Read Ebook: Liquid Drops and Globules Their Formation and Movements Three lectures delivered to popular audiences by Darling Charles R Charles Robert

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FIG. PAGE 1. Silver sheet floating on water . . . . . 4 2. Column and index of minimum thermometer . . . 6 3. Thread of golden syrup rising and forming a drop . . 8 4. Drops of different sizes resting on flat plate . . 10 5. Formation of a sphere of orthotoluidine . . . 12 6. Detached sphere floating under water . . . . 13 7. The centrifugoscope . . . . . . . 14 8. Aniline drops falling through cold water and ascending through hot water . . . . . . 17 9. Aniline skins enveloping water . . . . . 20 10, 11, 12. The "diving" drop. Three stages . . . . 23 13. Apparatus for forming ascending or descending drops of liquids 27 14-20. Formation of a drop of orthotoluidine, showing the droplet. Seven stages . . . . . 29-31 21, 22. Automatically formed aniline drops, showing the formation of droplets from the neck . . . 34, 35 23-25. Jets of orthotoluidine discharged under water . . 39 26. Water stretched between a tube and a plate . . . 40 27-30. A liquid column stretched upwards by addition of water until broken. Four stages . . . 43 31. A column of aceto-acetic ether in water . . . 44 32. Apparatus for communicating drops . . . . 45 33. Combined vapour and liquid drops . . . . 49 34. Spheroid of water on a hot plate . . . . 58 35. Forces acting on a floating globule . . . . 61 36. Aniline globules on a water surface . . . . 64 37. Orthotoluidine globules on a water surface . . . 66 38. Resolution of a floating skin into globules . . . 68 39. Network formed from a film of tar-oil . . . . 70 40. Quinoline rings and perforated plates . . . . 71 41. The expanding globule . . . . . . 72 42. The "devouring" globule. Five stages . . . 74 43. Photograph of one globule absorbing another . . . 75

PREFACE

The context maintains the form of the lectures delivered on this subject by the author at various places, and the method of presentation is such as may be followed by those who have not received a training in this branch of science. It is hoped, in addition, that the book may prove of some service to teachers of science and others interested in the properties of liquids.

CHAS. R. DARLING.

LIQUID DROPS AND GLOBULES

LECTURE I

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 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.

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.

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 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.

Cube . . . . 6 square feet.

Cylinder . . . . 5?86 ,, ,,

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

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 . 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 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.

Equal: 75 24 0?9973 0?997

Aniline 43

Olive Oil 32

Chloroform 27

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 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.

Now it so happens that we have to hand two liquids which, without any preparation, fulfil our requirements. Orthotoluidine, at temperatures below 75? F. 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 , and in order to make a large drop the end of the stem is widened to a diameter of 1 1/2 inches. It is best, when starting, to place the end of the stem 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 . And then follows the droplet; very small by comparison with the big drop, but plainly visible . 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 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 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.

LECTURE II

And now as to the explanation of this curious performance. When the aniline reaches the surface, and spreads out, it cools by contact with the air more rapidly than the water below. As it cools, its density increases, and soon becomes greater than that of the water, in which it then attempts to sink. The forces of surface tension prevent the whole of the aniline from falling--the water surface can sustain a certain weight of the liquid--but the surplus weight cannot be held, and therefore breaks away. But when the detached drop reaches the bottom of the vessel, it is warmed up again; and when its temperature rises above that of equi-density it floats up to the top. And so the cycle of operations becomes continuous, owing to cooling taking place at the top and heating at the bottom.

Perpetual motion, you might suggest. Nothing of the kind. Perpetual motion means the continuous performance of work without any supply of energy; it does not mean merely continuous movement. A steam-engine works so long as it is provided with steam, and an electric motor so long as it is fed with electricity; but both stop when the supply of energy is withdrawn. So with our aniline drop, which derives its energy from the heat of the water, and which comes to rest immediately the temperature falls below 147? F. or 64? C. But in order that the process of separation and reunion may continue, the cooling at the top is quite as necessary as the heating at the bottom. Our aniline drop is in essence a heat-engine--although it does no external work--and like all heat-engines possesses a source from which heat is derived, and a sink into which heat at a lower temperature is rejected. We might, with certain stipulations, work out an indicator diagram for our liquid engine, but that would be straying too far from our present subject.

It is possible, by using other liquids, and different diameters of vessels, to produce columns of a large variety of outlines. Some liquids spread over a greater area on the surface of water than others, and therefore produce columns with wider tops. Here we see a column of orthotoluidine, which has a top diameter of 2 inches; and here again, in contrast, is a column of aceto-acetic ether, the surface diameter of which is only 1/2 inch . Other liquids, such as aniline, give an intermediate result. The lower diameter is determined by the width of the vessel; and hence we are able to produce an almost endless number of shapes. It is interesting to note how workers in glass and pottery have unconsciously imitated these shapes; and I have here a variety of articles which simulate the outlines of one or other of the liquid columns you have just seen. It is possible that designers in these branches of industry might obtain useful ideas from a study of liquid columns, which present an almost limitless field for the practical observation of curved forms.

We have already seen that a drop of liquid possesses an elastic surface, and is practically the same thing as a soap-bubble filled with liquid instead of air. We might therefore expect the same results if two suspended drops of liquid were placed in communication as those observed in the case of soap-bubbles. And our reasoning is correct, as we may now demonstrate. The apparatus consists of two parallel tubes, each provided with a tap, and communicating with a cross-branch at the top, which contains a reservoir to hold the liquid used. About half-way down the parallel tubes a cross-piece, provided with a tap, is placed. We commence by filling the whole of the system with the liquid under trial, and the parallel tubes equal in length. Drops are then formed at the ends of each vertical tube by opening the taps on these in turn, and closing after suitable drops have been formed. Then, by opening the tap on the horizontal cross-piece, we place the drops in communication and watch the result.

I have chosen orthotoluidine as the liquid, and by placing the ends of the vertical tubes under water--which at the temperature of the room is slightly less dense than orthotoluidine--I am able to form much larger drops than would be possible in air. You now see a small and a large drop projected on the screen; and I now open the cross-tap, so that they may communicate. Notice how the little drop shrinks until it forms merely a slightly-curved prominence at the end of its tube. It attains a position of rest when the curvature of this prominence is equal to that of the now enlarged drop which has swallowed up the contents of the smaller one. So far the result is identical with that obtained with soap-bubbles; but we can extend the experiment in such a way as to reverse the process, and make the little drop absorb the big one. In order to do this I fasten an extension to one of the tubes, and form a small drop deep down in the water, and a larger one on the unextended branch near the top. When I open the communicating top, the system becomes a kind of siphon, the orthotoluidine tending to flow out of the end of the longer tube. The tendency of the large drop to siphon over is opposed by the superior pressure exerted by the skin of the smaller drop; but the former now prevails, and the big drop gradually shrinks and the little one is observed to grow larger. It is possible by regulating the depth at which the smaller drop is placed, to balance the two tendencies, so that the superior pressure due to the lesser drop is equalled by the extra downward pressure due to the greater length of the column of which it forms the terminus. Both pressures are numerically very small, but are still of sufficient magnitude to cause a flow of liquid in one or other direction when not exactly in equilibrium. In the case of communicating soap-bubbles, containing air and surrounded by air, locating the small bubble at a lower level would not reverse the direction of flow, which we succeeded in accomplishing with liquid drops formed in a medium of slightly inferior density.

The formation of vapour and its subsequent escape at the surface of the liquid, enable us to produce a very novel kind of drop; if, instead of allowing the bubbles to escape into air, we cause them to enter a second liquid. Here, for example, is a coloured layer of chloroform? at the bottom of a beaker, with a column of water above. I project the image of the beaker on the screen, and then heat it below. The chloroform vapour escapes in bubbles; but notice that each bubble carries with it a quantity of liquid, torn off, as it were, at the moment of separation. The vapour bubbles and their liquid appendages vary in size, but some of them, you observe, have an average density about equal to that of the water, and float about like weighted balloons. Some rise nearly to the surface, where the water is coldest; and then the vapour partially condenses, with the result that its lifting power is diminished, and hence the drops sink into the lower part of the beaker. But the water is warmer in this region, and consequently the vapour bubble increases in size and lifting power until again able to lift its load to the surface. So the composite drops go up and down, until finally they reach the surface, when the vapour passes into the air, and the suspended liquid falls back to the mass at the bottom of the beaker. Notice that the drop of liquid attached to each bubble is elongated vertically. This is because chloroform is a much denser liquid than water . There is a practical lesson to be drawn from this experiment. Whenever a bubble of vapour breaks through the surface of a liquid, it tends to carry with it some of the liquid, which is dragged mechanically into the space above. In our experiment the space was occupied by water, which enabled the bubble to detach a much greater weight than would be possible if the surface of escape had been covered by air, which is far less buoyant than water. But even when the bubbles escape into air, tiny quantities of liquid are detached; so that steam from boiling water, for example, is never entirely free from liquid. All users of steam are well acquainted with this fact.

? Mono-brom-benzene is better than chloroform for this experiment, but is more costly. It may be coloured with indigo. Chloroform may be coloured with iodine.

Whenever atmospheric moisture assumes the liquid form, drops are invariably formed. These may vary in size, from the tiny spheres which form a mist to the large raindrops which accompany a thunderstorm. But in every instance it is necessary that the air shall be cooled below its saturation point before the separation can commence; and keeping this fact in mind we can now proceed to demonstrate the production of mists and fogs. Here is a large flask containing some water, fitted with a cork through which is passed a glass tube provided with a tap. I pump some air into it with a bicycle pump, and then close the tap. As excess of water is present, the enclosed air will be saturated. Now a compressed gas, on expanding into the atmosphere, does work, and is therefore cooled; and consequently if I open the tap the air in the flask will be cooled, and as it was already saturated the result of cooling will be to cause some of the moisture to liquefy. Accordingly, when I open the tap, the interior of the flask immediately becomes filled with mist. If we examine the mist in a strong light by the aid of a magnifying glass, we observe that it consists of myriads of tiny spheres of water, which float in the air, and only subside very gradually, owing to the friction between their surfaces and the surrounding air preventing a rapid fall. The smaller the sphere, the greater the area of surface in proportion to mass, and therefore the slower its fall. And so in nature, the mists are formed by the cooling of the atmosphere by contact with the surface, until, after the saturation point is reached, the surplus moisture settles out in the form of tiny spheres, which float near the surface, and are dissipated when the sun warms up the ground and the misty air, and thus enables the water again to be held as vapour.

Fogs, like mists, are composed of small spheres of water condensed from the atmosphere by cooling; but in these unwelcome visitors the region of cooling extends to a higher level, and the lowering of temperature is due to other causes than contact with the cold surface of the earth. In our populous cities, the density of the fogs is accentuated by the presence of large quantities of solid particles in the atmosphere, which arise from the smoke from coal fires, and the abrasion of the roads by traffic. We can make a city fog in our flask. I blow in some tobacco smoke, and then pump in air as before. You will notice that the smoke, which is now disseminated through the air in the flask, is scarcely visible; but now, on opening the tap, the interior becomes much darker than was the case in our previous experiment. We have produced a genuine yellow fog, that is, a dense mist coloured by smoke. When we have learned how to abolish smoke, and how to prevent dust arising from the streets, our worst fogs will be reduced to dense mists, such as are now met with on the sea or on land remote from large centres of habitation.

There is one feature common to the spheres which compose a mist or fog, or indeed to any kind of drop resulting from the condensation of moisture in the atmosphere. As shown by the deeply interesting researches of Aitken and others, each separate sphere forms round a core or nucleus, which is usually a small speck of dust, and hence an atmosphere charged with solid particles lends itself to the formation of dense fogs immediately the temperature falls below the dew-point. But dust particles are not indispensable to the production of condensed spheres, for it has been shown that the extremely small bodies we call "ions," which are electrically charged atoms, can act as centres round which the water will collect; and much atmospheric condensation at high elevations is probably due to the aid of ions.? Near the surface, however, where dust is ever present, condensation round the innumerable specks or motes is the rule. Here, for example, is a jet of steam escaping into air, forming a white cloud composed of a multitude of small spheres of condensed water. If now I allow the steam to enter a large flask containing air from which the dust has been removed by filtration through cotton wool, no cloud is formed in the interior, but instead condensation takes place at the end of the jet, from which large drops fall, and on the cold sides of the flask. The cloud we see in dusty air is entirely absent, and the effect of solid particles in the process of condensation is thus shown in a striking manner. Clouds are masses of thick mist floating at varying heights in the atmosphere. On sinking into a warmer layer of dry air the particles of which clouds are composed will evaporate and vanish from sight. If the condensation continue, however, the spheres will grow in size until the friction of the atmosphere is unable to arrest their fall; and then we have rain. And whether the precipitation be very gentle, and composed of small drops falling slowly, as in a "Scotch mist," or in the form of rapid-falling large drops such as accompany a thunderstorm, the processes at work are identical. Every particle of a mist or cloud, and every raindrop, is formed round a nucleus, and owes its spherical shape to the tension at the surface.

? Mr. C. T. R. Wilson has recently devised an apparatus for making visible the tracks of ionizing rays, by the condensation of water vapour round the freshly liberated ions.

Let us study the forces at work on the floating globule a little more closely. Its upper surface is in contact with air, and the surface tension tends, as usual, to reduce the area to a minimum. The top of the globule is not flat, but curved , and its surface meets that of the water at an angle; and the counter-pull exerted against the stretching-pull of the water surface is not horizontal, but inclined in the direction of the angle of contact, as shown by the line B. The under part of the globule is also curved, and meets the water surface from below at an angle; and here also is exerted a pull in opposition to that of the water surface, different in magnitude to the force at the upper surface, but also directed at the angle of contact as shown by the line C. This tension at the joining surface of two liquids is called the "interfacial" tension, to distinguish it from that of a surface in contact with air. Acting against these two tensions is that of the water, which is directed horizontally along the surface, as shown by the line A. The lines A, B, and C indicate the forces acting at a single point; but the same forces are at work at every point round the circle of contact of the globule and the surface of the water. And therefore the tendency on the part of the water tension is to cause the globule to spread out in all directions, whereas the other two tensions tend to prevent any enlargement of its surface. The result depends upon the magnitudes and directions of the conflicting forces. We can imagine a kind of tug-of-war taking place, in which one contestant, A, is opposed to two others, B and C, all pulling in the directions indicated in Fig. 35. Although A is single-handed, he has the advantage of a straight pull, whereas B and C can only exert their strength at an angle, and the larger the angle the more they are handicapped. If A be more powerful than B and C, the globule will spread; but the result of the spreading is to diminish the angles at which the pulls of B and C are inclined to the surface, and hence their effective opposition to A will be increased. Moreover, the spreading of the liquid diminishes the surface tension of the water--that is, weakens A--and hence it becomes possible for B and C to prevail and draw back the surface of the globule which A had previously stretched. If, in spite of these disabilities, A should still be the stronger, the globule will be stretched until it covers the whole surface; whereas if B and C overcome A, the globule will shrink, increasing the angles at which B and C operate, and therefore reducing their effective pulls, until their combined strength is equal to that of A, when the globule will remain at rest. Bearing these facts in mind, we can understand why a small drop of oil placed on a clean water surface spreads across; for in this case A is stronger than B and C combined. But when the surface of the water is covered with a layer of oil, A is weakened, and can no longer overcome the opposing pulls of B and C. Hence a further drop of oil poured on to the surface remains in the form of a globule.

? These movements were first recorded by Romieu in 1748 and were ascribed by him to electricity.

Let us recall again the three forces at work at the edge of a floating globule . The surface tension of the water, acting horizontally, tends to stretch the globule, and is successful momentarily in overcoming the opposing tensions, each of which pulls at an angle to the surface. Enlargement of the upper surface of the globule, however, reduces the angles at which the tensions B and C act, and in consequence their effective strength is increased. The spreading of the aniline over the water surface diminishes the pull A, which B and C combined now overcome, and hence the surface of the globule shrinks again. For some unexplained reason both the stretching and recoil of the globule occur suddenly, there being an interval of repose between each, and these jerky movements result in small portions of the rim being detached, each of which forms a separate small globule. The aniline which spreads over the surface of the water dissolves, and the water tension A, which had been enfeebled by the presence of the aniline skin, recovers its former strength, and again stretches the globule; and so the whole process is repeated. When the surface of the water becomes permanently covered with a skin, which occurs when the top layer is saturated with aniline, the globule remains at rest, and has such a shape that the tensions B and C act at angles which enable them just to balance the weakened pull of A. Why the edge of the globule becomes indented during the movements, and why these movements are spasmodic instead of gradual, has not been clearly made out. It is interesting to recall that a spheroid of liquid on a hot plate also possesses a scalloped edge, and it may be that the two phenomena have something in common.

Now if I am asked to explain these extraordinary movements, I am bound to confess my inability to do so at present. Why should the globules become indented on one side only? The two tensions acting at the edge in opposition to the water tension are at work all round the globule, and it is not easy to see why they should prevail to such a marked degree at one spot only. The movement across the surface, if we followed our previous explanations, would be due to the superior pull of the water tension behind the globule, opposite the indented part; although to look at it would seem as if some single force produced the indentation and pushed the globule along bodily. Are there local weaknesses in the tension of the water, and, if so, why should such weak spots form simultaneously near each globule, causing each to move at the same moment? Any explanation we may give as to the origin of the cavity in the side of the globule does not suffice to account for the intermittent character of the movement, and its simultaneous occurrence over the whole surface. We must therefore leave the problem at present, and trust to future investigation to provide a solution.

It is not easy to see why the canals of water penetrate the film and split it up into small sections, nor why entry takes place at certain points on the edge in preference to others. Some orderly interplay of forces, not yet properly understood, gives rise to the action; and a satisfactory explanation has yet to be given.

? The breaking-up of films on the surface of water was first noticed by Tomlinson about 50 years ago. He used essential oils, and called the patterns "cohesion figures."

Apparatus and Materials required for Experiments on Drops and Globules.

For the formation of liquid columns, test-tubes, of diameter 1 to 2 inches, or small beakers, may be used. Test-tubes provided with a foot, which will stand upright, are most satisfactory; and the true shape may be seen by immersing the test-tube or beaker in water in a flat-sided vessel of the form described above. The effect of heat on the shape of the column may be observed by warming the water in the vessel. The centrifugoscope and the apparatus depicted in Figs. 8, 13, and 32, may be procured from the makers, Messrs. A. Gallenkamp & Co., Sun Street, E.C.

Experiments with skins and globules may be conducted in beakers of about 4 inches diameter, or in small porcelain photographic dishes. If intended for lantern projection shallow cells, with a bottom of plate glass, are necessary, and may be obtained from dealers in scientific apparatus.

Accessories such as glass rods, plates, tubing of various diameters, thin copper wire, and an aluminium plate for the spheroidal state, can be obtained from any dealer in apparatus; and the same applies to clamp-stands for holding funnels, etc.

INDEX

A PAGE

Aceto-acetic ether, automatic drops of, . . . 37 " columns of, . . . . . . 44 Aniline, automatic drops of, . . . . 33 " equi-density temperature of, . . . 17 " films or skins, . . . . . 19 " globules, movements of, . . . 63 Anisol, . . . . . . . . 19 Area of stretched surfaces, . . . . . 7

Boundary surface of two liquids, . . . . 6 Butyl benzoate, . . . . . . . 19

Camphor, movements of on the surface of water, . 63 Centrifugoscope, . . . . . . 14 Chloroform, composite drops of, . . . . 48

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