubtless, the slightest variation of temperature modifies the power of water to absorb air. We can also easily comprehend that the quantity of air dissolved in water increases as the external pressure becomes greater. Numerous applications are made of this property— in the manufacture of carbonic-acid waters, for instance. The air thus incorporated in the water is easily removed by warming the liquid, when infinite numbers of little bubbles may be seen adhering to the walls of the vessel or rising through the midst of the water. But to drive out all the dissolved air, the water should be subjected to a prolonged boiling, and this causes a wonderful increase in its cohesion after it is cooled; and water which has been treated thus will not boil except at temperatures considerably higher than the normal boiling point. Every engineer knows that the water from which he generates steam in his boilers must be aerated, if he would have the machine work regularly and avoid the danger of explosion.

Seeing so much effort displayed without relaxation on the confines of the water and the air in a simple gaseous globule, it is natural to inquire into the enormous sum of work that must be effected without interruption in the surface common to the whole atmosphere and all the rivers, streams, lakes, and seas of the globe; but the most brilliant imagination is confounded in the face of so prodigious an activity.

Who, indeed, shall measure the immense quantity of invisible vapor diffused in the atmosphere? In what balance shall we calculate the weight of the fogs and the clouds suspended above our heads? Who shall weigh the long streams of ice particles floating in the upper regions of the air? Who, in particular, shall adequately estimate the services that are rendered to mankind by those legions of liquid particles that are carried up to great heights in the atmosphere, and distribute warmth and fertility everywhere?

To return to our particles of air penetrating the free surface of the water, what should we see if we fancied everything sufficiently magnified? Gaseous particles gliding one behind another in the intervals of the upper liquid layer; here, particles of the pre-eminently vivifying gas, oxygen, whose mission it is to purify the water and give breath to the inhabitants of rivers and seas; there, molecules of another gas, the mission of which, among other things, is to modify the intensity of the action of its companion; argon, the office of which we hope to find out some day; and molecules of a fourth gas, carbonic acid, which is essential to the growth of plants. But this is not all, for we are further astonished to see penetrate the water infinite numbers of animal and vegetable germs only awaiting favorable conditions to grow and develop with wonderful rapidity. We all know that if water previously boiled be exposed to the light in an open vessel there will form on the sides of the vessel in the course of a week spots in which a powerful microscope will reveal the presence of millions of minute plants associated with legions of animalcules. The results of numerous and delicate observations show also that germs of plants and animals exist as universally in the air as in water; and when favorable conditions of light and temperature come, these germs at once grow, multiply, and become visible under the microscope.

Approaching the relations of our air particles with solids, we meet the question of what these minute bodies can have in common with compact masses of invariable form, incomparably denser than they, and all the particles of which seem to be too dense to permit the access of our gaseous particles. This, the hitherto prevalent idea of the structure of solid bodies, does not conform to the real condition; for, just as the superficial parts of liquids tend to diffusion in the ambient air, a like habit exists in the molecules of solids of being repelled from the interior toward the exterior, and they separate from one another, but only in an extremely thin exterior layer. Thus camphor, iodine, ice, and some other substances change into vapor at ordinary temperatures; and the exhalation of perfumes may be something of the sort.

Many other facts point to an exceptional constitution of the free surface of solid bodies, of which I need cite only the experiments of M. De Marçay on the vaporization of metals in vacuum at temperatures below their melting points, and especially the researches of M. Spring on the direct uniting of metals, either of the same or of different species. We conclude from all these evidences that there exists on the surface of solid bodies an extremely thin layer, the density of which diminishes the more nearly we approach the free surface. Let us assume, consequently, such a special constitution for the superficial layer of solids, and, by a new effort of our imagination, witness the unrelaxing work of our particles of air in the immediate vicinity of some solid body; we might thus see them dashing into the invisible intervals between the extreme molecules of the solid and opening passages for themselves through innumerable spaces, whence there results a whole formed of solid particles and more or less condensed aggregations of certain gases. Possibly this is the way in which has been developed that texture, doubtless very fine but still very resisting, which covers all solid bodies and is also very difficult to take away from them.

You ask, Of what interest to us is this incessant activity of the air? We answer that it has an interest of the very highest importance; for without this protecting layer covering solids, every object brought in contact with another would risk adhering to it so closely that they could not be separated without a great effort. It is this invisible layer that permits the workman to use his tools handily, the reader to turn the leaves of his book with ease, the writer to guide his pen at will, and the pedestrian to raise his feet from the ground; in fact, I should never get to an end if I should have to recall the principal examples of the utility of this microscopic cushion of air on the surface of solid bodies.

Long and patient observations by Moser and Waidele have made it extremely probable that every substance has its special gaseous envelope, which depends on the condition of the free surface, the temperature, the pressure, the vapors diffused in the surrounding space, etc. This is so true that it is enough to pass the finger over a plate of glass or metal to modify the minute molecular aggregate covering the surface. We can prove this by tracing, with the finger or any kind of rod or stick, invisible characters on the plate and breathing upon it, when all the tracings will immediately come out on it; for this reason, beyond a doubt, that the vapor of the breath deposits itself in different manners on the surface that has not been touched and on the parts followed by the tracings. Further, if we allow two metallic plates to remain for a considerable time slightly removed from one another, one of which is highly polished, and the other bears engraved characters such as may be found on a presentation watch, on separating them, say after two months, simply breathing on the surface of the smooth plate will cause the characters engraved upon the other to appear revealed. The cause of this appearance is, that the hollowed parts of one of the plates condense more air and moisture, and thus, by frequent changes of temperature and pressure, the parts of the smooth surface opposite the cavities are covered with a gaseous envelope different from that of the parts adjoining, and the difference is marked by a special condensation of the vapor of the breath.

Legions of grains of dust are known to be floating in the atmosphere, not near the ground alone, but miles above the sea level. We may form an estimate of the prodigious number of these solid particles suspended in the air by collecting snow during the earlier moments of a fall; the water resulting from the melting of it is nearly black with the corpuscles of every kind which the little ice crystals have brought down in the cavities of the snow. Later collections of snowflakes give clearer and clearer water. The snow has therefore been called the "broom of the atmosphere." The particles can not be held up in the atmosphere of themselves; for, taken one at a time and thoroughly dried, they will certainly weigh more than the air they displace. To learn the real cause of the phenomenon, we must recollect that the constitution of a solid particle is that of a minute kernel surrounded by a very thin layer of gradually decreasing density, into which the surrounding air infiltrates itself so as to make a kind of sponge; hence, the smaller the kernel the more notable the influence of the lighter sponge. Another perhaps more important cause may be found in the power of the cavities of a grain of dust to attract moisture from the air, by virtue of which an atmosphere of invisible vapor is gathered around the corpuscle so as to form a single system with it. The density of vapor being only about two thirds that of air at the same pressure, this vaporous envelope has great sustaining power, and its presence furnishes an adequate explanation of the suspension of so much solid matter.

Although the presence of these millions of particles may diminish the transparency of the atmosphere, they contribute to the illuminating power of the sun by reflecting its rays in every direction and causing all the space to be pervaded with light, and, intercepting the rays of heat as they pass from the earth, they prevent loss by too rapid radiation. A similar explanation accounts for the suspension of globules of water in clouds.

We come now to the powers of our particle of air to emit sounds, which are always curious and often imposing: manifested in the snap of the coachman's whiplash, when the particles suddenly thrown out of equilibrium execute sonorous vibrations in recovering it; in the resonance of artillery discharges, the roaring of the tempest, the moaning of the surf, and the rolling of the thunder—all reactions of air against forces which have compressed it.

When we bear in mind the power displayed by particles of air hurled in violent wind against a fixed obstacle, we are led to ask how these particles exhibit their energy when the air is traversed by a projectile—spherical it may be—moving with great velocity? Since the air, in spite of its extreme mobility, opposes a degree of resistance to any sudden displacement, the vacuum created behind the projectile is not instantly filled; and not all the particles in front of it being able to get out of the way as fast as it goes along, an accumulation takes place there which exerts considerable pressure against it. The situation then becomes the same as if there was a spring in front of the ball strong enough to nullify every instant a part of its velocity and to deform a solid obstacle placed in its course. Melsen was struck by this thought, and instituted a series of experiments that gained him great credit, the result of which was to prove that the air accumulated in front of a ball flying with sufficient velocity forms a gaseous layer capable of opposing the immediate contact of the projectile with a resisting medium, particularly at the point squarely opposed to the course of the missile. This view was very clearly confirmed by Prof. E. Mach, of the University of Prague, who obtained a photographic image of a projectile moving with great velocity and preceded by condensed gaseous waves. We can not, therefore, doubt that the cushion of greatly compressed air in front of the projectile causes considerable delay in its progress, and consequently a great beating of the ball. We know that this takes place with aëroliths which become incandescent and burst in flying through our atmosphere.

Melsen's experiments led me to suppose (in 1874) that the obstruction and heating of a projectile passing through the air might be notably diminished by driving a narrow and slightly conical channel through the ball and slipping into it a metallic obturator to fit it. "In this way," I said, in my lectures on thermodynamics, "the ball might be discharged without letting more gas escape than usual; once out of the chamber, it would condense the air in front of it, while the air behind it would be extremely rarefied. A difference of pressure would immediately be produced sufficient to force the conical tampon out of the projectile, and after that there would be no more projectile-air pressure." Under these conditions, I said, the velocity of projectiles could be kept up for much greater distances, and the heating would be considerably less. I was not so situated that I could verify these views by experiment; but the principle was applied about two years ago in Germany, in the Hebler Kruka ball, the axis of which is pierced with a small cylindrical channel, enlarged behind so as to be funnel-shaped, and closed with a small plug—the very device I had imagined twenty years before—which, when fired from a cannon, behaved just as I supposed my perforated ball would do.—Translated for the Popular Science Monthly from Ciel et Terre.

 

  1. Address before a public meeting of the Belgian Academy of Sciences, December 14, 1895.