Automatic Aniline Drops.—In the foregoing experiments the drop was enlarged until it broke away by feeding it with liquid; but it is possible to arrange that the formation shall be quite automatic. The experiment, as we shall see, is extremely simple, and yet it contains an element of surprise. Into a beaker containing water nearly boiling I pour a considerable quantity of aniline, which at first breaks up into a large number of drops. After a short time, however, all the aniline floats to the surface, having been warmed by contact with the water to a temperature higher than that of equi-density (147°F., or 64°C.)—which is exactly what we should expect to happen. There it remains for a brief period in the form of a large mass with the lower portion curved in outline. Soon, however, we observe the centre of the mass sinking in the water, and taking on the now familiar outline of a falling drop. Gradually, it narrows at the neck and breaks away; but as aniline is a viscous liquid, the neck in this case is long and therefore easily seen. The large drop breaks away and falls to the bottom of the beaker, its upper surface rising and falling for some time owing to the recoil of its skin after separation, [pg 34] finally becoming permanently convex. Immediately after the large drop has parted, the upper mass shrinks upwards, spreading out further on the surface of the water, with the result that the long neck is severed at the top, its own weight assisting the breakage. Now follows the resolution of the detached neck into two or more spheres, usually a large and a small (Fig. 22). And now, to those who view the experiment for the first time, comes the surprise. The large drop, which [pg 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 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, [pg 36] like its predecessor, rises to the surface, after remaining for a short time at the bottom of the vessel. And so long as we keep the temperature a few degrees above that of equi-density, the process of partition and reunion goes on indefinitely. The action is automatic and continuous, and the large size of the drop and of the neck, and the slowness of the procedure, enables us to follow with ease every stage in the formation of a parting drop.

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 [pg 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 170°F. (77°C.), but as this liquid is fairly soluble in hot water further quantities must be added during the progress of the experiment. Results equal to those obtained with aniline, however, may be secured by using nitrobenzene in nitric acid of specific gravity 1·2 at 59°F. (15°C.), the acid being heated to 185°F. (85°C.); and here you see the yellow drop performing its alternate ascents and descents exactly as in the case of aniline and water. Other examples might be given; but we may take it as a general rule that when the equi-density temperature of the liquid and medium is above [pg 38] 125°F. (52°C.), the phenomenon of the automatic drop may usually be observed when the temperature is raised by 30°F. (17°C.), above this point.

Liquid Jets.—So far we have been observing the formation of single drops, growing slowly at the end of a tube, or breaking away from a large mass of the floating liquid. If, however, we accelerate the speed at which the liquid escapes, the drop has no time to form at the outlet, and a jet is then formed. We are all familiar with a jet of water escaping from a tap; it consists of an unbroken column of the liquid up to a certain distance, depending upon the pressure, but the lower part is broken up into a large number of drops, which break away from the column at a definite distance from the tap. There are many remarkable features about jets which I do not intend to discuss here, as it is only intended to consider the manner in which the drops at the end are formed. To observe this procedure, it is necessary again to resort to our method of slowing down the rate of formation, by allowing the liquid to flow into a medium only slightly inferior in density. For this purpose, orthotoluidine falling into water at the ordinary room temperature is eminently satisfactory; and we see on the screen the projection of a pipe, with its end under water, placed so that a jet of orthotoluidine may be discharged vertically downwards from a stoppered funnel. I open the tap slightly at first, and we then merely form a single drop at the end. Now it is opened more widely, and you observe that the drop breaks away some distance below the outlet, being rapidly succeeded by another and another (Fig. 23). On still [pg 39] further opening the tap the drops form at a still greater distance from the end of the pipe, and succeed each other more rapidly, so that quite a number appear in view at any given moment (Figs. 24 and 25). Notice how the drop is distorted by breaking away from the [pg 40] 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 on the screen, and then bring up a flat plate of glass until it just touches the suspended drop. As soon as contact is established, the water spreads outwards over the plate, causing the drop to contract in diameter at or near its middle part, so that its outline resembles that of a capstan (Fig. 26). The end of the glass tube is now connected to the plate by a column of water of curved outline, which is quite stable if undisturbed. If, however, I gradually raise the tube, or lower the plate, the narrow part of the column becomes still narrower, and finally breaks across. In the same way we may produce columns of other [pg 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 [pg 42] 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 diameter of 2 inches; and here again, in contrast, is a column of aceto-acetic ether, the surface diameter of which is only ½ inch (Fig. 31). Other liquids, such as aniline, give an intermediate result. The lower diameter is determined by the width of the vessel; and hence we are able to produce an almost endless [pg 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 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 [pg 44] obtain useful ideas from a study of liquid columns, which present an almost limitless field for the practical observation of curved forms.

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 [pg 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 curvature, and we may therefore state that the pressure exerted 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 closing after suitable drops have been formed. Then, by opening the tap on the horizontal cross-piece, we [pg 46] 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 [pg 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 be familiar with this process, but it should be remembered that a liquid may give off large quantities of vapour without actually boiling. A dish of cold water, if exposed to the air, will gradually evaporate away; whilst other liquids, such as petrol and alcohol, will disappear rapidly under the same circumstances—and hence are called “volatile” liquids.

The formation of vapour and its subsequent escape at the surface of the liquid, enable us to produce a very novel kind of drop; if, instead of allowing the bubbles to escape into air, we cause them to enter a second liquid. Here, for example, is a coloured layer [pg 48] of chloroform¹ at the bottom of a beaker, with a column of water above. I project the image of the beaker on the screen, and then heat it below. The chloroform vapour escapes in bubbles; but notice that each bubble carries with it a quantity of liquid, torn off, as it were, at the moment of separation. The vapour bubbles and their liquid appendages vary in size, but some of them, you observe, have an average density about equal to that of the water, and float about like weighted balloons. Some rise nearly to the surface, where the water is coldest; and then the vapour partially condenses, with the result that its lifting power is diminished, and hence the drops sink into the lower part of the beaker. But the water is warmer in this region, and consequently the vapour bubble increases in size and lifting power until again able to lift its load to the surface. So the composite drops go up and down, until finally they reach the surface, when the vapour passes into the air, and the suspended liquid falls back to the mass at the bottom of the beaker. Notice that the drop of liquid attached to each bubble is elongated vertically. This is because chloroform is a much denser liquid than water (Fig. 33). There is a practical lesson to be drawn from this experiment. Whenever a bubble of vapour breaks through the surface of a liquid, it tends to carry with it some of the liquid, which is dragged mechanically into the space above. In our experiment the space was occupied by water, which enabled the bubble to detach [pg 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.

Condensation of Drops from Vapour,—Mists, Fogs and Raindrops.—The atmosphere is the great laboratory for the manufacture of drops. It is continually receiving water in the form of vapour from the surface of the sea, from lakes, from running water, and even from snow and ice. All this vapour is ultimately turned into drops, and returned again to [pg 50] the surface, and to this never-ceasing exchange all the phenomena connected with the precipitation of moisture are due. The atmosphere is only capable of holding a certain quantity of water in the form of vapour, and the lower the temperature the less the capacity for invisible moisture. When fully charged, the atmosphere is said to be “saturated”—a condition realized on the small scale by air in a corked bottle containing some water, which evaporates until the air can hold no more. The maximum weight of vapour that can be held by 1 cubic metre of air at different temperatures is shown in the table:—

Temperature.		Weight of water vapour (grammes) required to saturate 1 cubic metre.
Deg. C.	Deg. F.
0	32	4·8
5	41	6·8
10	50	9·3
15	59	12·7
20	68	17·1
25	77	22·8
30	86	30·0
35	95	39·2
40	104	50·6

It will be seen from the table that air on a warm day in summer, with a temperature of 77°F., can hold nearly five times as much moisture as air at the freezing point, or 32°F. The amount actually present, however, is usually below the maximum, and is recorded [pg 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 (16-9·3) = 6·7 grammes would separate out, as the maximum content at the lower temperature is 9·3 grammes. Precipitation would commence at 66°F., at which temperature 1 cubic metre is saturated by 16 grammes. And similarly for all states of the atmosphere with respect to moisture, cooling to a sufficient extent causes deposition of water to commence immediately below the saturation temperature, and the colder the air becomes afterwards the greater the amount which settles out. The temperature at which deposition commences is called the “dew point.”

Whenever atmospheric moisture assumes the liquid form, drops are invariably formed. These may vary in size, from the tiny spheres which form a mist to the large raindrops which accompany a thunderstorm. But in every instance it is necessary that the air shall be cooled below its saturation point before the separation can commence; and keeping this fact in mind we can now proceed to demonstrate the production of mists and fogs. Here is a large flask containing some water, fitted with a cork through which is passed a glass tube provided with a tap. I pump some air into it with a bicycle pump, and then close the tap. As excess of water is present, the enclosed air will be saturated. Now a compressed gas, on expanding into [pg 52] the atmosphere, does work, and is therefore cooled; and consequently if I open the tap the air in the flask will be cooled, and as it was already saturated the result of cooling will be to cause some of the moisture to liquefy. Accordingly, when I open the tap, the interior of the flask immediately becomes filled with mist. If we examine the mist in a strong light by the aid of a magnifying glass, we observe that it consists of myriads of tiny spheres of water, which float in the air, and only subside very gradually, owing to the friction between their surfaces and the surrounding air preventing a rapid fall. The smaller the sphere, the greater the area of surface in proportion to mass, and therefore the slower its fall. And so in nature, the mists are formed by the cooling of the atmosphere by contact with the surface, until, after the saturation point is reached, the surplus moisture settles out in the form of tiny spheres, which float near the surface, and are dissipated when the sun warms up the ground and the misty air, and thus enables the water again to be held as vapour.

Fogs, like mists, are composed of small spheres of water condensed from the atmosphere by cooling; but in these unwelcome visitors the region of cooling extends to a higher level, and the lowering of temperature is due to other causes than contact with the cold surface of the earth. In our populous cities, the density of the fogs is accentuated by the presence of large quantities of solid particles in the atmosphere, which arise from the smoke from coal fires, and the abrasion of the roads by traffic. We can make a city fog in our flask. I blow in some tobacco smoke, and [pg 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 probably due to the aid of ions.² Near the surface, however, where dust is ever present, condensation round the innumerable specks or motes is the rule. [pg 54] Here, for example, is a jet of steam escaping into air, forming a white cloud composed of a multitude of small spheres of condensed water. If now I allow the steam to enter a large flask containing air from which the dust has been removed by filtration through cotton wool, no cloud is formed in the interior, but instead condensation takes place at the end of the jet, from which large drops fall, and on the cold sides of the flask. The cloud we see in dusty air is entirely absent, and the effect of solid particles in the process of condensation is thus shown in a striking manner. Clouds are masses of thick mist floating at varying heights in the atmosphere. On sinking into a warmer layer of dry air the particles of which clouds are composed will evaporate and vanish from sight. If the condensation continue, however, the spheres will grow in size until the friction of the atmosphere is unable to arrest their fall; and then we have rain. And whether the precipitation be very gentle, and composed of small drops falling slowly, as in a “Scotch mist,” or in the form of rapid-falling large drops such as accompany a thunderstorm, the processes at work are identical. Every particle of a mist or cloud, and every raindrop, is formed round a nucleus, and owes its spherical shape to the tension at the surface.

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 [pg 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 throughout; and now the streaks become permanent, and the water becomes opaque, owing to the thick white cloud of precipitated aniline. The absence of the red colour characteristic of aniline is due to the extremely fine state of division assumed in the process. If left for some hours, the white cloud sinks through the water to the bottom of the beaker, where the small particles coalesce and form large drops, leaving the overlying water quite transparent. The process is quite analogous to the precipitation of moisture from the atmosphere in the form of small spheres, which, if undisturbed, would gradually sink to the ground and leave the air clear.

Overheated Drops.—The temperature at which a liquid boils, under normal conditions, depends only upon the pressure on its surface. Thus water boiling in air, when the pressure is 76 centimetres or 29·92 inches of mercury, corresponding to 14·7 pounds per square inch, possesses a temperature of 100°C. or 212°F. At higher elevations, where the pressure is less, the boiling point is lower; thus Tyndall observed that on the summit of the Finsteraarhorn (14,000 feet) water [pg 56] boiled at 86°C. or 187°F. Conversely, under increased pressure, the boiling point rises; so that at a pressure of 35 pounds per square inch water does not boil until the temperature reaches 125°C. or 257°F. There are certain abnormal conditions, however, under which the boiling point of a liquid may be raised considerably without any increase in the pressure at the surface; and it is then said to be “over-heated.” Dufour showed that when drops of water are floating in another liquid of the same density, they may become greatly overheated, and if very small in size may attain a temperature of 150°C. or 302°F., or even higher, before bursting into steam. In order to provide a medium in which water drops would float at these temperatures, Dufour made a mixture of linseed oil and oil of cloves, which possessed the necessary equi-density temperature with water. To demonstrate this curious phenomenon, I take a mixture of 4 volumes of ethyl benzoate and 1 volume of aniline, which at 125°C. or 257°F. is exactly equal in density to water at the same temperature. I add to the mixture two or three cubic centimetres of freshly-boiled water, the temperature being maintained at 125°C. by surrounding the vessel with glycerine heated by a flame. At first the water sinks, but on attaining the temperature of the mixture it breaks up with some violence, forming spheres of various sizes which remain suspended in the mixture. Any portion of the water which has reached the surface boils vigorously, and escapes in the form of steam; and some of the larger spheres may be observed to be giving off steam, which rises to the surface. Most of the spheres, however, remain [pg 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 immediately, with a rapid generation of steam. A complete explanation of this abnormal condition of water is difficult to follow, as a number of factors are involved. One of the contributory causes—though possibly a minor one—is the opposition offered to the liberation of steam by the tension at the surface of the spheres.

Floating Drops on Hot Surfaces.—If a liquid be allowed to fall in small quantity on to a very hot solid, it does not spread out over the surface, but forms into drops which run about and gradually evaporate. By careful procedure, we may form a very large, flattened drop on a hot surface, and on investigation we shall notice some remarkable facts. I take a plate of aluminium, with a dimple in the centre, and make it very hot by means of a burner. You see the upper surface of this plate projected on the screen. I now allow water to fall on the plate drop by drop, and you hear a hissing noise produced when each drop strikes the plate. The separate drops gather together in the depression at the centre of the plate, forming a very large flattened globule. You might have expected the water to boil vigorously, but no signs of ebullition are visible; and what is more remarkable, the temperature of the drop, in spite of its surroundings, is actually less than the ordinary boiling point. Notice now how the drop has commenced to rotate, and has been set into vibration, causing the edges to become scalloped (Fig. 34). The drop, although not actually [pg 58] boiling, is giving off vapour rapidly, and therefore gradually diminishes in size. And now I want to prove that the drop is not really touching the plate, but floating above it. To do this I make an electric circuit containing a cell and galvanometer, and connect one terminal to the plate and place the other in the drop. No movement is shown on the galvanometer, as would be the case if the drop touched the plate and thus completed the electric circuit. And at close range we can actually see a gap between the drop and the plate, so that the evidence is conclusive. If now I remove the flame—leaving the electric circuit intact—and allow the plate to cool, we notice after a time that the globule flattens out suddenly and touches the plate, as shown by the deflection of the galvanometer; [pg 59] and simultaneously a large cloud of steam arises, due to the rapid boiling which occurs immediately contact is made.

What we have seen in the case of water is shown by most liquids when presented to a surface possessing a temperature much higher than the boiling point of the liquid. A liquid held up in this manner above a hot surface is said to be in the spheroidal state, to distinguish it from the flat state usually assumed by spreading when contact occurs between the liquid and the surface. It is doubtful whether any satisfactory explanation of the spheroidal state has ever been given. Evidently, the layer of vapour between the plate and the drop must exert a considerable upward pressure in order to sustain the drop, but the exact origin of this pressure is difficult to trace.

[pg 60]