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diff --git a/37939-8.txt b/37939-8.txt deleted file mode 100644 index 205b871..0000000 --- a/37939-8.txt +++ /dev/null @@ -1,2489 +0,0 @@ - Liquid Drops and Globules - - -This eBook is for the use of anyone anywhere at no cost and with almost -no restrictions whatsoever. You may copy it, give it away or re-use it -under the terms of the Project Gutenberg License included with this -eBook or online at http://www.gutenberg.org/license. - -Title: Liquid Drops and Globules, their Formation and Movements - -Author: Chas. R. Darling - -Release Date: November 05, 2011 [EBook #37939] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK LIQUID DROPS AND GLOBULES *** - - - - -Produced by Chris Curnow, Enrico Segre, and the Online Distributed -Proofreading Team at http://www.pgdp.net. - -This file was produced from images generously made available by The -Internet Archive. - - - - - LIQUID DROPS AND GLOBULES - - - - - _BY THE SAME AUTHOR._ - - - -------------------------------- - - *PYROMETRY* - - A Practical Treatise on the Measurement - of High Temperatures. - - _With 60 Illustrations_, xii + 200 _pp._ - _Crown 8vo, cloth_ (1911). - - Price *5/-* net. - - ---------------- - - *HEAT FOR ENGINEERS* - - A Treatise on Heat, with special regard - to its Practical Applications. - - _Second Edition Revised_, with 110 _Illustrations_, - xiv + 430 _pp._ _Demy 8vo, cloth_ (1912). - - Price *12/6* net. - - -------------------------------- - _E. & F. N. SPON, Ltd., 57 Haymarket, London, S.W._ - - - - - LIQUID DROPS AND - GLOBULES - - Their Formation and Movements - - THREE LECTURES DELIVERED - TO POPULAR AUDIENCES - - BY - - CHAS. R. DARLING - - ASSOCIATE OF THE ROYAL COLLEGE OF SCIENCE, IRELAND; FELLOW OF THE - INSTITUTE - OF CHEMISTRY; FELLOW OF THE PHYSICAL SOCIETY, ETC.; LECTURER - IN PHYSICS AT THE CITY AND GUILDS OF LONDON - TECHNICAL COLLEGE, FINSBURY - - - - - WITH 43 ILLUSTRATIONS - - - - - - _London_ - E. & F. N. SPON, LIMITED, 57 HAYMARKET - - _NEW YORK_ - SPON & CHAMBERLAIN, 123 LIBERTY STREET - - 1914 - - - - - CONTENTS - - - PAGE -_List of Illustrations_ . . . . . . . vii -_Preface_ . . . . . . . . . ix - -_Lecture I._ - Introduction . . . . . . . . . 1 - General Properties of Liquids . . . . . . 2 - Properties of the Surface Skin of Water . . . . 3 - Elastic Skin of other Liquids-Minimum Thermometer . . . 5 - Boundary Surface of two Liquids . . . . . . 6 - Area of Stretched Surface . . . . . . . 7 - Shape of detached Masses of Liquid . . . . . 8 - Production of True Spheres of Liquids . . . . . 10 - The Centrifugoscope . . . . . . . . 14 - Effect of Temperature on Sphere of Orthotoluidine . . . 15 - Other Examples of Equi-Density . . . . . . 17 - Aniline Films or Skins . . . . . . . 19 - Surface Tension . . . . . . . . 21 - The "Diving" Drop . . . . . . . . 22 - Formation of Falling Drops of Liquid . . . . . 24 - Ascending or Inverted Drops . . . . . . 31 - -_Lecture II._ - Automatic Aniline Drops . . . . . . . 33 - Automatic Drops of other Liquids. . . . . . 37 - Liquid Jets . . . . . . . . . 38 - Liquid Columns . . . . . . . . 40 - Communicating Drops . . . . . . . . 44 - Combined Vapour and Liquid Drops . . . . . 47 - Condensation of Drops from Vapour . . . . . 49 - Liquid Clouds in Liquid Media . . . . . . 54 - Overheated Drops . . . . . . . . 55 - Floating Drops on Hot Surfaces . . . . . . 57 - -_Lecture III._ - Spreading of Oil on the Surface of Water . . . . 60 - Movements due to Solubility . . . . . . 63 - Movements of Aniline Globules on a Water Surface . . . 63 - Movements of Orthotoluidine and Xylidine 1-3-4 on a Water Surface 66 - Production of Globules from Films . . . . . 68 - Network formed from a Film . . . . . . 70 - Quinoline Rings . . . . . . . . 71 - Expanding Globules . . . . . . . . 71 - Attraction between Floating Globules . . . . . 73 - Analogies of Surface Tension Phenomena with Life . . . 75 - -_Conclusion_ . . . . . . . . . 76 - -_Appendix_ - Apparatus and Materials required for Experiments on Drops - and Globules . . . . . . . . 78 - -_Index_ . . . . . . . . . . 81 - - - - - LIST OF ILLUSTRATIONS - - -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 object of the present little volume is to reproduce in connected -form, an account of the many interesting phenomena associated with -liquid drops and globules. Much of the matter relates to experiments -devised by the author during the past four years, descriptions of which -have appeared in the _Proceedings of the Physical Society_; in the -columns of _Nature_ and _Knowledge_; and elsewhere. The exhibition of -these experiments at the conversazioni of the Royal Society and the -Royal Institution, and in the author's lectures, has evoked such -interest as to suggest the present publication. It may be added that all -the experiments described may be repeated by any intelligent reader at a -trifling cost, no special manipulative skill being required. - -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. - -A number of the illustrations used have appeared in the pages of -_Knowledge_ in connexion with the author's articles, and are here -reproduced by courtesy of the Editor. Other drawings have been provided -by Mr. W. Narbeth, to whom the author expresses his thanks. - - CHAS. R. DARLING. - - _City and Guilds Technical College,_ - _Finsbury, 1914._ - - - - - 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 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, 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 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. - -[Illustration: __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 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 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. - -[Illustration: __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 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 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. - -[Illustration: __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 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. - -[Illustration: __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 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. - -[Illustration: __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 -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 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 (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 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. - -[Illustration: __Fig._ 6.--The detached sphere floating under water._] - -[Illustration: __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 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 -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 - ------------------------------------------------------------------- - - -[Illustration: __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 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 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 (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, 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. - -[Illustration: __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 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. - -[Illustration: __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 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 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; 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 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. - -[Illustration: __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. 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 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 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. - -[Illustration: __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 respects identical with what we have just -seen on a large scale. - -[Illustration: __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, -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. - - - - - LECTURE II - - -*Automatic Aniline Drops.*--In the foregoing experiments the drop was -enlarged until it broke away by feeding it with liquid; but it is -possible to arrange that the formation shall be quite automatic. The -experiment, as we shall see, is extremely simple, and yet it contains an -element of surprise. Into a beaker containing water nearly boiling I -pour a considerable quantity of aniline, which at first breaks up into a -large number of drops. After a short time, however, all the aniline -floats to the surface, having been warmed by contact with the water to a -temperature higher than that of equi-density (147° F., or 64° C.)--which -is exactly what we should expect to happen. There it remains for a brief -period in the form of a large mass with the lower portion curved in -outline. Soon, however, we observe the centre of the mass sinking in the -water, and taking on the now familiar outline of a falling drop. -Gradually, it narrows at the neck and breaks away; but as aniline is a -viscous liquid, the neck in this case is long and therefore easily seen. -The large drop breaks away and falls to the bottom of the beaker, its -upper surface rising and falling for some time owing to the recoil of -its skin after separation, finally becoming permanently convex. -Immediately after the large drop has parted, the upper mass shrinks -upwards, spreading out further on the surface of the water, with the -result that the long neck is severed at the top, its own weight -assisting the breakage. Now follows the resolution of the detached neck -into two or more spheres, usually a large and a small (Fig. 22). And -now, to those who view the experiment for the first time, comes the -surprise. The large drop, which was more or less flattened when it came -to rest at the bottom of the beaker, becomes more and more rounded, and -finally spherical. Then, unaided, it rises to the top and mingles itself -with the aniline which remained on the surface. After a brief interval a -second drop falls, imitating the performance of the first one; and, like -its predecessor, rises to the surface, after remaining for a short time -at the bottom of the vessel. And so long as we keep the temperature a -few degrees above that of equi-density, the process of partition and -reunion goes on indefinitely. The action is automatic and continuous, -and the large size of the drop and of the neck, and the slowness of the -procedure, enables us to follow with ease every stage in the formation -of a parting drop. - -[Illustration: __Fig._ 21._] - -[Illustration: __Figs._ 21 and 22.--Automatically formed aniline drops, -showing the formation of droplets from the neck._] - -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. - -*Automatic Drops of other Liquids.*--Liquids which possess a low -equi-density temperature with water do not form automatic drops like -aniline, as the rate of cooling at the surface is too slow, and hence -the floating mass of liquid does not attain a density in excess of that -of the water beneath. Aceto-acetic ether, however, behaves like aniline, -if the temperature of the water be maintained at about 170° F. (77° C.), -but as this liquid is fairly soluble in hot water further quantities -must be added during the progress of the experiment. Results equal to -those obtained with aniline, however, may be secured by using -nitrobenzene in nitric acid of specific gravity 1·2 at 59° F. (15° C.), -the acid being heated to 185° F. (85° C.); and here you see the yellow -drop performing its alternate ascents and descents exactly as in the -case of aniline and water. Other examples might be given; but we may -take it as a general rule that when the equi-density temperature of the -liquid and medium is above 125° F. (52° C.), the phenomenon of the -automatic drop may usually be observed when the temperature is raised by -30° F. (17° C.), above this point. - -*Liquid Jets.*--So far we have been observing the formation of single -drops, growing slowly at the end of a tube, or breaking away from a -large mass of the floating liquid. If, however, we accelerate the speed -at which the liquid escapes, the drop has no time to form at the outlet, -and a jet is then formed. We are all familiar with a jet of water -escaping from a tap; it consists of an unbroken column of the liquid up -to a certain distance, depending upon the pressure, but the lower part -is broken up into a large number of drops, which break away from the -column at a definite distance from the tap. There are many remarkable -features about jets which I do not intend to discuss here, as it is only -intended to consider the manner in which the drops at the end are -formed. To observe this procedure, it is necessary again to resort to -our method of slowing down the rate of formation, by allowing the liquid -to flow into a medium only slightly inferior in density. For this -purpose, orthotoluidine falling into water at the ordinary room -temperature is eminently satisfactory; and we see on the screen the -projection of a pipe, with its end under water, placed so that a jet of -orthotoluidine may be discharged vertically downwards from a stoppered -funnel. I open the tap slightly at first, and we then merely form a -single drop at the end. Now it is opened more widely, and you observe -that the drop breaks away some distance below the outlet, being rapidly -succeeded by another and another (Fig. 23). On still further opening the -tap the drops form at a still greater distance from the end of the pipe, -and succeed each other more rapidly, so that quite a number appear in -view at any given moment (Figs. 24 and 25). Notice how the drop is -distorted by breaking away from the stream of liquid, and how it -gradually recovers its spherical shape during its fall through the -water. Finally, I increase the discharge to such an extent that the -formation of the terminal drops is so rapid as to be no longer visible -to the eye, but appears like the turmoil observed at the end of a jet of -water escaping into air. - -[Illustration: __Figs._ 23, 24, 25.--Jets of Orthotoluidine, discharged -under water._] - -[Illustration: __Fig._ 26.--Water stretched between a tube and a -plate._] - -*Liquid Columns.*--A simple experiment will suffice to illustrate what -is meant by a liquid column. Here is a drop of water hanging from the -end of a glass tube. I place it in the lantern and obtain a magnified -image on the screen, and then bring up a flat plate of glass until it -just touches the suspended drop. As soon as contact is established, the -water spreads outwards over the plate, causing the drop to contract in -diameter at or near its middle part, so that its outline resembles that -of a capstan (Fig. 26). The end of the glass tube is now connected to -the plate by a column of water of curved outline, which is quite stable -if undisturbed. If, however, I gradually raise the tube, or lower the -plate, the narrow part of the column becomes still narrower, and finally -breaks across. In the same way we may produce columns of other liquids; -those obtained with viscous liquids such as glycerine being capable of -stretching to a greater extent than water, but showing the same general -characteristics. A liquid column, then, is in reality a supported drop, -and the severance effected by lowering the support is similar to that -which occurs when a pendent drop breaks away owing to its weight. - -In our previous experiments we have seen that in order to produce large -drops of a given liquid, the surroundings should be of nearly the same -density, so as largely to diminish the effective weight of the suspended -mass. We might therefore expect that large columns of liquid could be -produced under similar conditions; and our conjecture is correct. We -may, for example, use the apparatus by means of which large drops of -orthotoluidine were formed (Fig. 13), using a shallow layer of water, so -that the lower end of the drop would come into contact with the bottom -of the vessel before the breaking stage was reached, and thus produce, -on a large scale, the same result as that we have just achieved by -allowing a hanging drop of water to touch a glass plate. This method, -however, restricts the diameter of the top of the column to that of the -delivery tube, and in this respect the shape is strained. The remedy for -this is to hang the drop from the surface of the water, when a degree of -freedom is conferred upon the upper part, which enables the column to -assume a greater variety of shapes. In order to show how this may be -accomplished, I pour a small quantity of water into the rounded end of a -wide test-tube, which is now seen projected on the screen, and then pour -gently down the side a quantity of _ethyl benzoate_, a liquid slightly -denser than water. You observe that the liquid spreads out on the -surface of the water, forming a hanging drop which at first is nearly -hemispherical in shape; but as I continue to add the liquid the drop -grows in size downwards, and finally reaches the bottom of the tube. -There is our liquid column (Fig. 27), which has formed itself in its own -way, free from the restriction imposed by a delivery tube. Notice the -graceful curved outline, produced by a beautiful balance between the -forces of surface tension and gravitation; and notice also how the -outline may be varied by the gradual addition of water, which causes the -upper surface to rise, and thus stretches the column (Fig. 28). The -middle becomes more and more narrow (Fig. 29), and finally breaks -across, leaving a portion of the former column hanging from the surface, -and the remainder, in rounded form (Fig. 30), at the bottom of the tube. -And, as usual, the partition was accompanied by the formation of a small -droplet. - -[Illustration: __Figs._ 27, 28, 29, 30.--A liquid column stretched -upwards until broken by addition of water. Four stages._] - -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 ½ inch (Fig. 31). 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. - -[Illustration: __Fig._ 31.--A column of aceto-acetic ether in water._] - -*Communicating Drops.*--There is a well-known experiment, which some of -you may have seen, in which two soap-bubbles are blown on separate -tubes, and are then placed in communication internally. If the bubbles -are exactly equal in size, no alteration takes place in either; but if -unequal, the smaller bubble shrinks, and forces the air in its interior -into the larger one, which therefore increases in size. Finally, the -small bubble is resolved into a slightly-curved skin which covers the -end of the tube on which it was originally blown. It is evident from -this experiment that the pressure per unit area exerted by the surface -of a bubble on the air inside is greater in a small than in a large -bubble. The internal pressure may be proved to vary inversely as the -radius of the bubble; thus by halving the radius we double the pressure -due to the elastic surface, and so on. The reciprocal of the radius of a -sphere is called its _curvature_, and we may therefore state that the -pressure exerted by the walls of the bubble on the interior vary -directly as the curvature. - -[Illustration: __Fig._ 32.--Apparatus for communicating drops, with -extensions of unequal length attached._] - -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 (Fig. 32) 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. - -[Illustration: __Fig._ 33.--Combined drops of vapour and liquid._] - -*Combined Vapour and Liquid Drops.*--All liquids when heated give off -vapour, the amount increasing as the temperature rises. The vapour -formed in the lower part of the vessel in which the liquid is heated -rises in the form of bubbles, which may condense again if the upper part -of the liquid be cold. When the liquid becomes hot throughout, however, -the vapour bubbles reach the surface and break, allowing the contents to -escape into the air above. Everyone who has watched a liquid boiling -will be familiar with this process, but it should be remembered that a -liquid may give off large quantities of vapour without actually boiling. -A dish of cold water, if exposed to the air, will gradually evaporate -away; whilst other liquids, such as petrol and alcohol, will disappear -rapidly under the same circumstances--and hence are called "volatile" -liquids. - -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[1] 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 (Fig. 33). 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. - - [1] 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. - -*Condensation of Drops from Vapour,--Mists, Fogs and Raindrops.*--The -atmosphere is the great laboratory for the manufacture of drops. It is -continually receiving water in the form of vapour from the surface of -the sea, from lakes, from running water, and even from snow and ice. All -this vapour is ultimately turned into drops, and returned again to the -surface, and to this never-ceasing exchange all the phenomena connected -with the precipitation of moisture are due. The atmosphere is only -capable of holding a certain quantity of water in the form of vapour, -and the lower the temperature the less the capacity for invisible -moisture. When fully charged, the atmosphere is said to be -"saturated"--a condition realized on the small scale by air in a corked -bottle containing some water, which evaporates until the air can hold no -more. The maximum weight of vapour that can be held by 1 cubic metre of -air at different temperatures is shown in the table:-- - - - ------------------------------------------------------------------ - Temperature. Weight of water vapour - (grammes) required to - Deg. C. Deg. F. saturate 1 cubic metre. - ------------------------------------------------------------------ - 0 32 4·8 - - 5 41 6·8 - - 10 50 9·3 - - 15 59 12·7 - - 20 68 17·1 - - 25 77 22·8 - - 30 86 30·0 - - 35 95 39·2 - - 40 104 50·6 - ------------------------------------------------------------------ - - -It will be seen from the table that air on a warm day in summer, with a -temperature of 77° F., can hold nearly five times as much moisture as -air at the freezing point, or 32° F. The amount actually present, -however, is usually below the maximum, and is recorded for -meteorological purposes as a percentage of the maximum. Thus if the -"relative humidity" at 77° F. were 70 per cent., it would imply that the -weight of moisture in 1 cubic metre was 70 per cent. of 22·8 grammes; -that is, nearly 16 grammes. If 1 cubic metre of air at 77° F., -containing 16 grammes of moisture, were cooled to 50° F., a quantity of -water equal to (16-9·3) = 6·7 grammes would separate out, as the maximum -content at the lower temperature is 9·3 grammes. Precipitation would -commence at 66° F., at which temperature 1 cubic metre is saturated by -16 grammes. And similarly for all states of the atmosphere with respect -to moisture, cooling to a sufficient extent causes deposition of water -to commence immediately below the saturation temperature, and the colder -the air becomes afterwards the greater the amount which settles out. The -temperature at which deposition commences is called the "dew point." - -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.[2] 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. - - [2] 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. - -*Liquid Clouds in Liquid Media.*--Just as the excess of moisture is -precipitated from saturated air when the temperature falls, so is the -excess of one liquid dissolved in another thrown down by cooling below -the saturation temperature. Moreover, a liquid when precipitated in a -second liquid appears in the form of myriads of small spheres, which -have the appearance of a dense cloud. Here is some boiling water to -which an excess of aniline has been added, so that the water has -dissolved as much aniline as it can hold. Aniline dissolves more freely -in hot water than in cold, so that if I remove the flame, and allow the -beaker to cool, the surplus of dissolved aniline will settle out. -Cooling takes place most rapidly at the surface, and you observe white -streaks falling from the top into the interior, where they are warmed up -and disappear. Soon, however, the cooling spreads throughout; and now -the streaks become permanent, and the water becomes opaque, owing to the -thick white cloud of precipitated aniline. The absence of the red colour -characteristic of aniline is due to the extremely fine state of division -assumed in the process. If left for some hours, the white cloud sinks -through the water to the bottom of the beaker, where the small particles -coalesce and form large drops, leaving the overlying water quite -transparent. The process is quite analogous to the precipitation of -moisture from the atmosphere in the form of small spheres, which, if -undisturbed, would gradually sink to the ground and leave the air clear. - -*Overheated Drops.*--The temperature at which a liquid boils, under -normal conditions, depends only upon the pressure on its surface. Thus -water boiling in air, when the pressure is 76 centimetres or 29·92 -inches of mercury, corresponding to 14·7 pounds per square inch, -possesses a temperature of 100° C. or 212° F. At higher elevations, -where the pressure is less, the boiling point is lower; thus Tyndall -observed that on the summit of the Finsteraarhorn (14,000 feet) water -boiled at 86° C. or 187° F. Conversely, under increased pressure, the -boiling point rises; so that at a pressure of 35 pounds per square inch -water does not boil until the temperature reaches 125° C. or 257° F. -There are certain abnormal conditions, however, under which the boiling -point of a liquid may be raised considerably without any increase in the -pressure at the surface; and it is then said to be "over-heated." Dufour -showed that when drops of water are floating in another liquid of the -same density, they may become greatly overheated, and if very small in -size may attain a temperature of 150° C. or 302° F., or even higher, -before bursting into steam. In order to provide a medium in which water -drops would float at these temperatures, Dufour made a mixture of -linseed oil and oil of cloves, which possessed the necessary -equi-density temperature with water. To demonstrate this curious -phenomenon, I take a mixture of 4 volumes of ethyl benzoate and 1 volume -of aniline, which at 125° C. or 257° F. is exactly equal in density to -water at the same temperature. I add to the mixture two or three cubic -centimetres of freshly-boiled water, the temperature being maintained at -125° C. by surrounding the vessel with glycerine heated by a flame. At -first the water sinks, but on attaining the temperature of the mixture -it breaks up with some violence, forming spheres of various sizes which -remain suspended in the mixture. Any portion of the water which has -reached the surface boils vigorously, and escapes in the form of steam; -and some of the larger spheres may be observed to be giving off steam, -which rises to the surface. Most of the spheres, however, remain -perfectly tranquil, in spite of the fact that the water of which they -are composed is many degrees above its normal boiling point. If I -penetrate one of these spheres with a wire, you notice that it breaks up -immediately, with a rapid generation of steam. A complete explanation of -this abnormal condition of water is difficult to follow, as a number of -factors are involved. One of the contributory causes--though possibly a -minor one--is the opposition offered to the liberation of steam by the -tension at the surface of the spheres. - -[Illustration: __Fig._ 34.--Spheroid of water on a hot plate._] - -*Floating Drops on Hot Surfaces.*--If a liquid be allowed to fall in -small quantity on to a very hot solid, it does not spread out over the -surface, but forms into drops which run about and gradually evaporate. -By careful procedure, we may form a very large, flattened drop on a hot -surface, and on investigation we shall notice some remarkable facts. I -take a plate of aluminium, with a dimple in the centre, and make it very -hot by means of a burner. You see the upper surface of this plate -projected on the screen. I now allow water to fall on the plate drop by -drop, and you hear a hissing noise produced when each drop strikes the -plate. The separate drops gather together in the depression at the -centre of the plate, forming a very large flattened globule. You might -have expected the water to boil vigorously, but no signs of ebullition -are visible; and what is more remarkable, the temperature of the drop, -in spite of its surroundings, is actually less than the ordinary boiling -point. Notice now how the drop has commenced to rotate, and has been set -into vibration, causing the edges to become scalloped (Fig. 34). The -drop, although not actually boiling, is giving off vapour rapidly, and -therefore gradually diminishes in size. And now I want to prove that the -drop is not really touching the plate, but floating above it. To do this -I make an electric circuit containing a cell and galvanometer, and -connect one terminal to the plate and place the other in the drop. No -movement is shown on the galvanometer, as would be the case if the drop -touched the plate and thus completed the electric circuit. And at close -range we can actually see a gap between the drop and the plate, so that -the evidence is conclusive. If now I remove the flame--leaving the -electric circuit intact--and allow the plate to cool, we notice after a -time that the globule flattens out suddenly and touches the plate, as -shown by the deflection of the galvanometer; and simultaneously a large -cloud of steam arises, due to the rapid boiling which occurs immediately -contact is made. - -What we have seen in the case of water is shown by most liquids when -presented to a surface possessing a temperature much higher than the -boiling point of the liquid. A liquid held up in this manner above a hot -surface is said to be in the _spheroidal state_, to distinguish it from -the flat state usually assumed by spreading when contact occurs between -the liquid and the surface. It is doubtful whether any satisfactory -explanation of the spheroidal state has ever been given. Evidently, the -layer of vapour between the plate and the drop must exert a considerable -upward pressure in order to sustain the drop, but the exact origin of -this pressure is difficult to trace. - - - - - LECTURE III - - -*Spreading of Oil on the Surface of Water.*--If a small drop of oil be -placed on the surface of water it will be observed to spread immediately -until it forms a thin layer covering the surface. If a further addition -of the oil be made, globules will be formed, which, as you now see upon -the screen, remain floating on the surface. The spreading of the oil in -all directions from the place on which the small quantity of oil was -dropped is due to the superior surface tension of water, which pulls the -oil outwards. The surface tension of the oil opposes that of the water, -and would prevent the drop from spreading were it not overcome by a -greater force. The result is the same as would be observed if the centre -or any other part of a stretched rubber disc were weakened; the weak -part would be stretched in all directions, and the rest of the disc -would shrink towards the sides. When the oil has spread out, however, -and contaminated, as it were, the surface of the water, the surface -tension is reduced, and is not sufficiently strong to stretch out a -further quantity of oil, which, if added, remains in the form of a -floating globule. - -[Illustration: __Fig._ 35.--Forces acting on a floating globule._] - -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 (Fig. 35), 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. - -*Movements due to Solubility.*--When small fragments of camphor are -placed on the surface of water some remarkable movements are seen.[3] -The bits of camphor move about with great rapidity over the surface, and -generally, in addition, show a rapid rotary motion. The explanation -usually given is that the camphor dissolves in the water at the points -of contact forming a solution which possesses a less surface tension -than pure water. This solution is in consequence stretched by the -tension of the rest of the surface, and the camphor floating on its -solution is therefore made to move in the direction of the line along -which the stretching force happens to be the greatest. But the camphor -continues to dissolve wherever it goes, and is therefore continuously -pulled about as a result of this interplay of tensions. Touching the -surface with a wire which has been dipped in oil immediately arrests the -movements, owing to the tension of the water being diminished to such an -extent by the skin of oil that it is no longer competent to stretch the -part on which the camphor floats. No doubt this explanation is correct -so far as it goes, but it is highly probable that when the floating -substance dissolves, other forces are called into action in addition to -the tensions. - - [3] These movements were first recorded by Romieu in 1748 and were - ascribed by him to electricity. - -[Illustration: __Fig._ 36.--Aniline globules on a water surface._] - -*Movements of Aniline Globules on a Water Surface.*--If we allow a small -quantity of aniline to run on to the surface of water, it forms itself -into a number of floating globules. I now project on the screen a water -surface on which a little aniline has been poured, and we are thus -enabled to watch the movements which occur. All the globules appear to -be twitching or shuddering; and if you observe closely you will notice -the surface of each globule stretching and recoiling alternately. The -recoil is accompanied by the projection of tiny globules from the rim, -which becomes scalloped when the globule is stretched. The small -globules thrown off appear to be formed from the protuberances at the -edge (Fig. 36), and after leaving the main globule they spread out over -the surface, or dissolve. This process continues for a long time, -gradually diminishing in vigour, until small stationary globules are -left floating on the surface, which is now covered with a skin of -aniline. This action is in striking contrast to the tranquil formation -of floating globules of oil, and calls for some special comment. - -Let us recall again the three forces at work at the edge of a floating -globule (Fig. 35). 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. - -[Illustration: __Fig._ 37.--Orthotoluidine globules on a water -surface._] - -*Movements of Orthotoluidine and Xylidine 1-3-4 on a Water Surface.*--We -will now observe, by the aid of the lantern, movements of globules more -striking, and certainly more puzzling, than those of aniline. I place on -the surface of the water a quantity of a special sample of -orthotoluidine, and you see that immediately a number of globules are -formed which are endowed with remarkable activity. They become indented -at one side, and then dart across the surface at a great speed, usually -breaking into two as a result of the violent action (Fig. 37). Then -follows a short period of rest, when suddenly, as if in response to a -signal, all the larger globules again become indented, forming shapes -like kidneys, and again shoot across the surface, breaking up into -smaller globules. Notice that the very small globules remain at rest; it -is only those above a certain size that display this remarkable -activity. A film of the liquid forms on the water, and the action -gradually becomes more intermittent, ceasing altogether when a skin is -well established, and the large globules have sub-divided into very -small ones. My sample of orthotoluidine is somewhat unique, as other -specimens of the liquid, obtained from the same and other sources, do -not show the same lively characteristics. As in the case of camphor, -touching the surface with a drop of oil arrests the movements -immediately. The organic liquid _xylidine_ 1-3-4, however, exhibits the -same movements, as you now see on the screen; and, if anything, is even -more active than the orthotoluidine previously shown. It may be added -that occasional samples of aniline show similar movements, but of less -intensity. - -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. - -[Illustration: __FIG. 38._--Resolution of a floating skin into -globules._] - -*Production of Globules from Films.*--When a film of oil spreads over a -water surface it sometimes remains as such indefinitely. Certain other -liquids, however, form films which after a short interval break up into -globules, and the process of transition is at once striking and -beautiful. In order to show it, I project a water surface on the screen, -and pour on to it a very small quantity of _dimethyl-aniline_--an oily -liquid related to but distinct from ordinary aniline. It spreads out -into a film of irregular outline, which floats quietly for a short time. -Soon, however, indentations are formed at the edges, which penetrate the -film, and from the sides of the indentations branches spread which in -turn become branched; and shortly the whole film becomes ramified, -resembling a mass of coral, or, to use a more homely illustration, a -jig-saw puzzle (Fig. 38). The various branches join in numerous places, -cutting off small islands from the film; and immediately each island -becomes circular in outline--and the resolution into globules is -complete. We have witnessed one of the beauty-sights of Nature. - -The same method of globule formation is shown by nitro-benzol and -_quinoline_, and as the action is more gradual in the case of the latter -substance, I show it in order that we may study the process in greater -detail. Notice the formation of the indentations and their subsequent -branching; and also that holes form in the skin from which branchings -also proceed. In this instance the film is broken up in sections, but -the action continues until nothing but globules remain on the -surface.[4] - -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. - - [4] 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." - -*Network formed from a Film.*--A further example of the breaking up of a -film is furnished by certain oils derived from coal-tar, the result in -this case being the formation of a network or cellular structure. I -place on the surface of water in a glass dish a small quantity of -tar-oil, and project it on the screen. It spreads out at first into a -thin film, which, by reflected light, shows a gorgeous display of -colours. After a short time, little holes make an appearance in the -film, and these holes gradually increase in size until the whole of the -film is honeycombed (Fig. 39), the oil having been heaped up into the -walls which divide the separate compartments. Here again the accepted -views on surface tension do not appear competent to explain the action. -It appears to be the case that most films on the surface of water show -this tendency to perforation, which may be due to inequalities in the -thickness of the film, or in the distribution of the strain to which it -is subjected.[5] - - [5] An interesting discussion on cellular structures of this type may - be found in _Nature_, April 16 to June 11, 1914. - -[Illustration: __Fig._ 39.--Network formed from a film of tar-oil on the -surface of water._] - -[Illustration: __Fig._ 40.--Quinoline rings and perforated plates._] - -*Quinoline Rings.*--Reference has already been made to the breaking-up -of a quinoline film into globules. But if we examine the surface about -half an hour after the formation of these globules, we find that each -has been perforated in the centre, forming a ring or annulus (Fig. 40). -Some of the larger globules have undergone perforation in several -places, forming honeycombed plates. These rings and plates, which you -now see projected on the screen, remain unchanged, and apparently -represent the final stage of equilibrium under the action of the various -forces. Quinoline, so far as observations have been made, appears to be -unique in respect to the formation of stable rings from globules. - -[Illustration: __Fig._ 41.--The expanding globule._] - -*Expanding Globules.*--I now wish to show, by an experiment, how -sensitive a floating globule is to disturbances in the existing -tensions, which maintain it at rest. On the screen is projected a -globule of dimethyl-aniline, floating tranquilly on the surface of -water. I now allow a small drop of quinoline to fall upon it, and -immediately it spreads out over the surface, forming a hole in its -centre (Fig. 41), after which it gradually resumes its former shape. -Sometimes the action is so violent that the globule is split up into -several portions, which, however, join together again after a short -time. In order to explain this action, we must again refer to the three -tensions operating on the globule (Fig. 35). When in equilibrium, A is -balanced by the joint pull of B and C; and hence if either of the latter -be weakened, A will predominate and stretch the globule. In our -experiment it is the interfacial tension, C, which has been diminished -in strength, as we may now prove by a second experiment. In this -instance I float on the water surface a globule of lubricating oil, with -which quinoline does not readily mix, and which does not act so -immediately as dimethyl-aniline. On allowing the drop of quinoline to -fall into it, no action is observed until the drop has rested on the -junction of the oil and water for a short time; but when it has -penetrated the interface the oil globule suddenly spreads over the water -surface, and with such violence as to detach several portions from the -main globule. Merely touching the upper surface of the oil with a rod -moistened with quinoline has no effect, and hence the result is due to -the weakening of the interfacial tension. A similar effect is obtained -when quinoline is dropped into a globule of aniline, and may be obtained -with various other liquids. - -*Attraction between Floating Globules.--The "Devouring" Globule.* When -globules of different liquids are floating on the same water surface, a -tendency to coalesce is sometimes noticed, but is by no means general. I -will show one example which possesses striking features, showing as it -does the remarkable results which may be brought about by surface -forces. First of all, we form a number of active orthotoluidine globules -on the surface of a dish of water, which you see wriggling about in -their characteristic fashion. After their activity has subsided -somewhat, I float on to the surface a large globule of dimethyl-aniline. -Attraction of some kind is at once apparent, for the nearest globule of -orthotoluidine immediately approaches the intruder. And now comes the -process of absorption. The large globule of dimethyl-aniline develops a -protuberance in the direction of its victim (Figs. 42 and 43), and the -small globule of orthotoluidine coalesces with this "feeler," which then -shrinks back into the large globule, conveying with it the entangled -orthotoluidine. This, however, by no means satisfies the devouring -globule, as a second victim is at once appropriated in the same manner; -and you will notice a nibbling process at work round the edges -continuously, which is due to the absorption of the smaller globules of -orthotoluidine. The action continues until the whole of the surface has -been cleared of orthotoluidine, after which the large globule floats -tranquilly in the centre of the vessel, apparently resting after its -heavy meal. The interaction of the forces which gives rise to this -phenomenon is difficult to fathom; there are no doubt several tensions, -constantly changing in magnitude, which in the result cause the liquids -of the large and small globules to intermingle. Separate globules of a -single liquid sometimes unite in this manner, but this is not common, it -being more usual for the scattered units to remain apart. - -[Illustration: __Fig._ 42.--The "devouring" globule. Five stages._] - -[Illustration: __Fig._ 43.--Photograph of one globule absorbing -another._] - -*Analogies of Surface Tension Phenomena with Life.*--When we watch the -movements of globules on the surface of water, the resemblance to the -antics of the lower forms of life immediately occurs to our minds. Now I -do not intend here to intrude any opinion on the much-discussed subject -of the Origin of Life, but merely to point out that certain phenomena, -usually supposed to be associated only with living things, may result -from the interplay of surface tensions. In our experiments we have -witnessed expansive and contractile motion (aniline globules on water); -movement of translation, of a very vigorous kind (xylidine and -orthotoluidine globules); incorporation of external matter, or feeding -(dimethyl-aniline absorbing orthotoluidine)--we are getting quite -familiar with these long names now--, splitting up of masses, or -division (skins of quinoline, etc., breaking up into branched portions, -and sub-division of large globules); and formation of cellular structure -(tar-oil on water). And the conclusion we may legitimately draw is this: -that mechanical forces may account for many observed phenomena in -connexion with life which formerly were attributed to the action of -"vital" forces. Modern biological research all points in the same -direction, and it seems probable that the operations of the animate and -inanimate are controlled by the same forces. But the mystery of Life -still remains. - -*Conclusion.*--I have endeavoured in these lectures to bring to your -notice some of the remarkable results which may be produced by the use -of water and a few other liquids, and the scientific conclusions which -may be drawn from them. It may be that the phenomena we have considered -have little or no commercial application; but science has other uses in -addition to its fruitful alliance with commerce. The study of the -methods by which Nature achieves her ends stimulates the imagination and -quickens the perceptions, and is therefore of the highest educational -value. It is a great scientific achievement to run a railway to the -summit of the Jungfrau, but we should not envy the mental condition of -the individual to whom that glorious mountain appealed only through the -railway dividends. And I trust that we shall never become so imbued with -the industrial aspects of science, as to lessen our appreciation of the -works of Nature, whether manifested in the snow-clad peak or the equally -wonderful drop of water. - - - - - APPENDIX - - -Apparatus and Materials required for Experiments on Drops and Globules. - - -*Vessels.*--For direct observation of liquid spheres, large drops, etc., -beakers about 6 inches in height and 4 inches in diameter are suitable. -It must be remembered, however, that a beaker containing water behaves -like a cylindrical lens, and hence objects in the interior appear -distorted in shape. In order to observe the true dimensions, flat-sided -vessels must be used, in which the faces are of uniform thickness. Glass -battery-vessels, which are made of a single piece of glass, have sides -of irregular thickness, and are not to be recommended. A useful form of -vessel is one in which the bottom and edges are made of copper, the -sides being formed of windows of plate glass cemented to the copper -framework. Water may be boiled in such a vessel without danger to the -glass, starting with cold water; it is not advisable to pour hot water -into the cold vessel, however, as the glass may crack. Suitable -dimensions for a vessel of this kind are 6 inches high, and 4 inches in -width and thickness. A beaker containing water, in which drops are -formed may be placed in this square vessel, and surrounded by water, -when distortion will be absent; and the whole of the contents may be -kept hot--as required, for example, with the automatic aniline drop. It -is best to conduct the experiments in beakers immersed as described, as -the materials used may then be easily recovered without having to clean -out the flat vessel. - -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 (Fig. 7) 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. - -*Materials.*--Sufficient quantities of the various liquids used may be -procured from dealers in chemicals at a small cost. Aniline and -orthotoluidine, which figure largely in the experiments, should be -obtained in the "commercial" form, which is the cheapest and most -suitable. The remaining liquids should be of the variety described as -"pure" in the catalogues. When used for the formation of films, they -should be kept in bottles in which the glass stopper is prolonged into a -tapered rod, which dips into the liquid, and which, on removal, carries -a convenient quantity of liquid to drop on to the water surface. - -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. - -*Water.*--Ordinary tap-water suffices for all the experiments described, -and for work with films and globules is superior to distilled water, -which often possesses a surface so greasy as to retard or even entirely -prevent the desired result. All experiments conducted on the surface of -water should be performed in a clean vessel which has been rinsed out -several times with tap-water before filling. - -*Lantern Projection.*--In demonstrating the phenomena to an audience, a -lantern may be used to advantage. It should be of the type now -procurable, which is arranged for the projection of experiments -conducted either in a horizontal or vertical position, by the use of the -electric arc or other suitable source of light. Flat-sided vessels are -essential for the successful projection of views of objects in a -vertical position; and for showing globules, etc., on the surface of -water, better definition is secured if cells with plate-glass bottoms -are used instead of vessels made of a single piece of glass. The author -has generally used a "Kershaw" lantern for lecture purposes, with quite -satisfactory results. This lantern may also be adapted for projecting -solid objects by reflected light--as, for example, a hot plate on which -a spheroid of water is floating (Fig. 34). The contrivance known as the -"Mirrorscope" may also be used, with slight modification, for producing -a magnified image of solid objects on the screen. - - - - - 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 - - B - -Boundary surface of two liquids, . . . . 6 -Butyl benzoate, . . . . . . . 19 - - C - -Camphor, movements of on the surface of water, . 63 -Centrifugoscope, . . . . . . 14 -Chloroform, composite drops of, . . . . 48 - - D - -Dimethyl-aniline, skin of on water, . . . 68 -"Diving" drop, . . . . . . . 22 -Droplet, formation of, . . . . 28, 34 -Drops of liquid, apparatus for, . . . . 27 - " " automatic, . . . 33, 37 - " " combined with vapour, . . 47 - " " communicating, . . . 44 - " " condensation of from vapour, . 49 - " " floating on hot surface, . . 57 - " " formation of, . . 24, 33, 37 - " " overheated, . . . . 55 - " " shapes of, . . 10, 29, 30, 31 - - E - -Elastic skin of liquids, . . . . . 5 -Equi-density temperatures, . . . 16, 17, 19 -Ethyl benzoate, columns of, . . . . 42 - - F - -Fogs, . . . . . . . . 52 - - G - -Globule, forces acting on, . . . . . 61 - " the "devouring", . . . . . 74 -Globules, attraction between, . . . . 73 - " expanding, . . . . . . 72 - " production from films, . . . . 69 - " surface movements on water, . . 63, 66 -Golden syrup, experiment with, . . . . 8 - - I - -Interfacial tension, . . . . . 22, 61 -Ions, condensation on, . . . . . 53 - - J - -Jets of liquid, . . . . . . . 38 - - L - -Liquid clouds in liquid media, . . . . 54 - " columns, . . . . . . 40 - " jets, . . . . . . 38 -Liquids, general properties of, . . . . 2 - " origin of, . . . . . . 1 - " properties of surface of, . . . 3 - - M - -Minimum thermometer, . . . . . . 6 -Mists, . . . . . . . . 49 -Mono-brom-benzene, . . . . . . 48 - - N - -Network formed from film, . . . . . 70 -Nitrobenzene, drops of, . . . . 29, 37 - " films, . . . . . . 69 - - O - -Orthotoluidine columns, . . . . . 42 - " drops, . . . . . . 27 - " equi-density temperature of, . . . 16 - " globules, movements of, . . . . 66 - " jets, . . . . . . . 39 - " spheres, . . . . . 11, 14 - - P - -Petroleum, boundary surface with water, . . . 6 -Plateau's spherule, . . . . . . 25 - - Q - -Quinoline, formation of globules of, . . . 69 - " rings of, . . . . . . 71 - - R - -Raindrops, . . . . . . . 54 - - S - -Shape of detached masses of liquid, . . . 8 -Silver floating on water, . . . . . 4 -Solubility, movements due to, . . . . 63 -Spheres of liquids, effect of temperature on, . 15 - " " " production of, . . 10 -Spheroidal state of liquids, . . . . 59 -Spreading of oil on water, . . . . . 60 -Surface skin of water, properties of, . . . 3 - " tension, definition of, . . . . 21 - " " phenomena, analogies to life, . 75 - " " value for various liquids, . . 22 - - W - -Water, column of, . . . . . . 40 - " surface tension of, . . . . 21 - " beetle, . . . . . . 4 - - X - -Xylidine 1-3-4, movements of globules of, . . 66 - - - - - - (_Pr 1266_) - -------- - Butler & Tanner Frome and London - - - - - Transcription note - - -The following minor typographical flaws have been corrected: - - - Fig. 7: _missing period at the end of the caption_ - - "feeler,"' _unnecessary additional closing quote_ - - *Index:* Drops of liquid, shapes of, 10, 29, 30, 31 _missing commas_ - - *Index:* Mono-brom-benzene _added hyphen to conform with reference - in text_ - -Footnotes have been renumbered progressively throughout the book. - - - - - -*** END OF THIS PROJECT GUTENBERG EBOOK LIQUID DROPS AND GLOBULES *** - - - - - A Word from Project Gutenberg - - -We will update this book if we find any errors. - -This book can be found under: http://www.gutenberg.org/ebooks/37939 - -Creating the works from public domain print editions means that no one -owns a United States copyright in these works, so the Foundation (and -you!) can copy and distribute it in the United States without permission -and without paying copyright royalties. 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