LIQUID DROPS AND GLOBULES

LECTURE I

Introduction.—In choosing a subject for a scientific discourse, it would be difficult to find anything more familiar than a drop of liquid. It might even appear, at first sight, that such a subject in itself would be quite inadequate to furnish sufficient material for extended observation. We shall find, however, that the closer study of a drop of liquid brings into view many interesting phenomena, and provides problems of great profundity. A drop of liquid is one of the commonest things in nature; yet it is one of the most wonderful.

Apart from the liquids associated with animal or vegetable life, water and petroleum are the only two which are found in abundance on the earth; and it is highly probable that petroleum has been derived from the remains of vegetable life. Many liquids are fabricated by living organisms, such as turpentine, alcohol, olive oil, castor oil, and all the numerous vegetable oils with which we are all familiar. But in addition to these, there are many liquids produced in the laboratory of the chemist, many of which are of great importance; for example, nitric acid, sulphuric acid, and aniline. The progress of chemical science [pg 2] has greatly enlarged the number of liquids available, and in our experiments we shall frequently utilize these products of the chemist's skill, for they often possess properties not usually associated with the commoner liquids.

General Properties of Liquids.—No scientific study can be pursued to advantage unless the underlying principles be understood; and hence it will be necessary, in the beginning, to refer to certain properties possessed by all liquids, whatever their origin. The most prominent characteristic of a liquid is mobility, or freedom of movement of its parts. It is owing to this property that a liquid, when placed in a vessel, flows in all directions until it reaches the sides; and it is this same freedom of movement which enables water, gathering on the hills, to flow under the pull of gravitation into the lowlands, and finally to the sea. If we drop a small quantity of a strongly-coloured fluid—such as ink—into a large volume of water, and stir the mixture for a short time, the colour is evenly distributed throughout the whole mass of water, because the freedom of movement of the particles enables the different portions to intermingle readily. This property of mobility distinguishes a liquid from a solid; for a solid maintains its own shape, and its separate parts cannot be made to mix freely. Mobility, however, is not possessed in equal degree by all liquids. Petrol, for example, flows more freely than water, which in turn is more mobile than glycerine or treacle. Sometimes a substance exhibits properties intermediate between those of a solid and a liquid, as, for instance, butter in hot weather. We shall not be concerned, [pg 3] however, with these border-line substances, but shall confine our attention to well-defined liquids.

There is another feature, however, common to all liquids, which has a most important bearing on our subject. Every liquid is capable of forming a boundary surface of its own; and this surface has the properties of a stretched, elastic membrane. Herein a liquid differs from a gas or vapour, either of which always completely fills the containing vessel. You cannot have a bottle half full of a vapour or gas only; if one-half of that already present be withdrawn, the remaining half immediately expands and distributes itself evenly throughout the bottle, which is thus always filled. But a liquid may be poured to any height in a vessel, because it forms its own boundary at the top. Let us now take a dish containing the commonest of all liquids, and in many ways the most remarkable—water—and examine some of the properties of the upper surface.

Properties of the Surface Skin of Water.—Here is a flat piece of thin sheet silver, which, volume for volume, is 10½ times as heavy as water, in which it might therefore be expected to sink if placed upon the surface. I lower it gently, by means of a piece of cotton, until it just reaches the top, and then let go the cotton. Instead of sinking, the piece of silver floats on the surface; and moreover, a certain amount of pressure may be applied to it without causing it to fall to the bottom of the water. By alternately applying and relaxing the pressure we are able, within small limits, to make the sheet of silver bob up and down as if it were a piece of cork. If we look closely, we [pg 4] notice that the water beneath the silver is at a lower level than the rest of the surface, the dimple thus formed being visible at the edge of the floating sheet ([Fig. 1]). If now I apply a greater pressure, the piece of silver breaks through the surface and sinks rapidly to the bottom of the vessel. Or, if instead I place a thick piece of silver, such as a shilling, on the surface of the water, we find that this will not float, but sinks immediately. All these results are in agreement with the supposition that the surface layer of water possesses the properties of a very thin elastic sheet. If we could obtain an extremely fine sheet of stretched rubber, which would merely form a depression under the weight of the thin piece of silver, but would break under the application of a further pressure or the weight of a heavier sheet, the condition of the water surface would then be realized. We may note in passing that a sheet of metal resting on the surface of water is a phenomenon quite distinct from the floating of an iron ship, or hollow metal vessel, which sinks until it has displaced an amount of water equal in weight to itself.

Fig. 1.—Silver sheet floating on water.

We can now understand why a water-beetle is able to run across the surface of a pond, without wetting its legs or running any risk of sinking. Each of its [pg 5] legs produces a dimple in the surface, but the pressure on any one leg is not sufficient to break through the skin. We can imitate this by bringing the point of a lead pencil gently to the surface of water, when a dimple is produced, but the skin is not actually penetrated. On removing the pencil, the dimple immediately disappears, just as the depression caused by pushing the finger into a stretched sheet of indiarubber becomes straight immediately the finger is removed.

Elastic Skin of other Liquids—Minimum Thermometer.—The possession of an elastic skin at the surface is not confined to water, but is common to all liquids. The strength of the skin varies with different liquids, most of which are inferior to water in this respect. The surface of petroleum, for example, is ruptured by a weight which a water surface can readily sustain. But wherever we have a free liquid surface, we shall always find this elastic layer at the boundary, and I will now show, by the aid of lantern projection, an example in which the presence of this layer is utilized. On the screen is shown the stem of a minimum thermometer—that is, a thermometer intended to indicate the lowest temperature reached during a given period. The liquid used in this instrument is alcohol, and you will observe that the termination of the column is curved ([Fig. 2]). In contact with the end of the column is a thin piece of coloured glass, with rounded ends, which fits loosely in the stem, and serves as an index. When I warm the bulb of the thermometer, you notice that the end of the column moves forward, but the index, round [pg 6] which the alcohol can flow freely, does not change its position. On inclining the stem, the index slides to the end of the column, but its rounded end does not penetrate the elastic skin at the surface. I now pour cold water over the bulb, which causes the alcohol to contract, and consequently the end of the column moves towards the bulb. In doing so, it encounters the opposition of the index, which endeavours to penetrate the surface; but we see that the elastic skin, although somewhat flattened, is not pierced, but is strong enough to push the index in front of it. And so the index is carried towards the bulb, and its position indicates the lowest point attained by the end of the column—that is, the minimum temperature. Obviously, a thermometer of this kind must be mounted horizontally, to prevent the index falling by its own weight.

Fig. 2.—Column and index of minimum thermometer.

Boundary Surface of two Liquids.—So far we have been considering surfaces bounded by air, or—in the case of the alcohol thermometer—by vapour. It is possible, however, for the surface of one liquid to be bounded by a second liquid, provided the two do not mix. We may, for example, pour petroleum on to water, when the top of the water will be in contact with the floating oil. If now we lower our piece of [pg 7] silver foil through the petroleum, and allow it to reach the surface of the water, we find that the elastic skin is still capable of sustaining the weight; and thus we see that the elastic layer is present at the junction of the two liquids. What is true of water and oil in this respect also holds good for the boundary or interface of any two liquids which do not mix. Evidently, if the two liquids intermingled, there would be no definite boundary between them; and this would be the case with water and alcohol, for example.

Area of Stretched Surface.—We will not at present discuss the nature of the forces which give rise to this remarkable property of a liquid surface, but will consider one of the effects. The tendency, as in the case of all stretched membranes, will be to reduce the area of the surface to a minimum. If we take a disc of stretched indiarubber and place a weight upon it, we cause a depression which increases the area of the surface. But on removing the weight, the disc immediately flattens out, and the surface is restored to its original smallest dimension. Now, in practice, the surface of a liquid is frequently prevented from attaining the smallest possible area, owing to the contrary action of superior forces; but the tendency is always manifest, and when the opposing forces are absent or balanced the surface always possesses the minimum size. A simple experiment will serve to illustrate this point. I dip a glass rod into treacle or “golden syrup,” and withdraw it with a small quantity of the syrup adhering to the end. I then hold the rod with the smeared end downwards, and the syrup falls from it slowly in the form of a long, tapered column. When [pg 8] the column has become very thin, however, owing to the diminished supply of syrup from the rod, we notice that it breaks across, and the upper portion then shrinks upwards and remains attached to the rod in the form of a small drop ([Fig. 3]). So long as the column was thick, the tendency of the surface layer to reduce its area to the smallest dimensions was overpowered by gravity; but when the column became thin, and consequently less in weight, the elastic force of the outer surface was strong enough to overcome gravitation, and the column was therefore lifted, its area of surface growing less and less as it rose, until the smallest area possible under the conditions was attained.

Fig. 3.—Thread of golden syrup rising and forming a drop.

Shape of Detached Masses of Liquid.—Let us now pay a little attention to the small drop of syrup which remains hanging from the rod. It is in contact with the glass at the top part only, and the lower portion is only prevented from falling by the elastic skin around it, which sustains the weight. We may compare it to a bladder full of liquid, in which case also the weight is borne by the containing skin. Now suppose we could separate the drop of syrup entirely [pg 9] from the rod; what shape would it take? We know that its surface, if not prevented by outside forces from doing so, would become of minimum area. Assuming such extraneous forces to be absent or counterbalanced, what would then be the shape of the drop? It would be an exact sphere. For a sphere has a less surface-area in proportion to its volume than any other shape; and hence a free drop of liquid, if its outline were determined solely by its elastic skin, would be spherical. A numerical example will serve to illustrate this property of a sphere. Supposing we construct three closed vessels, each to contain 1 cubic foot, the first being a cube, the second a cylinder of length equal to its diameter, and the third a sphere. The areas of the surfaces would then be:—

Cube . . . . 6 square feet.
Cylinder . . . . 5·86 ,, ,,
Sphere . . . . 4·9 ,, ,,

And whatever shape we make the vessel, it will always be found that the spherical form possesses the least surface.

Fig. 4.—Drops of different sizes resting on flat plate.

Now let us examine some of the shapes which drops actually assume. I take a glass plate covered with a thin layer of grease, which prevents adhesion of water to the glass, and form upon it drops of water of various sizes by the aid of a pipette. You see them projected on the screen ([Fig. 4]). The larger drops are flattened above and below, but possess rounded sides and resemble a teacake in shape. Those of intermediate size are more globular, but still show signs of [pg 10] flattening; whilst the very small ones, so far as the eye can judge, are spherical. Evidently, the shape depends upon the size; and this calls for some explanation. If we take a balloon of indiarubber filled with water, and rest it on a table, the weight of the enclosed water will naturally tend to stretch the balloon sideways, and so to flatten it. A smaller balloon, made of rubber of the same strength, will not be stretched so much, as the weight of the enclosed water would be less; and if the balloon were very small, but still had walls of the same strength, the weight of the enclosed water would be incompetent to produce any visible distortion. It is evident, however, that so long as it is under the influence of gravitation, even the smallest drop cannot be truly spherical, but will be slightly flattened. The tendency of drops to become spherical, however, is always present.

Fig. 5.—Formation of a sphere of orthotoluidine.

Production of True Spheres of Liquids.—Now it is quite possible to produce true spheres of liquid, even of large size, if we cancel the effect of gravity; and we may obtain a hint as to how this may be accomplished by considering the case of a soap-bubble, which, when floating in air, is spherical in shape. Such a bubble is merely a skin of liquid enclosing air; but being surrounded by air of the same density, there is no tendency for the bubble to distort, nor would it [pg 11] fall to the ground were it not for the weight of the extremely thin skin. The downward pull of gravity on the air inside the bubble is balanced by the buoyancy of the outside air; and hence the skin, unhampered by any extraneous force, assumes and retains the spherical form. And similarly, if we can arrange to surround a drop of liquid by a medium of the same density, it will in turn become a sphere. Evidently the medium used must not mix with the liquid composing the drop, as it would then be impossible to establish a boundary surface between the two. Plateau, many years ago, produced liquid spheres in this manner. He prepared a mixture of alcohol and water exactly equal in density to olive oil, and discharged the oil into the mixture, the buoyancy of which exactly counteracted the effect of gravity on the oil, and hence spheres were formed. The preparation of an alcohol-water mixture of exactly correct density is a tedious process, and we are now able to dispense with it and form true spheres in a more convenient way. There is a liquid known as orthotoluidine, which possesses a beautiful red colour, does not mix with water, and which has exactly the same density as water when the temperature of both is 75° F. or 24° C. At this temperature, therefore, if orthotoluidine be run into water, spheres should be formed; and there is no reason why we should not be able to make one as large as a cricket-ball, or even larger. I take a flat-sided vessel for this experiment, in order that the appearance of the drop will not be distorted as it would be in a beaker, and pour into it water at 75° F. until it is about two-thirds full. I now take a pipette containing [pg 12] a 3 per cent. solution of common salt, and discharge it at the bottom of the water. Being heavier, the salt solution will remain below the water, and will serve as a resting-place for the drop. The orthotoluidine is contained in a vessel provided with a tap and wide stem, which is now inserted in the water so that the end of the stem is about 1 inch above the top of the salty layer. I now open the tap so as to allow the orthotoluidine to flow out gradually; and we then see the ball of liquid growing at the end of the stem [pg 13] ([Fig. 5]). By using a graduated vessel, we can read off the quantity of orthotoluidine which runs out, and thus measure the volume of the sphere formed. When the lower part reaches the layer of salt solution, we raise the delivery tube gently, and repeat this as needed during the growth of the sphere. We have now run out 100 cubic centimetres, or about one-sixth of a pint, and our sphere consequently has a diameter of 5¾ centimetres, or 2¼ inches. To set it free in the water we lift the delivery tube rapidly—and there is the [pg 14] sphere floating in the water ([Fig. 6]). We could have made it as much larger as we pleased, but the present sphere will serve all our requirements.

Fig. 6.—The detached sphere floating under water.

Fig. 7.—The Centrifugoscope.

The Centrifugoscope.—I have here a toy, which we may suitably call the centrifugoscope, which shows in a simple way the formation of spheres of liquid in a medium of practically equal density. It consists of a large glass bulb attached to a stem, about three-quarters full of water, the remaining quarter being occupied by orthotoluidine. This liquid, being slightly denser than water at the temperature of the room, rests on the bottom of the bulb. When I hold the stem horizontally, and rotate it—suddenly at first, and steadily afterwards—a number of fragments are detached from the orthotoluidine, which immediately become spherical, and rotate near the outer side of the bulb. The main mass of the red liquid rises to the centre of the bulb, and rotates on its axis ([Fig. 7]), and we thus get an imitation of the solar system, with the planets of various sizes revolving round the central [pg 15] mass; and even the asteroids are represented by the numerous tiny spheres which are always torn off from the main body of liquid along with the larger ones. When the rotation ceases, the detached spheres sink, and after a short time join the parent mass of orthotoluidine. We can therefore take this simple apparatus at any time, and use it to show that a mass of liquid, possessing a free surface all round, and unaffected by gravity, automatically becomes a sphere. After all, this is only what we should expect of an elastic skin filled with a free-flowing medium.

Effect of Temperature on Sphere of Orthotoluidine.—I will now return to the large sphere formed under water in the flat-sided vessel, and direct your attention to an experiment which teaches an important lesson. By placing a little ice on the top of the water, we are enabled to cool the contents of the vessel, and we soon notice that the red-coloured sphere becomes flattened on the top and below, and sinks a short distance into the saline layer. Evidently the cooling action, which has affected both liquids, has caused the orthotoluidine to become denser than water. I now surround the vessel with warm water, and allow the contents gradually to attain a temperature higher than 75° F. You observe that the flattened drop changes in shape until it is again spherical; and as the heating is continued elongates in a vertical direction, and then rises to the surface, being now less dense than water. So sensitive are these temperature effects that a difference of 1 degree on either side of 75° F. causes a perceptible departure from the spherical shape in the case of a large drop. It therefore follows that [pg 16] orthotoluidine may be either heavier or lighter than water, according to temperature, and this fact admits of a simple explanation. Orthotoluidine expands more than water on heating, and contracts more on cooling. The effect of expansion is to decrease the density, and of contraction to increase it; hence the reason why warm air rises through cold air, and vice versa. Now if orthotoluidine and water, which are equal in density at 75° F., expanded or contracted equally on heating above or cooling below this temperature, their densities would always be identical. But inasmuch as orthotoluidine increases in volume to a greater extent than water on heating, and shrinks more on cooling, it becomes lighter than water when both are hotter than 75° F., and heavier when both are colder. We call the temperature when both are equal in density the equi-density temperature. Here are some figures which show how the densities of these two liquids diverge from a common value on heating or cooling, and which establish the conclusions we have drawn:—

Temperature.

Density.

Deg. F.

Deg. C.

Water.

Orthotoluidine.

50

10

0·9997

1·009

59

15

0·9991

1·005

68

20

0·9982

1·001

Equal: 75

24

0·9973

0·997

86

30

0·9957

0·992

95

35

0·9940

0·988

104

40

0·9923

0·983

[pg 17]

Fig. 8.—Aniline drops falling through cold water and ascending through hot water.

Other Examples of Equi-Density.—There are many other liquids which, like orthotoluidine, may be heavier or lighter than water, according to temperature, and I now wish to bring to your notice the remarkable liquid aniline, which falls under this head. Aniline is an oily liquid, which, unless specially purified, has a deep red colour. It forms the basis of the beautiful and varied colouring materials known as the aniline dyes, which we owe to the skill of the chemist. The equi-density temperature of water and aniline is 147° F. or 64° C.; that is, aniline will sink in water if both be colder than 147° F., and rise to the surface if this temperature be exceeded. We may illustrate this fact by a simple but striking experiment. Here [pg 18] are two tall beakers side by side, and above them a cistern containing aniline ([Fig. 8]). The stem of the cistern communicates with the two branches of a horizontal tube, the termination of one branch being near the top of one of the beakers, whilst the other branch is prolonged to the bottom of the second beaker, and is curved upwards at the end. Both branches are provided with taps to regulate the flow of liquid, and to commence with are full of aniline. Cold water is poured into the beaker containing the shorter branch until the end is submerged; and water nearly boiling is placed in the second beaker to an equal height. I now open the taps, so that the aniline may flow gradually into each beaker; and you notice that the drops of aniline sink through the cold water and rise through the hot. We have thus the same liquid descending and ascending simultaneously in water, the only difference being that the water is cold on the one side and hot on the other. Prolonging the delivery-tube to the bottom of the beaker containing the hot water enables the rising drops to be observed throughout the length of the column of water; and in addition enables the cold aniline from the cistern to be warmed up on its way to the outlet, so that by the time it escapes its temperature is practically the same as that of the water. If this temperature exceed 147° F., the drops will rise. We might, in this experiment, have used orthotoluidine instead of aniline; or, indeed, any other liquid equal in density to water at some temperature intermediate between those of the hot and cold water—always provided that the liquid chosen did not mix with water. Amongst such other liquids may be [pg 19] mentioned anisol; butyl benzoate; and aceto-acctic ether; but none of these possess the fine colour of aniline or its chemical relative orthotoluidine, and in addition are more costly liquids. Besides these are a number of other liquids rarer still, practically only known to the chemist, which behave in the same way. These liquids are all carbon compounds, and more or less oily in character. There is a simple rule which may be used to predict whether any organic liquid will be both lighter and heavier than water, according to temperature. Here it is: If the density of the liquid at 32° F. or 0° C. be not greater than 1·12, the liquid will become less dense than water below 212° F. or 100° C., at which temperature water boils. This rule is derived from a knowledge of the extent to which the expansion of organic liquids in general exceeds that of water. I have considered it necessary to enter at some length into this subject of equi-density, as much that will follow involves a knowledge of this physical relation between liquids.

Aniline Films or Skins.—We have previously concluded, largely from circumstantial evidence, that a liquid drop is encased in a skin or what is equivalent to a skin, and I propose now to show by experiments with aniline how we can construct a drop, commencing with a skin of liquid. Here is some aniline in a vessel, covered by water. I lower into the aniline a circular frame of wire, which I then raise slowly into the overlying water; and you observe that a film of aniline remains stretched across the frame. By lifting the frame up and down in the water the skin is stretched, forming a drop which is constricted near the frame [pg 20] ([Fig. 9]). On lifting the wire more suddenly, the skin of aniline closes in completely at the narrow part, and a sphere of water, encased in an aniline skin, then falls through the water in the beaker, and comes to rest on the aniline below—into which, however, it soon merges. You were previously asked to regard a drop of liquid as being similar to a filled soap-bubble; and this experiment realizes the terms of the definition. And it requires only a little imagination to picture a drop surrounded by its own skin instead of that of another liquid. It is easy to make one of these enclosed water-drops by imitating the blowing of a soap-bubble—using, however, water instead of air. In order to do this I take a piece of glass tubing, open at both ends, [pg 21] and pass it down the vessel, until it reaches the aniline. Water, in the meantime, has entered the tube, to the same height as that at which it stands in the vessel. On raising the tube gently, a skin of aniline adheres to the end; and as we raise it still further, the water in the tube, sinking so as to remain at the level in the vessel, expands the skin into a sphere ([Fig. 9])—the equivalent of a filled soap-bubble. On withdrawing the tube gradually, the composite sphere is left hanging from the surface of the water.

Fig. 9.—Aniline skins enveloping water.

Surface Tension.—Before proceeding further, it will be advisable to introduce and explain the term “surface tension.” We frequently use it, without attaching to it any numerical value, to express the fact that the free surface of a liquid is subjected to stretching forces, or is in a state of tension; and thus we say that certain phenomena are “due to surface tension.” But the physicist does not content himself with merely observing occurrences; he tries also to measure, in definite units, the quantities involved in the phenomena. And hence surface tension is defined as the force tending to pull apart the two portions of the surface on either side of a line 1 centimetre in length. That is, we imagine a line 1 centimetre long on the surface of the liquid, dividing the surface into two portions on opposite sides of the line, and we call the force tending to pull these two portions away from each other the surface tension. Experiments show that this force, in the case of cold water, is equal to about 75 dynes, or nearly 8/100 of a gramme. If we choose a line 1 inch long on the surface of water, the surface tension is represented by about 3 1/6 grains. It is always [pg 22] necessary to specify the length when assigning a value to the surface tension; and unless otherwise stated a length of 1 centimetre is implied. The values for different liquids vary considerably; and it is also necessary to note that the figure for a given liquid depends upon the nature of the medium by which it is bounded—whether, for example, the surface is in contact with air or another liquid. The following table gives the values for several liquids when the surfaces are in contact with air:—

Liquid. Tension at 15° C. (59° F.), dynes per cm.
Water 75
Aniline 43
Olive Oil 32
Chloroform 27
Alcohol 25

When one liquid is bounded by another, the interfacial tension, as it is called, is generally less than when in contact with air. Thus the value for water and olive oil is about 21 dynes per centimetre at 15° C.

We are now in a position to speak of surface tension quantitatively, and shall frequently find it necessary to do so in order to explain matters which will come under our notice later.

Figs. 10, 11 and 12.—The Diving Drop. Three stages.

The “Diving” Drop.—In order to illustrate the tension at the boundary surface of two liquids, I now show an experiment in which a drop is forcibly projected downwards by the operation of this tension. I pour some water into a narrow glass vessel, and float [pg 23] upon it a liquid called dimethyl-aniline, so as to form a layer about 1 inch in depth. A glass tube, open at both ends, is now passed down the floating liquid into the water, and then raised gradually, with the result that a skin of water adheres to the end, and is inflated by the upper liquid, forming a sphere on the end of the tube ([Fig. 10]). On withdrawing the tube from the upper surface, the sphere is detached and falls to the boundary surface, where it rests for a few seconds, and is then suddenly shot downwards into the water ([Figs. 11 and 12]). It then rises to the interface; breaks through, and mingles with the floating liquid, thereby losing its identity. Why should the drop, which is less dense than water, dive below in [pg 24] this manner? The explanation is that the drop (which consists of a skin of water filled with dimethyl-aniline), after resting for a time on the joining surface, loses the under part of its skin, which merges into the water below. The shape of the boundary of the two liquids is thereby altered, the sides now being continuous with the skin forming the upper part of the drop. This is an unstable shape; and accordingly the boundary surface flattens to its normal condition, and with such force as to cause the drop beneath it to dive into the water, although the liquid is lighter than water and tends to float. The result is the same as that which would occur if a marble were pressed on to a stretched disc of rubber, and then released, when it would be projected upwards owing to the straightening of the disc. I now repeat the experiment, using paraffin oil instead of dimethyl-aniline; but in this case the drop is only projected to a small depth, and the effect is not so marked. The experiment furnishes conclusive evidence of the existence of the interfacial tension.

Formation of Falling Drops of Liquid.—We will now direct our attention to one of the most beautiful of natural phenomena—the growth and partition of a drop of liquid. Let us observe, by the aid of the lantern, this process in the case of water, falling in drops from the end of a glass tube. The flow of water is controlled by a tap, and you observe that the drop on the end gradually grows in size, then becomes narrower near the end of the tube, and breaks across at this narrow part, the separated drop falling to the ground. Another drop then grows and breaks away; [pg 25] but the process is so rapid that the details cannot be observed. None of you saw, for example, that each large drop after severance was followed by a small droplet, formed from the narrowed portion from which the main drop parted. But the small, secondary drop is always present, and is called, in honour of its discoverer, Plateau's spherule. Nor did any of you observe that the large drop, immediately after separation, became flattened at the top, nor were you able to notice the changing shape of the narrow portion. To show all these things it will be necessary to modify the experimental conditions.

Mr. H. G. Wells, in one of his short stories, describes the wonderful effects of a dose of a peculiarly potent drug, called by him the “Accelerator.” While its influence lasted, all the perceptions were speeded up to a remarkable degree, so that occurrences which normally appeared to be rapid seemed absurdly slow. A cyclist, for example, although travelling at his best pace, scarcely appeared to be making any movement; and a falling body looked as if it were stationary. Now if we could come into possession of some of this marvellous compound, and take the prescribed quantity, we should then be able to examine all that happens when a drop forms and falls at our leisure. But it is not necessary to resort to such means as this to render the process visible to the eye. We could, for example, take a number of photographs succeeding each other by very minute intervals of time—a kind of moving picture—from which the details might be gleaned by examining the individual photographs. This procedure, however, would be troublesome; and evidently [pg 26] the simplest plan, if it could be accomplished, would be to draw out the time taken by a drop in forming and falling. And our previous experiments indicate how this may be done, as we shall see when we have considered the forces at work on the escaping liquid.

A liquid issuing from a tube is pulled downwards by the force of gravitation, and therefore is always tending to fall. At first, when the drop is small, the action of gravity is overcome by the surface tension of the liquid; but as the drop grows in size and increases in weight, a point arrives at which the surface tension is overpowered. Then commences the formation of a neck, which grows narrower under the stretching force exerted by the weight of the drop, until rupture takes place. Now if we wish to make the process more gradual, it will be necessary to reduce the effect of gravity, as we cannot increase the surface tension. We have already seen how this may be done in connexion with liquid spheres—indeed, we were able to cancel the influence of gravity entirely, by surrounding the working liquid by a second liquid of exactly equal density. We require now, however, to allow the downward pull of the drop ultimately to overcome the surface tension, and we must therefore form the drop in a less dense liquid. If this surrounding liquid be only slightly less dense, we should be able to produce a very large drop; and if we make its growth slow we may observe the whole process of formation and separation with the unaided eye.

Fig. 13.—Apparatus for forming ascending or descending drops of liquids.

Now it so happens that we have to hand two liquids which, without any preparation, fulfil our requirements. Orthotoluidine, at temperatures below 75° F. [pg 27] or 24° C., is denser than water of equal temperature. At 75° F. their densities are identical; and as the ordinary temperature of a room lies between 60° and 70° F., water, under the prevailing conditions, will be slightly the less dense of the two, and will therefore form a suitable medium in which to form a large drop of orthotoluidine. I therefore run this red-coloured liquid into water from a funnel controlled by a tap ([Fig. 13]), and in order to make a large drop the end of the stem is widened to a diameter of 1½ inches. It is best, when starting, to place the end of the stem [pg 28] in contact with the surface of the water, as the first quantity of orthotoluidine which runs down then spreads over the surface and attaches itself to the rim of the widened end of the stem. The tap is regulated so that the liquid flows out slowly, and we may now watch the formation of the drop. At first it is nearly hemispherical in shape; gradually, as you see, it becomes more elongated; now the part near the top commences to narrow, forming a neck, which, under the growing weight of the lower portion, is stretched until it breaks, setting the large drop free ([Figs. 14 to 18]). And then follows the droplet; very small by comparison with the big drop, but plainly visible ([Figs. 19 and 20]). The graceful outline of the drop at all stages of the formation must appeal to all who possess an eye for beauty in form; free-flowing curves that no artist could surpass, changing continuously until the process is complete.

Slow as was the formation of this drop, it was still too rapid to enable you to trace the origin of the droplet. It came, as it always does come, from the drawn-out neck. When the large drop is severed, the mass of liquid clinging to the delivery-tube shrinks upwards, as the downward pull upon it is now relieved. The result of this shrinkage—which, as usual, reduces the area of surface to the minimum possible—is to cut off the elongated neck, at its upper part, thus leaving free a spindle-shaped column of liquid. This column immediately contracts, owing to its surface tension, until its surface is a minimum—that is, it becomes practically a sphere; and this constitutes the droplet. In a later experiment, in which the formation is slower still, and [pg 29] the liquid more viscous, the origin of the droplet will be plainly seen, and the correctness of the description verified. The recoil due to the liberation of the stretching force after rupture of the neck was visible on the top of the large drop, and also on the bottom of the portion of liquid which remained attached to the tube, both of which were momentarily flattened ([Figs. 19 and 20]) before assuming their final rounded shape. This is exactly what we should expect to happen if a filled skin of indiarubber were stretched until it gave way at the narrowest part.

Fig. 14.

As a variation on the two liquids just used, I now take the yellow liquid nitrobenzene, and run it into nitric acid (or other suitable medium) of specific gravity 1·2, and you observe the same sequence of events as in the previous experiment, even to the details. Very rapid photography shows that the breaking away of a drop of water from the end of a tube in air is in all [pg!30] [pg 31] respects identical with what we have just seen on a large scale.

Figs. 14 to 20.—Formation of a drop of orthotoluidine, showing the droplet. Seven stages.

Ascending or Inverted Drops.—If we discharge orthotoluidine into water when both are hotter than 75° F., the former liquid will rise, as its density is now less than that of water. If, therefore, I take a funnel with the stem bent into a parallel branch, so as to discharge upwards (A, [Fig. 13]) and raise the temperature of both liquids above 75° F., we see that the drop gradually grows towards the top of the water, finally breaking away and giving rise to the droplet. Everything, [pg 32] in fact, was the same as in the case of a falling drop, except that the direction was reversed. A slight rise in temperature has thus turned the whole process topsy-turvy, but the action is really the same in both cases. When, on heating, the water acquired the greater density, its buoyancy overcame the pull of gravitation on the orthotoluidine, and accordingly the drop was pushed upwards, the result being the same as when it was pulled downwards. An inverted drop may always be obtained by discharging a light liquid into a heavier one, e.g. olive oil into water, or water into any of the liquids mentioned on p. 19, below the equi-density temperature.

[pg 33]