LIQUID DROPS AND GLOBULES 
 
BY THE SAME AUTHOR. 
 
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 A Practical Treatise on the Measurement 
 of High Temperatures. 
 
 With 60 Illustrations, xii + 200 pp. 
 Crown 8vo, cloth (1911). 
 
 Price 5/- net. 
 
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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. J LECTURER 
 
 IN PHYSICS AT THE CITY AND GUILDS OF LONDON 
 
 TECHNICAL COLLEGE, FINSBURY 
 
 WITH 43 ILLUSTRATIONS 
 
 Honbon 
 
 E. & F. N. SPON, LIMITED, 57 HAYMARKET 
 
 ^eto gorb 
 
 SPON & CHAMBERLAIN, 123 LIBERTY STREET 
 1914 
 
3 
 
CONTENTS 
 
 PAGl! 
 
 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 
 
 358334 
 
vi CONTENTS 
 
 Lecture II continued. PAGE 
 
 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 Kings . . . . . .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 Experi- 
 ments 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, show- 
 ing 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 addi- 
 tion of water until broken. Four stages 43 
 
 31 , A column of aceto-acetic ether in water , 44 
 
 vii 
 
viii LIST OF ILLUSTRATIONS 
 
 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 publi- 
 cation. 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, 
 
x PREFACE 
 
 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 th/3 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 
 
 1 B 
 
GLOBULES 
 
 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 under- 
 lying principles be understood ; and hence it will be 
 necessary, in the beginning, to refer to certain pro- 
 perties 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 gravita- 
 tion 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 dis- 
 tributed 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. Some- 
 times 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, 
 
LECTURE I 3 
 
 however, with these border-line substances, but shall 
 confine our attention to well-defined liquids. 
 
 There is another feature, however, common to all 
 liquids, which has a most important bearing on our 
 subject. Every liquid is capable of forming a boun- 
 dary surface of its own ; and this surface has the pro- 
 perties 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 pro- 
 perties 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 10 \ 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 
 
4 LIQUID DROPS AND GLOBULES 
 
 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, 
 
 FIG. 1. Silver sheet floating on water. 
 
 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. 
 
 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 
 
LECTURE I 5 
 
 legs produces a dimple in the surface, but the pressure 
 on any one leg is not sufficient to break through the 
 skin. We can imitate this by bringing the point of a 
 lead pencil gently to the surface of water, when a 
 dimple is produced, but the skin is not actually pene- 
 trated. On removing the pencil, the dimple imme- 
 diately disappears, just as the depression caused by 
 pushing the finger into a stretched sheet of india- 
 rubber 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 ex- 
 ample, 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 ther- 
 mometer 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 curve/i (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 
 
6 LIQUID DROPS AND GLOBULES 
 
 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 en- 
 deavours 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, 
 
 FIG. 2. Column and index of minimum thermometer. 
 
 and its position indicates the lowest point attained by 
 the end of the column that is, the minimum temper- 
 ature. Obviously, a thermometer of this kind must 
 be mounted horizontally, to prevent the index falling 
 by its own weight. 
 
 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 
 
LECTURE I 7 
 
 silver foil through the petroleum, and allow it to reach 
 the surface of the water, we find that the elastic skin 
 is still capable of sustaining the weight ; and thus we see 
 that the elastic layer is present at the junction of the 
 two liquids. What is true of water and oil in this 
 respect also holds good for the boundary or interface 
 of any two liquids which do not mix. Evidently, 
 if the two liquids intermingled, there would be no 
 definite boundary between them ; and this would 
 be the case with water and alcohol, for example. 
 
 Area of Stretched Surface. We will not at 
 present discuss the nature of the forces which give rise 
 to this remarkable property of a liquid surface, but 
 will consider one of the effects. The tendency, as in 
 the case of all stretched membranes, will be to reduce 
 the area of the surface to a minimum. If we take a 
 disc of stretched indiarubber and place a weight upon 
 it, we cause a depression which increases the area of the 
 surface. But on removing the weight, the disc im- 
 mediately flattens out, and the surface is restored to 
 its original smallest dimension. Now, in practice, the 
 surface of a liquid is frequently prevented from attain- 
 ing the smallest possible area, owing to the contrary 
 action o: 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 
 
8 
 
 LIQUID DROPS AND GLOBULES 
 
 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 over- 
 powered by gravity ; but when 
 the column became thin, and 
 consequently less in weight, the 
 elastic force of the outer sur- 
 face was strong enough to over- 
 come 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.* 
 
 Shape of Detached Masses 
 FIG. 3. Thread of golden o f Liquid. Let us now pay a 
 
 syrup rising and form- ,..., .. J . * _. 
 
 ing a drop little attention to the small 
 
 drop of syrup Avhich 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 
 
LECTURE I 9 
 
 from the rod ; what shape would it take ? We know 
 that its surface, if not prevented by outside forces 
 from doing so, would become of minimum area. 
 Assuming such extraneous forces to be absent or coun- 
 terbalanced, what would then be the shape of the drop ? 
 It would be an exact sphere. For a sphere has a less 
 surf ace- 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. 
 
 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 pro- 
 jected 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 inter- 
 mediate size are more globular, but still show signs of 
 
10 LIQUID DROPS AND GLOBULES 
 
 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 ex- 
 planation. If we take a balloon of indiarubber filled 
 
 FIG. 4. Drops of different sizes resting on flat plate. 
 
 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 hal 
 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. 
 
 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 accom- 
 plished 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 
 
LECTURE I 11 
 
 fall to the ground were it not for the weight of the 
 extremely thin skin. The downward pull of gravity 
 on the air inside the bubble is balanced by the buoy- 
 ancy of the outside air ; and hence the skin, unham- 
 pered 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 com- 
 posing 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 ortkctoluidine, which pos- 
 sesses 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 appear- 
 ance 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 con- 
 
12 LIQUID DROPS AND GLOBULES 
 
 fcaining a 3 per cent, solution of common salt, and dis- 
 charge 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 orthoto- 
 luidine is contained in a vessel provided with a tap 
 
 FIG. 5. Formation of a sphere of orthotoluidine. 
 
 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 
 
LECTURE I 13 
 
 (Fig. 5). By using a graduated vessel, we can read 
 off the quantity of orthotoluidine which runs out, and 
 thus measure the volume of the sphere formed. When 
 the lower part reaches the layer of salt solution, we 
 raise the delivery tube gently, and repeat this as 
 
 FIG. 6. The detached sphere floating under water. 
 
 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 5f 
 centimetres, or 2J inches. To set it free in the water 
 we lift the delivery tube rapidly and there is the 
 
14 
 
 LIQUID DROPS AND GLOBULES 
 
 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. 
 
 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 
 
 FIG. 7. The Centrifugcscope 
 
 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 
 
LECTURE I 15 
 
 mass ; and even the asteroids are represented by the 
 numerous tiny spheres which are always torn off from 
 the main body of liquid along with the larger ones. 
 When the rotation ceases, the detached spheres sink, 
 and after a short time join the parent mass of orthoto- 
 luidine. 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 Ortho- 
 toluidine. 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 affectad 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 
 
16 
 
 LIQUID DROPS AND GLOBULES 
 
 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 tem- 
 perature, 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 1 0-988 
 
 104 
 
 40 
 
 0-9923 
 
 0-983 
 
LECTUKE 1 
 
 17 
 
 Other Examples of Equi -Density. There are 
 many other liquids which, like orthotoluidine, may be 
 heavier or lighter than water, according to tempera- 
 ture, and I now wish to bring to your notice the re- 
 markable 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 
 
 PIG. 8. Aniline drops falling through cold water and 
 ascending through hot water. 
 
 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 
 
18 LIQUID DROPS AND GLOBULES 
 
 are two tall beakers side by side, and above them a cis- 
 tern 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 pro- 
 vided 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 gradu- 
 ally 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 tempera- 
 ture 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 
 
LECTURE I 19 
 
 mentioned anisol ; butyl benzoate ; and aceto-acctic 
 ether ; but none of these possess the fine colour of 
 aniline or its chemical relative orthotoluidine, and in 
 addition are more costly liquids. Besides these are a 
 number of other liquids rarer still, practically only 
 known to the chemist, which behave in the same way. 
 These liquids are all carbon compounds, and more or 
 less oily in character. There is a simple rule which 
 may be used to predict whether any organic liquid 
 will be both lighter and heavier than water, according 
 to temperature. Here it is : If the density of the 
 liquid at 32 F. or C. be not greater than 1-12, the 
 liquid will become less dense than water below 21 2 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 over- 
 lying 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 
 
20 
 
 LIQUID DROPS AND GLOBULES 
 
 (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 
 
 FIG. 9. Aniline skins enveloping water. 
 
 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 a,t both ends, 
 
LECTURE I 21 
 
 and pass it down the vessel, until it reaches the aniline. 
 Water, in the meantime, has entered the tube, to the 
 same height as that at which it stands in the vessel. 
 On raising the tube gently, a .skin of aniline adheres 
 to the end ; and as we raise it still further, the water 
 in the tube, sinking so as to remain at the level in the 
 vessel, expands the skin into a sphere (Fig. 9) the 
 equivalent of a filled soap-bubble. On withdrawing 
 the tube gradually, the composite sphere is left hanging 
 from the surface of the water. 
 
 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 T | ^ of a gramme . If we choose 
 a line 1 inch long on the surface of water, the surface 
 tension is represented by about 3-jl r grains. It is always 
 
22 LIQUID DROPS AND GLOBULES 
 
 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 U. (59 ,F.), dynes 
 per cm. 
 
 Water .... 
 
 75 
 
 Aniline 
 Olive Oil 
 Chloroform 
 Alcohol 
 
 43 
 32 
 
 27 
 25 
 
 
 
 When one liquid is bounded by another, the inter facial 
 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. 
 
 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 pro- 
 jected downwards by the operation of this tension. 
 I pour some water into a narrow glass vessel, and float 
 
LECTURE I 23 
 
 upon it a liquid called dimethyl-aniline, so as to form 
 a layer about 1 inch in depth. A glass tube, open at 
 both ends, is now passed down the floating liquid into 
 the water, and then raised gradually, with the result 
 that a skin of water adheres to the end, and is inflated 
 by the upper liquid, forming a sphere on the end of 
 the tube (Fig. 10). On withdrawing the tube from 
 
 FIGS. 10, 11 and 12. The Diving Drop. Three stages. 
 
 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 inter- 
 face ; 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 
 
24 LIQUID DROPS AND GLOBULES 
 
 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 con- 
 tinuous 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 straighten- 
 ing 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 beauti- 
 ful 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 ; 
 
LECTURE I 25 
 
 but the process is so rapid that the details cannot be 
 observed. None of you saw, for example, that each 
 large drop after severance was followed by a small 
 droplet, formed from the narrowed portion from which 
 the main drop parted. But the small, secondary drop 
 is always present, and is called, in honour of its dis- 
 coverer, Plateau's spherule. Nor did any of you 
 observe that the large drop, immediately after separa- 
 tion, 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 proce- 
 dure, however, would be troublesome ; and evidently 
 
26 LIQUID DROPS AND GLOBULES 
 
 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 con- 
 nexion with liquid spheres indeed, we were able to 
 cancel the influence of gravity entirely, by surrounding 
 the working liquid by a second liquid of exactly equal 
 density. We require now, however, to allow the 
 downward pull of the drop ultimately to overcome 
 the surface tension, and we must therefore form the 
 drop in a less dense liquid. If this surrounding liquid 
 be only slightly less dense, we should be able to produce 
 a very large drop ; and if we make its growth slow we 
 may observe the whole process of formation and 
 separation with the unaided eye. 
 
 Now it so happens that we have to hand two liquids 
 which, without any preparation, fulfil our require- 
 ments. Orthotoluidine, at temperatures below 75 F. 
 
LECTURE I 
 
 27 
 
 or 24 C., is denser than water of equal temperature. 
 At 75 F. their densities are identical ; and as the 
 ordinary temperature of a room lies between 60 and 
 70 F., water, under the prevailing conditions, will be 
 slightly the less dense of the two, and will therefore 
 
 A 
 
 * 
 
 FIG. 13. Apparatus for forming ascending or descending 
 drops of liquids. 
 
 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 1^ inches. It 
 is best, when starting, to place the end of the stem 
 
28 LIQUID DROPS AND GLOBULES 
 
 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 
 (Pigs. 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 deli very- 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 surf ace 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 
 
LECTURE I 
 
 29 
 
 the liquid more viscous, the origin of the droplet will 
 be plainly seen, and the correctness of the description 
 verified. The recoil due to the liberation of the stretch- 
 ing 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 
 
 FIG. 14. 
 
 skin of indiarubber were stretched until it gave way 
 at the narrowest part, 
 
 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 aJl 
 
30 LIQUID DROPS AND GLOBULES 
 
 FIG. 17. 
 
 FIG. 18. 
 
 FIGS. J4 to 30. Formation of a drop of orthotoluidine, showing the 
 droplet. Seven stages,, 
 
LECTURE I 
 
 31 
 
 respects identical with what we have just seen on a large 
 scale. 
 
 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 
 
 FIG. 19, 
 
 FIG. 20. 
 
 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 tempera- 
 ture 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, Every- 
 
32 LIQUID DROPS AND GLOBULES 
 
 thing, 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, how- 
 ever, 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, 
 
 33 D 
 
34 
 
 LIQUID DROPS AND GLOBULES 
 
 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 
 
 lillBIBIIIBlBIBK^^^M ^^^^^W| 
 
 I 
 
 FIG. 21. 
 
 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 
 
LECTURE II 
 
 35 
 
 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 
 
 I 
 
 FIG. 22. 
 
 FIGS. 21 and 22. Automatically formed aniline drops, showing the 
 formation of droplets from the neck. 
 
 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, 
 
36 LIQUID DROPS AND GLOBULES 
 
 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 re- 
 union goes on indefinitely. The action is automatic 
 and continuou , 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. 
 
 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 
 
LECTURE II 37 
 
 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 170F. (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 nitro- 
 benzene in nitric acid of specific gravity 1-2 at 59F. 
 (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 
 
38 LIQUID DROPS AND GLOBULES 
 
 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 forma- 
 tion, by allowing the liquid to flow into a medium only 
 slightly inferior in density. For this purpose, orthotolui- 
 dine falling into water at the ordinary room tempera- 
 ture 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 
 
LECTURE II 39 
 
 further opening the tap the drops form at a still greater 
 distance from the end of the pipe, and succeed each 
 
 FIG. 23. FIG. 24. FIG. 25. 
 
 FIGS. 23, 24, 25. Jets of Orthotoluidine, discharged under 
 
 water. 
 
 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 
 
40 LIQUID DROPS AND GLOBULES 
 
 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. 
 
 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 
 
 FIG. 26. Water stretched between a tube and a plate. 
 
 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 dia- 
 meter 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 
 
LECTURE II 41 
 
 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 
 
42 LIQUID DROPS AND GLOBULES 
 
 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. 
 
 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 dia- 
 meter of 2 inches ; and here again, in contrast, is a 
 column of aceto-acetic ether, the surface diameter 
 of which is only 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 
 
LECTURE II 
 
 43 
 
 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 
 
 FIG. 27. 
 
 FIG. 28. 
 
 FIG. 29. 
 
 FIG. 
 
 FIGS. 27, 28, 29, 30. A liquid column stretched upwards until broken 
 by addition of water. Four stages. 
 
 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 
 
44 LIQUID DROPS AND GLOBULES 
 
 obtain useful ideas from a study of liquid columns, 
 which present an almost limitless field for the practical 
 observation of curved forms. 
 
 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 
 
LECTURE II 
 
 45 
 
 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 curvatire, and we may 
 therefore state that the pressure ex- 
 erted by the walls of the bubble on the 
 interior vary directly as the curvature. 
 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 clos- 
 ing after suitable drops have been formed. Then, by 
 opening the tap on the horizontal cross-piece, we 
 
 FIG. 32. 
 Apparatus for 
 com m u nicating 
 drops, with ex- 
 tensions of un- 
 equal length 
 attached. 
 
46 LIQUID DROPS AND GLOBULES 
 
 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 
 
LECTURE II 47 
 
 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. 
 
 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 fce 
 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 
 
48 LIQUID DROPS AND GLOBULES 
 
 of chloroform l 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 experi- 
 ment. 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 
 
 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. 
 
LECTURE II 
 
 49 
 
 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. 
 
 
 
 
 3 
 
 9 
 
 
 
 
 
 o fl O 
 
 
 
 
 
 X 
 
 8 
 
 
 6 
 
 e e 
 
 Q 
 
 ) n 
 
 n_ 
 
 Pfe^^iE^:, 
 
 FIG. 33. Combined drops of vapour and liquid, 
 
 Condensation of Drops from Vapour, Mists, 
 Fogs and Raindrops. The atmosphere is the great 
 laboratory for the manufacture of drops. It is con- 
 tinually 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 
 
 E 
 
50 
 
 LIQUID DROPS AND GLOBULES 
 
 the surface, and to this never-ceasing exchange all the 
 phenomena connected with the precipitation of mois- 
 ture 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 'in visible moisture. When fully charged, 
 the atmosphere is said to be "saturated" a condi- 
 tion 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 tempera- 
 tures is shown in the table : 
 
 Temperature. 
 
 Weight of water vapour 
 (grammes) required to satu- 
 rate 1 cubic metre. 
 
 Deg. C. 
 
 Deg. F. 
 
 
 
 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, how- 
 ever, is usually below the maximum, and is recorded 
 
LECTURE II 51 
 
 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 (1 6 9-3 ) = 
 6-7 grammes would separate out, as the maximum con- 
 tent at the lower temperature is 9-3 grammes. Precipita- 
 tion would commence at 66 F., at which temperature 1 
 cubic metre is saturated by 1 6 grammes. And similarly 
 for all states of the atmosphere with respect to mois- 
 ture, cooling to a sufficient extent causes deposition of 
 water to commence immediately below the saturation 
 temperature, and the colder the air becomes after- 
 wards 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 separa- 
 tion 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 
 at lira, ted. Now a compressed gas, on expanding into 
 
52 LIQUID DROPS AND GLOBULES 
 
 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 su.face, 
 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 tempera- 
 ture 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 
 
LECTURE II 53 
 
 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 pro- 
 bably due to the aid of ions. 1 Near the surface, 
 however, where dust is ever present, condensation 
 round the innumerable specks or motes is the rule. 
 
 1 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. 
 
54 LIQUID DROPS AND GLOBULES 
 
 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 con- 
 densation 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 preci- 
 pitation 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 thunder- 
 storm, 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. 
 
 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 
 
LECTURE II 55 
 
 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 through- 
 out ; 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 
 
56 LIQUID DROPS AND GLOBULES 
 
 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 surround- 
 ing 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 
 
LECTURE II 57 
 
 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 im- 
 mediately, with a rapid generation of steam. A com- 
 plete 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. 
 
 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 evapor- 
 ate. 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 ex- 
 pected 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 
 
58 LIQUID DROPS AND GLOBULES 
 
 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 con- 
 nect one terminal to the plate and place the other in 
 the drop. No movement is shown on the galvano- 
 
 FIG. 34. Spheroid of water on a hot plate. 
 
 meter, 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 galvano- 
 
LECTURE II 59 
 
 meter ; 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 satisfac- 
 tory 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, how- 
 ever, and contaminated, as it were, the surface of the 
 water, the surface tension is reduced, and is not suffi- 
 ciently strong to stretch out a further quantity of oil, 
 which, if added, remains in the form of a floating 
 globule. 
 
 Let us study the forces at work on the floating 
 globule a little more closely. Its upper surface is in 
 
 60 
 
LECTURE III 61 
 
 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 
 
 FIG. 35. Forces acting on a floating globule. 
 
 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 " ten- 
 sion, 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 sur- 
 face, 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 wa,ter 
 
62 LIQUID DROPS AND GLOBULES 
 
 tension is to cause the globule to spread out in all 
 directions, whereas the other two tensions tend to pre- 
 vent 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, arid 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. More- 
 over, 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 
 
LECTURE III 63 
 
 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. 1 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. 
 
 Movements of Aniline Globules on a Water Sur- 
 face. If we allow a small quantity 6f 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 
 
 1 These movements were first recorded by Romieu in 1748 
 apd were ascribed by him to electricity. 
 
64 LIQUID DROPS AND GLOBULES 
 
 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 
 
 FIG. 36. Aniline globules on a water surface. 
 
 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 s,kin of Aniline, This action is in 
 
LECTURE III 65 
 
 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 ten- 
 sion of the water, acting horizontally, tends to stretch 
 the globule, and is successful momentarily in over- 
 coming the opposing tensions, each of which pulls at an 
 angle to the surface. Enlargement of the upper sur- 
 face 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 move- 
 ments, and why these movements are spasmodic 
 instead of gradual, has not been clearly made out. It 
 
66 
 
 LIQUID DROPS AND GLOBULES 
 
 is interesting to recall that a spheroid of liquid on a hot 
 plate also possesses a scalloped edge, and it maybe 
 that the two phenomena have something in common. 
 
 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 
 
 FIG. 37. Orthotoluidine globules on a water surface. 
 
 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 
 
LECTURE III 67 
 
 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 remark- 
 able 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 
 
68 LIQUID DROPS AND GLOBULES 
 
 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 move- 
 ment, and its simultaneous occurrence over the whole 
 
 FIG. 38. Resolution of a floating skin into globules. 
 
 surface. We must therefore leave the problem at 
 present, and trust to future investigation to provide 
 a solution. 
 
 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 
 
LECTURE III 69 
 
 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, inden- 
 tations 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 numer- 
 ous 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 in- 
 stance the film is broken up in sections, but the action 
 continues until nothing but globules remain on the 
 surface. 1 
 
 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 pre- 
 
 1 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." 
 
70 LIQUID DROPS AND GLOBULES 
 
 ference 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. 
 
 Network formed from a Film. A further exam- 
 ple 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 
 
 FIG. 39. rNetwork formed from a film of tar-oil on the 
 surface of water. 
 
 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 per- 
 
LECTURE III 71 
 
 foration, which may be due to inequalities in the 
 thickness of the film, or in the distribution of the strain 
 to which it is subjected. 1 
 
 Quinoline Rings. Reference has already been 
 made to the breaking-up of a quinoline film into glo- 
 bules. 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 
 
 FIG. 40. Quinoline rings and perforated plates. 
 
 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. 
 
 Expanding Globules. I now wish to show, by an 
 
 1 An interesting discussion on cellular structures of this 
 type may be found in Nature, April 16 to June 11, 1914. 
 
72 LIQUID DROPS AND GLOBULES 
 
 experiment, how sensitive a floating globule is to dis- 
 turbances 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 sur- 
 face, 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 
 
 FIG. 41. The expanding globule. 
 
 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 
 
LECTURE III 73 
 
 instance I float on the water surface a globule of lubri- 
 cating oil, \\ ith 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 ten- 
 dency to coalesce is sometimes noticed, but is by no 
 means general. I will show one example which pos- 
 sesses striking features, showing as it does the re- 
 markable results which may be brought about by sur- 
 face 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 char- 
 acteristic 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 direc- 
 
74 LIQUID DROPS AND GLOBULES 
 
 tion 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, convey- 
 ing with it the entangled orthotoluidine. This, how- 
 ever, by no means satisfies the devouring globule, as a 
 second victim is at once appropriated in the same 
 
 FIG. 42. The "devouring" globule. Five stages. 
 
 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 inter- 
 
LECTURE III 75 
 
 action 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 
 
 FIG. 43. Photograph of one globule absorbing another. 
 
 not common, it being more usual for the scattered units 
 to remain apart. 
 
 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 
 
76 LIQUID DROPS AND GLOBULES 
 
 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 ten- 
 sions. In our experiments we have witnessed expan- 
 sive 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 applica- 
 tion ; but science has other uses in addition to its 
 fruitful alliance with commerce. The study of the 
 
LECTURE III 77 
 
 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 wonder- 
 ful 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 remem- 
 bered, however, that a beaker containing water be- 
 haves 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 recom- 
 mended. 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 with- 
 out 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 
 
 78 
 
APPENDIX 79 
 
 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 ob- 
 served by warming the water in the vessel. The cen- 
 trifugoscope (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.G. 
 
 Experiments with skins and globules may be con- 
 ducted 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. 
 
kr APPENDIX 
 
 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 con- 
 ducted 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 pheno- 
 mena to an audience, a lantern may be used to advan- 
 tage. 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 pro- 
 jection 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 satis c actory 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 
 
 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. 
 
 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 
 
 81 G 
 
82 INDEX 
 
 E PAGE 
 
 Elastic skin of liquids ...... 5 
 
 Equi-density temperatures . . . 16, 17, 19 
 
 Ethyl benzoate, columns of . . . . .42 
 
 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 
 
 Interfacial tension . . . . . . 22, 61 
 
 Ions, condensation on ...... 53 
 
 Jets of liquid ........ 38 
 
 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 
 
 Monobrom-benzene . . . . . . .48 
 
 N 
 Network formed from film ...... 70 
 
 Nitrobenzene, drops of ..... 29, 37 
 
 films . 69 
 
INDEX 83 
 
 O PAGE 
 
 Orthotoluidine columns . . . . . .42 
 
 drops . . . . . .27 
 
 equi-density temperature of . . .16 
 
 globules, movements of ... 66 
 
 jets 39 
 
 spheres . . . . . 11, 14 
 
 Petroleum, boundary surface with water ... 6 
 
 Plateau's spherule . . . . . . .25 
 
 Q 
 
 Quinoline, formation of globules of . . ,69 
 
 ,, rings of . . . . . .71 
 
 R 
 Raindrops ........ 54 
 
 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 
 
 (PK1266) 
 
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SCIENTIFIC BOOKS. 19 
 
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SCIENTIFIC BOOKS. 23 
 
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SCIENTIFIC BOOKS. 27 
 
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SCIENTIFIC BOOKS. 33 
 
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SCIENTIFIC BOOKS. 35 
 
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SCIENTIFIC BOOKS. 37 
 
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SCIENTIFIC BOOKS. 43 
 
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SCIENTIFIC BOOKS. 49 
 
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