Chapter III: The Nature of Liquids

THE difference between a gas and a liquid is that in the former the atoms and molecules move to and fro in an independent existence, whereas in the latter they are always in touch with one another though they are changing partners continually. In the rivalry between motion and attractive forces the motion is no longer in complete control: the attractive forces have now sufficient power to keep the general body of molecules in touch with one another, or at least so many of them so that they form a definite volume of liquid, having a surface that we can see. Yet the control of the attractive forces is not absolute: there is a continual process which we call evaporation. Suppose a bowl of water to be placed in an empty room. The molecules of the water are all in movement--vibrating, turning, shifting, and changing partners all the time. But their motion is not enough to make them break away from one another, except at the surface, where the conditions are of a special nature. In consequence the molecules hold together as a body having a definite volume, and there are boundaries to that volume. Only at the surface there are breakaways: in the constant interchange of motion it will happen that some of the outlying molecules have impulses given to them which are big enough to break their connection with the molecules below, and they leave the surface for good. If this had happened to molecules within the liquid, they would have been recaptured. Thus the room in which the bowl of water has been placed will contain a gradually increasing number of water molecules flying about independently as a gas. If the room is closed, the increase will not go on forever, because there will come a time when the number of free water molecules in the room is so great that molecules strike the surface of the water and re-enter it as fast as others leave it. The room has become saturated with water vapor. That may happen before the bowl is empty; but if the air in the room is continuously removed, carrying the water vapor with it, the water in the bowl will all evaporate in time.

The molecules that leave the surface will always be those that possess more than the average amount of energy, part of which they spend in tearing themselves away from their fellows; the average energy of the main body will fall steadily as evaporation proceeds. In other words, the water becomes colder and colder. We all know this effect well. If we wave our hands when they are wet we feel the chill: we are, in fact, using somewhat excessively a process which nature employs to cool our bodies to the proper temperature. Our bodies are called on to make good the excess of energy which the evaporating molecules have carried away with them. The rate of chilling may be increased by the use of a liquid which evaporates more rapidly than water; so, for example, the surgeon at one time used an ether spray to cause local freezing. In hot, dry countries drinking-water is cooled by putting it into a bag made of porous canvas, which is hung so as to be shaded from the sun but exposed to the wind, and the hotter and drier the wind the better.

The lower bulb, which is empty, is immersed in liquid air. The upper bulb contains water which quickly freezes.]

In the hot Australian summer it is usual to see the bag hanging under the veranda of the house or the roof of the railway station of a country township. The water leaks through the canvas and is quickly evaporated by the passing air, so that the water which is left grows cool.

We can carry out the experiment in a very striking manner on the lecture-room table. The two bulbs shown in the figure contain water only, no air. The water is first brought to the upper bulb, and the lower is then immersed in liquid air; in two or three minutes the water is frozen, although the upper bulb has been nowhere near the liquid air. The explanation is that the water molecules which fling themselves from the surface of the water make their way down the tube and so to the lower bulb. This would happen whether or no there were any liquid air round the lower vessel; but then they would come back again, most of them at least, and return to the water carrying their superabundant energy with them. Thus the water would be very little cooled. If, however, the lower bulb is reduced in temperature by the liquid air, the molecules do not return. Their motion is taken away from them and they collect first as water and then as ice in the lower bulb. The water in the upper bulb is rapidly cooled, and soon frozen. The reason for removing the air from the bulbs is that it is necessary to give the water molecules a clear road, so that the evaporation may take place quickly. If the operation were too slow, heat would leak in from the outside air at such a rate that the freezing would not take place. The presence of the air does not stop the energetic molecules from leaving the surface, but it hampers their subsequent movements, reducing the action to the process of diffusion which we have already considered.

When a liquid boils, the temperature has been raised to such a pitch that the evaporating molecules are sufficient in number and speed to lift off the air from the surface of the liquid and push it back _en masse_. It is no longer the case that the individual molecules have to thread their way through a crowd. The whole process is so strikingly different in appearance from that of evaporation that the essential similarity is apt to be overlooked. The temperature at which a liquid boils depends on the pressure which the evaporating molecules have to overcome: at the top of Mount Blanc boiling water is 27° F. cooler than it is at the base.

The heat that is wanted if a liquid is to be evaporated is a measure of the energy required to tear the molecules away from one another. Perhaps that does not impress the mind with a sense of the importance of these forces, which, though individually minute, are so powerful in the gross. We may, however, remind ourselves of the heat required to convert water into steam and of the amount of work that the steam can do. The forces are manifested to us more directly in every hanging drop of water or other liquid. The molecules are clinging to one another like bees in a swarm. The links with which the molecules of the last layer are attached to the surface from which the drop is hanging are carrying the whole weight of the drop.

Again the impression of the magnitude and importance of the forces is not fairly conveyed by this simple effect; but the experiment can be developed into a more impressive form.

Here is a bent glass tube containing water and no air. The water is made to fill one limb entirely; if a little bubble of air is to be found in it, it must be made to pass over into the other by holding the tube in a suitable position and gently tapping it on the table. When this has been done, the tube can be held so that the level of the free end of the water column is far below the level of the other, where it is clinging to the end of the glass tube. The weight of the excess column on one side is all borne by the attachment of the water molecules to the glass tube at the other side, and of other water molecules to them. In fact, we have a drop of water about a foot long. We cannot make a drop of this length hang from a finger, for the reason that the water can break away by changing its shape. If that is prevented, as it is in the glass tube, the magnitude of these molecular forces is more obvious. When we try to stretch a bar of iron, the great difficulty makes us realize the magnitude of the forces that keep the molecules of solid iron together; we are apt to think that it is easy to stretch a mass of water, but it is not so. It is easy to make the water change its shape, but not to pull a layer of molecules directly away from another with which it is in contact. Water is, in reality, just as hard to stretch as to compress.

We may here make a little digression from our main line of argument, because we come across a curious effect while we handle this bent tube. If the tube is tilted so that the water runs along the tube and is brought up sharply at one end, the sound of a blow is heard, as of two hard substances striking each other; the blow is felt by the hand that holds the tribe. The effect is sometimes described as an example of water hammer; the explanation is simple enough. There is no air in the tube, and the water strikes the end of the tube as if it were a rigid body; and indeed it behaves as such because it is as incompressible. It is necessary to be very careful in the handling of the tubes, because it is so easy to knock out an end: it is just as if we struck a glass vessel with a hammer. A very curious, and as it happens very serious, example of this effect has manifested itself of recent years in the wearing away of the propellers of ships that are driven by rapidly turning screws. The illustration shows the erosion in a propeller blade of the Mauretania (Plate IX a). The effect first appeared when the adoption of the Parsons steam turbine increased the rate of revolution of the screw and boats began to move faster, and it was at first the cause of great loss, financial and otherwise. The explanation was found in the fact that the steamship was advancing so fast and the screws revolving so rapidly that the water could not fill up entirely the holes that the blades left behind them.

A.

The right-hand figure shows erosion in a portion of an early propeller blade of the “Mauretania.” Notice the small bite in the edge of the blade shown in full in the left-hand figure.

B. Photograph of the cavitations formed by a propeller in the experimental tank of the Parsons Marine Steam Turbine Co. The propeller is the disc-shaped object at the left centre of the photograph. There are three blades on it, and each is leaving a corkscrew line of bubbles in the water which goes past the blade. We can see where the spirals begin at the screw. (_Reproduced by courtesy of the Manganese Bronze and Brass Co., Ltd._)]

The illustration shows the cavities as they are formed in a model tank in the Turbinia Works at Newcastle (Plate IX b). They are arranged in spirals; we can trace in the figure the spiral belonging to each of the propeller blades.

Now these cavities close up under the pressure of the surrounding water, and since there is no air in them, the sides of the cavity strike one another as hard and smartly, when they come together, as the water in our tube could be made to strike the glass. If part of the propeller blade forms part of a cavity wall, the blow may be so great as to tear away pieces of the metal. It has cost much labor to arrive at the full explanation and to provide a cure; propeller blades are now made of an alloy specially designed to withstand erosion, and at the same time the design of the blade has been improved. A very striking experiment made in the course of these researches is illustrated in Fig. 15. The strong metal vessel shown in the center of the figure is filled with water and allowed to fall to the bottom of a tank, also full of water, where its motion is suddenly stopped. The momentum of the water in the cone aided by that of the heavy weight _W_ is sufficient to make the water carry on its motion and leave a cavity at the top of the cone at _V_. This fills up again immediately afterward, on account of the pressure of the surrounding water, and as it does so the water in the cone, increasing its velocity as it rushes up into the narrowing space, strikes the top of the cavity so hard that it punches a hole in the brass plate inserted at _P_.

The collapsing of the cavities formed by the screw makes quite a noise in the water, so that a ship can be heard at great distances by the use of an underwater receiver of sound.

Many of my readers will be familiar with a less serious example of the blow that a mass of water can give because of its inelasticity: it hurts considerably if one dives from any height and does not make a proper entry into the water!

Since the molecules of a liquid all try to draw together under their united attractions, they will bunch themselves together into a sphere if they are allowed to do so, and this happens obviously when mercury is dropped upon the table and breaks up into round drops which run about as if they were round and hard.

Fig. 16.

The small drops are almost perfect spheres. The large drops are flattened out.]

Water will do the same thing if it does not wet the table: generally it does wet the solid on which it rests, but we see many exceptions, as, for example, when it is spilt on a dusty surface. What wetting means and implies we have yet to consider: it is a very important part of our subject. Gravitation also interferes with the tendency of a liquid to gather into spheres. When the drops of mercury are very small they look perfectly round, but larger masses are more like thick disks with rounded edges. If we seek for good examples of the formation of spheres by the general attraction of the molecules for one another, we must contrive to avoid the influences both of wetting and of gravitation. The small drops of mercury are a successful illustration. Another is to be found in the manufacture of lead shot. The molten lead is allowed to fall in a shower from the top of the shot tower, and gathers into round drops as it falls, just as the rain does.

Perhaps it seems as if this were inconsistent with what has gone before: gravitation has not been avoided and is indeed in full action, yet the drops are formed. But the evil effect of gravitation is indirect: it is the resistance to gravity which spoils the formation of spheres. In the case of the large drops of mercury, the flattening is due, not to gravitation directly, but to the upward pressure of the table, which is resisting gravitation. When the drops of lead arrive at the bottom of the tower they fall into water, which freezes them in the shape they have acquired.

Here is an experiment to illustrate our point. The dark-looking liquid, ortho-toluidine, does not mix with water, or, in other words, water does not “wet” it, and its density is such that it floats conveniently in a layer of pure water riding on a layer of salt water (Plate X a). It is, even when spherical, supported at all points by the surrounding water: it is not held up at one point only, as a mercury drop would have to be were it a sphere resting on a hard surface. In the circumstances of our experiment neither wetting nor gravitation has any influence, and a large drop is formed, as we see: it is perhaps a couple of inches in diameter. If it is pulled about by a glass rod it sluggishly recovers itself; or, after wabbling heavily through a variety of strange shapes, may break into smaller spheres. When the rod is pressed gently against the sphere it makes a depression or dimple on the surface; the toluidine’s effort to form a spherical drop is for the moment interfered with, but it adapts itself as well as possible to the circumstances. So also if we float some solid body--an iron ball let us say--on the surface of mercury, a dimple is formed; the surface of the mercury near the ball has the form shown in the figure. If we look at the form of the mercury surface close to the wall of the containing vessel, we see the same outline.

It is different when the liquid wets the wall of the vessel which holds it or of the body which floats in it. If we put clean water into a clean glass vessel, we see the water heaping itself against the side.

This is a more complicated effect than the other; evidently there are attractive forces between the glass and the water. If a glass plate is forced down into a dish full of mercury and made to touch the bottom so that the mercury is squeezed out and none remains between the plate and the bottom of the dish, it will stay where it is put, and indeed great force is required to remove it. The explanation is simple and in accordance with the principles which we have been considering.

If the plate is to rise, the mercury must be made to get under it again, otherwise the plate cannot rise far, because if there is not a vacuum under the plate there is at most only a little air, which would fall rapidly in pressure if the plate were raised.

A. A large drop of liquid ortho-toluidine floating in water on a layer of brine.

B. One bubble rests inside another, but as in Fig. 21 and for the same reason the two do not coalesce.]

As the pressure on the top of the plate is more than that of the atmosphere, the downward forces on the plate are greater than those which try to lift it. At the edge of the plate the form of the mercury surface is as shown in the figure: the mercury refuses to allow itself to be drawn out into a thin sheet between the plate and the bottom of the vessel.

A drop of fluid which tries to draw itself together into a sphere looks as if it were being held in an elastic bag. The atoms of mercury in the surface are not quite in the same circumstances as those in the interior, because they are exposed on one side, but it is only in this sense that there is a surface film. We use the idea of a surface film, nevertheless, finding it a convenient term; and we speak of its tendency to contract and of its tension. Sometimes, however, there is a real film on the surface which is different in composition from the liquid of the interior, and then we find many strange and beautiful consequences. The example most familiar to us is, no doubt, that of the soap bubble. We put into the water a little soap, and at once we find it easy to churn the soapy water into a pile of froth or blow it out into bubbles. What has the soap to do with this effect? The answer is to be found in the properties of the soap molecule. It is of very curious shape, many times as long as it is broad; and it is made up of a chain of carbon atoms fringed along its length with hydrogens, and ending, at one end, in a little bunch of three hydrogen atoms, at the other in a little group consisting of oxygen and sodium. The former of these bunches is very self-contained: its attractions for other atoms and molecules are small. But the latter is by no means so unsociable: it is an active group tending to enter into association with others, and especially it has a strong desire to join up with molecules of water, for which reason the soap dissolves in the water. Because, however, it is only one end of the chain which is very active in this respect--the other end and the sides of the chain behave differently--the soap molecules are apt to stay on the outer fringe of the water if they come there in the course of their wanderings. In this way a real film forms on the surface of the water, consisting of soap molecules standing on end, so to speak, one end rooted in the water, and the other exposed to the air. They are packed together side by side like the corn in a field, or the pile on a piece of velvet. They are not as free, however, as the hairs of the pile: they are tied together side by side, because there is some force of attraction between them when so laid alongside. We find that effect displayed under other circumstances, as we shall see later. Thus they form a sort of chain mail over the surface of the water--a real envelope. The sheet can be stretched in the sense that if it has to be extended other long molecules will come out of the body of the liquid and take their place with the rest.

The soap bubble is a thin-walled sphere of solution bounded within and without by the soap films; it holds together so well because the films are there. It shrinks if the air within it is allowed to escape: evidently the long molecules would gather together with the water molecules as closely as possible. But there must be an outside, of course, and where both kinds of molecules are present it is the long chain molecules that form the outside layer. A very simple experiment will illustrate still further the tendency to shrink. A wire ring is dipped into some soap solution, and when lifted out carries a soap film stretched across it.

In the film floats a ring of fine cotton shows. It is clear that which was attached to the wire ring before the latter was put into the solution. If the film inside the cotton loop is burst by touching it with a hot needle, the loop flies instantly into the form of a perfect circle, as the figure shows. It is clear that the whole film is under tension and is trying to contract.

A very curious feature of the soap bubble is its reluctance to join up with another bubble. If we blow a bubble on a ring (see Fig. 21), we may take a second bubble and knock it against the first with force, one would think, enough to break them. But the bubbles bounce from one another like india-rubber balloons. Perhaps the explanation lies in the fact that in both cases the outer layer consists of those ends of the molecules which, as we saw before, have very little tendency to associate with other molecules or parts of molecules.

The two bubbles are pressing each other, and may be rubbed on each other, but do not coalesce, because their liquids do not mix; it is only the inactive ends of chain molecules that come into contact. (By courtesy of Prof. C. V. Boys.)]

There is no tendency for the one bubble to coalesce with the other when the two are pressed together, because the parts that come first into contact do not attract each other.

This is very clearly seen in another of the wonderful experiments of C. V. Boys.[4] A bubble is blown on a ring held in a stand (Fig. 21a).

A small ring carrying a tiny weight is attached to the under part of the bubble as shown. A glass pipe is charged with solution and pushed through the top of the bubble; when blown, a second bubble appears within the first, and when it has attained a suitable size is released by a skillful twist of the pipe. The inner bubble falls gently to rest on the lower part of the outer, which it touches along a ring, not at the bottom point. This is intentional, and was the purpose for which the weight was attached to the outer bubble. The two do not tend to coalesce, although in contact all along a line, no doubt because they are presenting to each other surfaces composed of the inactive or unsociable ends of the chains. If the outer bubble had not been pulled out of shape, the inner and outer would have touched each other at their lowest points. Now there is generally a drop of solution at the lowest point of the inner bubble. When this comes into contact with the outer, the bubbles generally coalesce. The drop of solution in some way forms a bridge between the two. If the glass pipe be pushed through the outer and made to touch the bottom point of the inner, and so drain it, the weight hanging from the outer may be peeled off, and now the two can touch each other at this lowest point without disaster (Plate X b.)

The frothing of liquids is often caused by the presence of molecules which have the same property of forming a skin over the surface. When the foam gathers on a brook it is due to the presence of such molecules as those of the various saponines, chain-like formations which are found in many plants and trees. So also the foam that gathers on the shore is believed to be due to the presence of similar molecules formed in the seaweeds.

We have learned much about the form of these long-chain molecules within recent years. In particular we are indebted to the late Lord Rayleigh, to Devaux in France, to Langmuir in America, to Hardy and to Adam in England for the examination of what happens when oils are allowed to spread on water surfaces. We can repeat one or two of the experiments in order to get an idea of the magnitude of the effects of which we are speaking. We take a clean water surface, that is to say, a surface free from any contamination by oil or grease. It is convenient to attach a rubber tube to the tap and let the free end of the tube lie at the bottom of a basin so that the water wells up and overflows the edges, carrying away any dirt that has settled on its surface. We now spread on the water a thin dusting of talc powder or anything else that is convenient. Next we take a fine-drawn glass point or needle and dip it into oil--olive oil will do--and then, after wiping nearly all the oil off, dip the point of the slightly greasy needle into the water surface. Instantly a circle is cleared round the needle (Plate XI a). It appears that the long molecules range themselves side by side on the surface as before; to the soap bubble they came from within, now we apply them from without. Each molecule hastens to root itself in the water by its active end, and stands upright, as if it were a water plant rooted and growing in the water. In the end all the molecules are successful, and a thin sheet, one molecule thick, covers the surface of the water; its thickness is of the order of a ten-millionth of an inch. By measuring the weight of the oil that has been placed on the water—a difficult task, since it is so small—and the area covered, it is possible to find a measure of the thickness of the film.

A. Circles cleared by minute drops of oil.

B. The camphor boat.

A small piece of camphor is fastened at the stern of a very light boat, and as it dissolves in the water the solution forms a him on the surface. It is so eager to do so that it drives the boat away so as to make room for itself If a little oil is put on the water and makes a film all over it, the boat stops. If the oil partly covers the water, the boat stops as soon as the ruler which is held by the operator in the picture is pushed so far forward that the oil covers the surface left to it.]

This is, in fact, the method that has been followed by the workers mentioned. More recently it has been possible to apply a new method, based on the use of X-rays, to the exact measurement of the same quantity, and I hope to show you presently how this is done. On the results of the earlier work it was possible to assert that the thickness of the layer was such as would be expected if it were one molecule thick; and the argument was greatly strengthened by the fact that when different substances, known by chemists to be chain molecules of different length, were placed upon the water, the thickness varied with the length, as it ought to do.

If the drop of oil is small enough, and the dust is finely scattered, the cleared spot is exactly circular. If we prick the water surface somewhere else, another circle is formed. Each circle is totally unaffected by the presence of others. This was relied on by Devaux to show that the action of each drop was concerned only with the surface round it over which the oil was spread: it was not a general effect on the body of the liquid. It was just what one would expect if the drop of oil had spread out until it was drawn down to a certain thickness and could then spread no farther. By putting on a larger drop, we can see that larger spaces are cleared. We may, for example, pour a few drops into a large bath, and clear the whole surface. When the dust layer on the surface of the water is broken up into little patches by several applications of minute drops, in different places, and when the surface is not covered all over with the oil film, we can observe the quickness of the spreading by touching the surface with the oiled needle at some little distance from a floating patch, and watching how suddenly the patch is hurried away from the spot. The impulses that are given in this way are the cause of the lively movements of camphor fragments when they are dropped on the surface of the water, an old experiment. As the camphor dissolves, the solution shoots over the surface in a film, and the camphor itself recoils like a gun when it is fired, or a rocket when the heated gases stream from its tail. Sometimes the fragments dart to and fro and sometimes spin round merrily. A tiny boat can be made to sail about on the water by fastening a little piece of camphor on its stern in such a way as to touch the water (Plate XI b). When a number of camphor boats and pieces of camphor are all on the move, it is quaint to see how suddenly it all goes dead when a little oil is poured on the water. The oil film has covered the water in an instant, and the dissolved camphor no longer spreads over the surface.

We have all heard of the stilling of the waves by pouring oil upon the sea. We can watch the effect by making a series of waves run along the long tank which Lord Rayleigh once used here for the same purpose; a vacuum cleaner serves to provide the wind, and you see there is quite a heavy storm on the water (Plate XII a, b). It is magically stilled if a few drops of oil are allowed to fall in the center of the storm; after a few moments the oil sheet is blown to the end of the tank and the waves rise once more. We can repeat the experiment again and again. We must suppose in this case that the wind has no “bite” on the water. The latter is covered, as we know, with a film of oil, the top surface of which is formed of the inactive ends of the long-chain molecules; and it may well be that the molecules of the air when they strike it recoil as from a smooth surface. A rough surface would be driven forward by the impacts of the air molecules--rough, that is to say, in the sense that the spaces between the exposed molecules are of the same size as the molecules that strike. But if the surface of the oil film is very smooth and has little tendency to hold on to any molecules that strike it, the air cannot push it and make it rise in little waves which afterward grow to great ones. So the oil stills the waves by stopping the action of the wind, and the motion of the waves dies out in their own friction.

We now come to the problem of the “wetting” of a surface. We know, for example, that a clean glass surface is wetted by water, but not when it is smeared with grease, even if the film is almost invisible. The water molecules clearly refuse to associate with the molecules of the grease. That is not surprising, perhaps, because we have seen that in some cases at least the long molecules that make the fats and oils present to the outside their inactive ends, which have very little attraction for the water molecules. So water spilled on a greasy surface gathers into drops, just as mercury when it is spilled on the table; the form of the water is due to the general attraction of its molecules for one another. An oiled needle can be gently laid on water without sinking more than to make a depression in the surface, just as if there were a skin on the water which gave slightly under the weight. Still more striking, perhaps, is the floating of a greased wire sieve. (Fig. 22).

The sieve is dipped in melted paraffin wax, shaken so as to clear the pores, and allowed to dry; it is well not to touch it with the fingers. It will float readily and carry quite a lot of cargo, as Boys showed at the Christmas lectures many years ago. Or it may be filled with water; but the water must not be poured in roughly; it must be allowed to flow in gently on to a piece of paper which can afterward be removed. To show that the pores are quite open we can give the sieve a sharp movement, when the water film gives way and the water falls in a heavy shower on the floor.

When soda water is poured out into a clean, smooth tumbler, very few bubbles come to the surface; but if the surface of the tumbler is at all dirty or rough we may see streams of bubbles rising. There is a beautiful old experiment which illustrates this effect, that of “the grape and champagne.” We must use soda water instead of champagne. A grape is not wetted by water, and so when it is put into the tumbler it sinks to the bottom of the soda water, where it collects bubbles at a great rate (Plate XII c). Soon it is covered over with a sheet of bubbles that look like seed pearls, and these bring it by their buoyancy to the surface. The grape is not much heavier than the water, and does not require much to lift it. At the surface the grape parts with some of its bubbles, which burst into the open air, and this goes on until it sinks again, only to collect a few more bubbles and once more be made buoyant. The process will repeat itself continually for many minutes until the soda water is “dead.”

It is interesting to put in two glass beads instead of the grape. They have been cleaned: washing with soap and water is efficient. No bubbles form on them and they stay at the bottom.

A. Stormy water.

B. Oil stilling the storm.

C. Grape in soda water.

A and B. The blower raises a storm and the waves run along the tank. A drop of oil stills the waves at once, and the water surface becomes level. Notice the blank space in B. After a while, the oil is blown to the end of the tank, and then the waves rise again. The photographs have been retouched so as to bring out the reflection of the light by the waves.

C. The grape and the greasy glass sinker are carried to the top by bubbles; the clean glass sinker has no bubbles and stays at the bottom.]

We take one of them out, rub it over with a greasy finger, and now it behaves like the grape, collecting bubbles, rising, parting with some of them, falling, and so on.

We must realize that when a bubble of carbonic-acid gas forms in the soda water the particles of the gas have to collect and push back the water all round. Now the water molecules are holding on to one another tightly, and resist being torn asunder. For this reason we do not see the bubbles forming in the middle of the water. At the edge, when the glass is clean, the water wets the glass, or, in other words, the water molecules are clinging to the glass even harder than they cling to one another. Bubbles cannot under those circumstances form here, either, for they would have to tear away the molecules from the glass. But it is different if the surface is greasy and the molecules are not really holding on to the glass--merely pressed against it by the pressure of the rest of the water which is behind them. In that case the gas bubbles find somewhere to grow, and quickly increase in size. It is easier to push back the surrounding water when the bubbles have grown somewhat. One of the most beautiful ways of showing that is by another of Boys’ soap-bubble experiments. Two bubbles of different sizes are blown on the two ends of the same tube; when they are allowed, through the opening of a tap, to communicate with one another, the little bulb blows out the big one and disappears. Of the mass of bubbles in the soda water which lie side by side on the wall or the grape, the larger ones tend to take up the smaller, and all of them to amalgamate.

The little streams of bubbles that we sometimes see rising from definite points on the surface of the tumbler are due to some irregularity in the glass--a tiny protuberance, perhaps--on which, if a bubble tends to form, it already is past the earliest stages of small diameters.

This tendency of bodies under water to collect bubbles and rise to the surface has of recent years become the basis of a great metallurgical industry. Various metal ores when crushed into powder form a mixture of particles of rock material, such as quartz and various silicates, and of metallic sulphides. It is found possible to treat the mixture so as to cover the particles containing metal with a thin oil film, which is not wetted by water, while at the same time the particles of rock are still clean and the water wets them. The mass is then churned up into a froth. All the metal-bearing particles are made buoyant by the adherence of bubbles and rise to the top in a thick frothy scum; the rest of the ore stays at the bottom of the vat, and the two parts are easily separated.

There is one other experiment which will help to illustrate these principles. We know that water heaps itself up against the side of a clean glass vessel which contains it. The molecules cling to the glass, and as it were climb up the wall on one another’s shoulders in their eagerness to affix themselves thereto. If we dip two glass plates side by side in the water, the water rises higher to the space between them than it does outside. Those that are climbing one wall now help those that are climbing the other. The effect is spoken of as being due to “capillary action,” the name being given to it because it is so marked in the case of a fine or “capillary” tube. The water in the fine bore is lifted up to a great height, one inch in the case of a tube of one-twentieth-inch diameter. If we float a small hollow glass ball on the surface of the water, the water rises up the sides of the ball. If two floating balls are made to approach each other, they will, when within a short distance--perhaps half an inch--of each other, move together, at the end quite violently.

The pressure at _Q_ is less than the pressure at _R_ because it is at a higher level in the water. The pressure at _R_ is the same as at _S_, because _R_ and _S_ are on the same level. The pressure at _S_ is that of the atmosphere, which is the same as the pressure at _P_. Hence the pressure at _P_ is greater than the pressure at _Q_, and the one glass ball is forced toward the other.]

We shall understand that if we consider the diagram in Fig. 23. Two glass balls are floating in the water. The pressure at _Q_ is less than the pressure at the level of the dotted line, because _Q_ is at a higher level in the water. The pressure at the level of the dotted line is the pressure of the atmosphere, because the line continues the level of the water without. So the two pressures marked _P_, both sensibly equal to the pressure of the atmosphere, overcome the two pressures marked _Q_ and drive the balls together.

If we float on the water two balls made of paraffin, or two glass balls coated with paraffin, the two attract each other as the clean glass balls did, though the action is somewhat different.

The pressure at _P_ is greater than that of the atmosphere and therefore than that of _Q_, and the balls are forced together.]

As the figure shows, the balls combine in making a dimple in the water, and again if we study the forces acting on the balls we find that the pressures are such as to force the balls together. But the clean glass balls avoid the paraffin balls. This action is a little more complicated, but it can be followed from the figure, which shows the forces that are in action. When the vessel containing the water is clean, and the water is heaped up against the side, the clean glass balls are attracted to the side, just as they are attracted by each other. On the other hand, the paraffin balls avoid the side of the vessel. If now we carefully fill up the vessel with water until it tends to brim over, so that the edge of the water no longer curls up against the side, but curls down toward the edge of the vessel, the clean balls leave for the middle and the paraffin balls come to the side and stay there.

The left-hand ball is wetted by the water; the right hand ball is greased and is not wetted. The pressure at _P_ is greater than that at _Q_, and at _P_' greater than at _Q_'. Thus the balls are forced apart.]

All these facts which we have been considering are illustrations of the one principle on which the formation of a liquid depends, namely, the strength of the attractions between the atoms and the molecules which are strong enough to keep them in constant association with one another, though they are not so strong as to bind them together into a rigid, solid body. And it is important to remember that molecule attaches itself to molecule at special points; one part of a molecule may be able to exert a strong hold on a special part of another. Presented differently to each other, there may be little or no tendency for the two to join together.