SURFACE OF LIQUIDS. 23 3. If a bottle be filled with water to the top of the neck, and then a glass stopper quickly forced into the bottle, in which direction will the water in the neck move? What property of liquids does this illustrate? 4. Water is poured on the middle of a table, and runs towards one particular side; what inference can you draw from this? 5. If two drops of water are brought close together in one place, and two pieces of glass close together in another place, what difference will be observable in the two cases? 6. What property of liquids enables them to be sold by measured volume (gallon, pint, &c.) more satisfactorily than solids? 7. Explain why a piece of lead is melted when it is desired to cast it into any particular shape. LESSON 11. SURFACE OF LIQUIDS. It is It was stated in the last lesson, that liquids in vessels arrange themselves so that their surfaces are level or horizontal. In speaking of the horizontal surface of a liquid, we mean that if we could measure the lengths of a number of lines, drawn from the centre of the earth to different parts of the surface of the liquid, these lines would all have exactly the same length. Strictly speaking, therefore, the surface of water is not flat but curved. This is very easily observed at the seaside, where we see vessels in the distance gradually sinking out of our sight-first the lower, then the upper, parts disappearing. But the surface of a plate of water, or even of a pond, is so little curved, that we may think of it for all our purposes as being plane. easy to understand why a liquid cannot heap itself up in one part of a vessel, when we remember that if on any side a liquid is unsupported, it will flow or spread out in that direction, and will move to as low a position as it possibly can. In small portions, however, liquids do not take level surfaces, but break up into little rounded masses, which we call drops. This may be seen when mercury is spilt on a plate, or when water is dropped on to something which it does not easily wet. The pressure of the upper portion of the drop tends to make the liquid spread out into a flat sheet; but this pressure is not so great as the cohesion, which tends to draw together the molecules of the liquid as closely as possible. We all know that when water is poured into the body, or wide part, of a kettle or teapot, some of it passes into the spout. And we may all see, if we notice carefully, that the water rises just as high in the spout as in the body, notwithstanding the great difference in the size and shape of the two parts. What is true of the kettle, is true of all pipes or vessels which consist of two portions. When a liquid is poured into such vessels, it runs first to the lowest part of the vessel, and then rises just as high on the one side as on the other, whatever may be the shapes and sizes of the two portions. This has been known for a very long time, and people commonly express the fact by saying, that "water finds its own level," though we must remember that all other liquids besides water will do just the same. Many applications are made of this principle, one of the most interesting, perhaps, being its application to the production of fountains. Let us suppose that a tank or cistern of water is placed on the top of a building, or on a hill. If a leaden pipe were carried from the Fig. 6. tank to the ground, and then bent up again as high as the cistern, the water would descend the pipe on one side and rise up on the other, until it reached the same height as the surface of the water in the cistern. If the pipe were cut off against the ground, as in the illustration (fig. 6), the water would issue from the pipe, with force enough to carry it nearly as high as the surface of the water in the cistern. This would constitute a fountain, and the height of the jet of water might be increased, by placing the cistern on a higher tower or hill. A more useful application of the same principle, however, is afforded by the water supply of many towns, where a reservoir placed on some neighbouring hill corresponds to the cistern in the illustration, and the pipe in the illustration represents the pipes laid along the streets. The water will be forced from any opening in these street pipes to a certain height, by the pressure of the water in the reservoir above it, and will thus be available in the case of fires, &c. The water will not rise to exactly the same height as the reservoir, since some of the force is lost in passing through the pipes, by the rubbing of the water against their sides; and again, when it comes from the pipe, part of the force is spent in beating the air out of the way of the ascending column of water. Another interesting application is seen in the case of the water-gauge on a steam-engine, by means of which the engineman can see how high the water stands SURFACE OF LIQUIDS. 25 in the boiler. The gauge consists of a small tube connected with the boiler, somewhat like the spout of a kettle is connected with the body of the kettle, only that it opens into the boiler at the top as well as at the bottom. Suppose that we wish to fasten a piece of string horizontally from one point to another, we can assure ourselves that it is horizontal in several ways. In the first place, we might get a large glass vessel of water, and ascertain whether the level of the water coincided with the level of the string. Or we might get a bent piece of glass tube containing some water, and see that the string passed exactly at the level of the water in both parts of the tube. The instrument which is most generally used to ascertain whether surfaces are horizontal is that known as the spirit-level. It consists of a glass tube slightly curved, as shown in fig. 7, (though not curved quite so much as there shown), nearly filled Fig. 7. with alcohol (or spirit, as it is frequently called), leaving, however, a small bubble of air in the tube. The alcohol always lies in the lower parts of the tube, and the bubble of air in the highest part. When the two ends rest on a horizontal surface, the bubble is seen in the middle of the upper portion of the tube, but if one end be raised ever so little, the bubble moves up nearer to that end. Alcohol is used in preference to water, partly because it does not freeze even in the coldest weather, and partly because the bubble moves more freely in alcohol than in water, for a reason already given. EXERCISES. 1. If an open glass tube be pushed vertically downwards into a vessel of water, how high will the water rise inside the tube? 2. Explain why the water comes from the pipes in the upper rooms of a house with less force than from those in the lower rooms. 3. Describe several methods by which you could determine a point on a rod set up in one part of a field, which should be at the same horizontal level as a point on a second rod in another part of the field not far distant. 4. If the upper portion of the spout of a kettle were cut off, to what height could the kettle be filled with water? How might such a kettle be placed so that the water could reach the lid? 5. Describe and explain the working of "lock gates" on canals. LESSON 12. CAPILLARY PHENOMENA OF LIQUIDS. When a piece of loaf sugar is laid in a small quantity of water or tea, say in a teaspoon, the liquid is seen to make its way upwards to the top of the lump sugar. Similarly, when a piece of blotting paper is dipped into a drop of ink, the ink rises up into the blotting paper and remains there. The force which causes the liquid in each of these cases to make its way into and through the solid, is called capillary force. It is sometimes called capillary attraction, because in most cases there seems to be some force in the solid which attracts, or draws nearer, the liquid. If we examine carefully the surface of the water in a glass vessel, we shall see that at the sides of the vessel the water is raised up against the glass, and stands slightly higher there than the surface of the rest of the water. This can be well seen with water in an ordinary flat-sided glass bottle, especially at the corners of the bottle. Mercury in a tumbler or bottle behaves differently; where it comes against the glass, the edge is turned downwards, so as to stand slightly lower than the surface of the rest of the mercury. And if we try a number of liquids in succession, we shall find this rule to hold good:-Those liquids which are able to wet (that is, adhere to) the glass, such as water and alcohol, rise up against the glass: while those which do not wet the glass, such as mercury, sink down where they come in contact with it. Again, suppose we take two pieces of sheet glass, place them nearly close together face to face, and then plunge them into water, alcohol, glycerine, turpentine, or any other liquid which wets the plates, we shall find that the liquid rises up between the plates some distance higher than the liquid in the vessel on the outside of them. And a few very simple experiments will soon show us, that the narrower the space between the plates, the higher the liquid rises; and of course the wider the plates are apart, the less is the rise of the liquid. The same effects may be seen in glass tubes, and the same rule holds good, viz., that the smaller the diameter of the tube the greater is the rise of the liquid. In tubes, and between plates, where the space is no larger than the thickness of a hair, the effects of capillary force are very plainly seen. Such spaces are called capillary spaces, from a Latin word capillaris, meaning hair-like. If the glass plates or tubes were plunged into mercury, or if they were greased before being plunged into water, the liquid would be depressed, or forced down, against the glass; and the narrower the space the greater would be this depression. Now a piece of loaf sugar is simply a collection of crystals, having small spaces between them. And blotting paper is really composed of a large number of fibres matted together, having also very small PRESSURE OF LIQUIDS. 27 spaces between the numerous fibres. When these substances are placed in water or ink, the liquid rises up the capillary spaces in them. The force, then, which causes the elevation or depression of liquids, where they come into contact with solid bodies, is called capillary force, and it is so called because its action is most striking in hair-like or capillary spaces. The wicks of candles and lamps are masses of cotton fibres, which contain between the fine fibres a large number of capillary spaces. The lower portion of the wick dips into a quantity of melted tallow, or wax, or oil, or other combustible liquid, which rises up the wick by reason of the capillary force acting on it. The roots, stems, and leaves of plants, all contain numbers of very fine tubes-vessels they are called-through which the sap rises up the plant. It is supposed that the rise of the sap in plants is due, for the most part, to the action of capillary force. EXERCISES. 1. In a narrow glass tube the surface of water has the form of a hollow cup, while the surface of mercury is rounded like part of a ball. Explain this. 2. Why do many liquids, when kept in bottles, rise up between the neck of the bottle and the glass stopper? 3. People sometimes water plants in flower pots by letting the pot stand in a saucer of water. Describe and explain the manner in which the water gets to the roots of the plant, and even to the top of the soil. 4. Explain why ink spreads when written on paper which has not been sized or glazed, most kinds of brown paper for example, and does not so spread when written on similar paper after it has been sized. 5. Explain why a piece of dry bread will soak up a considerable quantity of water or tea. 6. When a tallow candle is put into a vessel of water, the liquid is depressed round the sides of the candle. Explain this effect. LESSON 13. PRESSURE OF LIQUIDS. A liquid contained in a vessel exerts a pressure, not only on the base of the vessel, but also on its sides. And we have learned that any additional pressure which may be put on the surface of the liquid, will be transmitted by it to the base and sides of the vessel. Suppose that we have a tumbler half full of water, the water will be pressing on the base of the tumbler, and on the sides as far as it reaches. If we then pour an additional quantity of water into the tumbler, there will be an increase of pressure on the parts just named. The water last poured in will press on the water below it, |