Part 4

But the distance which the water will travel during this second will be the difference between the distance which it would have ascended if there had been no gravity forcing it down, and the distance which it would have descended if there had been no momentum driving it up; and, at the end of the second, the rate of its motion upwards, or its velocity, will be proportionally slower. Thus, at the end of the first second, the water has spent a certain portion of its momentum in overcoming its gravity. And as there is nothing to make good the loss, it would, if left to itself, travel more slowly, or over a less distance, in the second second than it tended to do in the first. But though the momentum of the water is diminished, its gravity, weight, or tendency to fall downwards, for a given distance in a second, remains exactly what it was, and operates in the course of the second second to exactly the same extent as in the first. Hence, at the end of the second second, the distance through which the water travels upwards is still smaller, and its velocity is still more diminished. It is obvious that, however great the disproportion between momentum and gravity to start with, gravity must gain the day in the long run under these circumstances. The store of momentum will be used up; and, after a momentary rest, the water, reduced to the condition of a body without support, will begin to be carried downwards by the unopposed action of gravity.

The case is similar to that of a boy sculling a boat, the bows of which are suddenly seized and the boat thrust violently backwards by a strong man. The boat will go stern-foremost rapidly, at first, but every stroke of the boy’s oar at the stern will retard its backward motion; until, at length, the stock of momentum conferred upon it by the man’s thrust will be completely exhausted in working against the boy, and the boat, after a momentary rest, will resume its onward course. The distance to which the boat will be propelled backwards will evidently depend upon the amount of muscular power which the man, as it were, suddenly capitalizes in the boat, and which the boat then slowly pays out.

We call people who possess much muscular or other power energetic; and we estimate their =energy= by the obstacles they overcome, or, in other words, by the =work= they do. In the present illustration the man’s energy would be measured by the distance to which the boat was propelled before it stopped.

It is easy to transfer this conception of energy, as the power of doing work, to inanimate things; and thus when a body in motion overcomes any kind of obstacles in its way, parting with its momentum and more or less coming to rest in the process, we say that it has =energy= and that it does =work=.

The energy of moving water is thus measured by the intensity of the opposing forces which it can overcome multiplied into the distance which it can travel before that energy is exhausted; that is to say, by the work it does before it is itself reduced to a state of rest. In the case under consideration, the energy by which gravity is overcome, for a greater or less time, depends upon the velocity of the stream; and this again depends upon the height of the water in the vat above the tap. Just as the energy of the horizontal stream diminished as the level of the water became lower, so does the energy of the vertical stream diminish. Hence, as the vat empties, the jet becomes shorter and shorter, until at last it sinks down to nothing.

The energy of moving water makes it, under some circumstances, one of the most destructive of natural agents; and, under others, one of the most useful servants of man. A stream is water falling down hill with a velocity depending upon the inclination of its bed. As it falls it acquires momentum and, hence, energy; and thus a mountain stream, suddenly swollen by rain or melting snow, will tear away masses of rock and sweep everything before it. Nothing can look softer or more harmless than a calm sea, but if the wind sweeping over its surface puts the water in motion, it strikes upon the shore with terrific force; and its energy is expended in throwing up great waves, which lift vast blocks or drive masses of shingle up the beach.

In all kinds of watermills it is the energy of more or less rapidly falling water which is turned to account. The water is made to flow against buckets or floats attached to the circumference of a wheel. Each bucket or float is therefore an obstacle to which the water transfers some of its own motion; it moves away and thus makes the wheel to which it is fastened turn. But the turning of the wheel brings a new obstacle in the way of the stream. This is treated in the same fashion, and the wheel turns still further, thus introducing another obstacle in the way of the stream upon which the same effect is produced. Thus each float, or bucket, is a means by which some of the momentum of the stream is, as it were, caught and transferred to the water-wheel, which consequently turns round with a certain velocity.

But this water-wheel is now a mass of matter in motion, and therefore itself contains a store of energy or power doing work. If a cord with a weight at the end of it were fastened to the axle of the wheel, the cord would be wound upon the axle, and the weight could be raised, or, in other words, so much work would be done by the turning of the wheel and we should thus have a rough measure of the amount of energy which had been given up by the stream to the wheel.

The machinery of the mill is simply a set of contrivances for transferring the energy stored up in the water-wheel to the place in which work has to be done. In a flour-mill, for example, a series of wheels carries it from the water-wheel to the grindstones, which it sets in motion.

30. =The Properties of Water are Constant.=

If, whenever there is a shower, you catch some rain-water, you will find that it possesses all the properties which have been described. It will be found to be an almost incompressible liquid, an imperial pint of which weighs about a pound and a quarter. It would make no difference if the rain-water were collected in Africa or in New Zealand; or if it had been obtained centuries ago and kept bottled up ever since. And there is every reason to believe that rain-water will have exactly the same properties a hundred or a thousand years hence. So far as the properties of rain-water are concerned =the order of nature is constant=.

This, however, is by no means the same thing as saying that the properties of water are always the same. In fact the properties of the substance, water, vary immensely according to the conditions to which it is exposed; but, under the same conditions, they are the same, so that we may still say that so far as water is concerned, the order of nature is constant.

31. =Increase of Heat at first causes Water to Increase in Volume.=

It has been seen that a certain weight of water always has the same volume under the same conditions. The most important of these conditions is the heat or cold to which it is exposed. Water which has stood for some time in a warm room becomes less in volume, or =contracts=, if it is taken into a cool place; while its volume increases, or it =expands=, if it is made hot. The same thing is true of quicksilver, of spirit, and of liquids in general. A =thermometer= is simply a small flask—the bulb—with a long and narrow neck—the tube—filled with as much mercury or spirit as will rise a short distance into the neck. If the liquid in the bulb is warmed, its volume is increased and it overflows into the tube, increasing the height of the column of liquid in the tube. If, on the other hand, the liquid in the bulb is cooled, its volume is diminished; and, as it shrinks, the column of liquid in the tube flows back into the bulb, and the level of the top of the column is lowered.

If a mark is made on the tube, or on a scale fixed to it, at the point which the liquid reaches when the bulb is placed in boiling water; and another mark at the point to which it sinks when the bulb is in melting ice; and the space between the two marks is divided into 180 equal parts, each of these parts is what is called a “degree” in the thermometers ordinarily used in this country (called Fahrenheit). And if the boiling-point is counted as 212° the freezing-point must be 32° (212 - 32 = 180). With the same amount of heat the fluid in the tube always stands at the same degree, and hence the instrument measures =temperature=.

That hot water is lighter than cold is easily seen when a bath is filled from two taps, one of hot and one of cold water, which run at the same time. Unless care is taken to stir the water, the top of the bath will be very much hotter than the bottom. Thus, an imperial pint of water weighs a pound and a quarter only at a certain temperature or degree of warmth, namely at 62°; if it is made hotter its volume increases, and therefore its specific gravity diminishes.

It was for this reason that in § 22 the weight of the same volume of water was said to be constant =under the same conditions=; and, of course, the same qualification must be borne in mind when we speak of the weight of a cubic inch of water being about 252 and a half grains. Its weight is in fact 252·45 grains only when Fahrenheit’s thermometer stands at 62°—but as this is the temperature of ordinary mild weather, and the expansion or contraction of water for a degree about this temperature amounts to less than 1/3000th of its volume, the weight of a cubic inch may for all practical purposes be taken as 252 and a half grains.

32. =Increase of Heat at length causes Water to become Steam.=

Thus a change is effected in the properties of water by heating it ever so little. If it is more strongly heated a still greater change takes place. You know what happens when a saucepan containing water is put on the fire. The water gets hotter and hotter, then it begins to simmer, and finally, when it reaches 212°, it boils away into steam, which passes into the air and disappears. If the boiling is carried on long enough all the water vanishes. It looks at first as if the water had been destroyed by the heat. In reality, however, not a particle of water has been destroyed. It has merely changed its state. The heat has altered it from the state of liquid water into that of gaseous water, =vapour= or =steam=.

Try the same experiment with a tea-kettle instead of a saucepan, but only put a little water in the tea-kettle, and shut the lid well down. Then, as soon as the water begins to boil, the steam will shoot out of the spout in a jet; and this will go on as long as any water remains in the kettle.

The steam, as it comes out of the spout, is so hot that it will scald you if you put your finger in it. But you may satisfy yourself that it is very hot, without scalding your fingers, by holding a stick of sealing-wax in it. The wax will soften, just as if you held it before the fire. Moreover, if you look through the steam, just where it leaves the spout, you will see that it is quite transparent; it is only at some little distance from the spout that it loses its transparency, changes into a white opaque cloud, and rapidly vanishes in the air.

33. =The taking away of Heat from Steam causes the steam to change into Hot Water.=

Now take a cold spoon, or a cold plate, and hold it against the jet of steam, for a moment or two. When you take it away, you will find that it is quite wet, being covered with drops of warm water, and, moreover, the cold spoon, or plate, has become warm. And if you fit a long cold metal pipe to the nozzle of the tea-kettle, you will find that no steam at all issues from the end of the pipe, but only water, while the pipe becomes warmed.

Thus the heat passes from the fire into the saucepan, or kettle, and thence to the water which they contain; the water gets hotter and hotter, and, when it has taken in a certain quantity of heat, it becomes steam, or vapour of water. When the steam comes against the cold plate, or passes through the cold pipe, it gives up the heat it has taken in to the plate, or the metal of the pipe. They carry off the heat which kept the water in a condition of a =vapour=, and so it passes back into the condition of =liquid=.

Thus steam and water are two conditions of the same thing, =water=; they are effects of the quantity of heat which the water has taken in.

34. =When Water is changed into Steam, its Volume becomes about 1,700 times greater that it was at first.=

If you could measure and weigh the water in your kettle to begin with, and then measure and weigh all the steam into which the heat of the fire changes it, you would find that the bulk of the steam was nearly 1,700 times as great as the bulk of the water, though the weight of the steam would be exactly the same as that of the water. If you had a small square cup like a die, the inside measure of which was exactly one inch each way, it would hold one cubic inch of water. If this cup full of water were heated till all the water was turned into steam, the steam would nearly occupy a cubic foot; since there are 1,728 cubic inches in a cubic foot. A cubic inch of water weighs 252½ grains, and the steam into which it is converted has just the same weight. Thus we may say that steam is water expanded by heat into a vapour which is of 1,700 times less specific gravity than water. On the other hand, a pint of steam allowed to cool, becomes converted into a quantity of water, which measures only 1/1700th of a pint, though it weighs just as much as the whole pint of steam did. The steam, therefore, is =condensed= to a 1/1700th of its volume of water.

The power with which water expands when it is converted into steam is very great. If you were to stop up the nozzle of the tea-kettle, the steam, inside the kettle, in trying to expand, would burst open the lid; and if you were to fasten down the lid, it would pretty soon burst the kettle itself. You sometimes hear of the strong boilers of steam-engines being burst in this way.

35. =Gases or Elastic Fluids. Air.=

Here is a glass flask with a long neck and an open mouth. If we pour water in at the mouth until it rises to the lip we say that the flask is full of water. If we now pour the water out we say that the flask is empty. But is it empty? Press the flask mouth downwards into a glass jar full of water. If the flask were empty there would be no reason why the water should not enter the neck of the flask and stand at the same height inside the neck as it does outside. If you take an “empty” glass tube open at each end and press it down into the water, the water inside and the water outside will stand at the same level. But if you put your finger on the upper end of the tube so as to convert it into a closed vessel, the water will enter the lower end only a little way. So with the flask, the water enters the neck only a little way. Hence there is something inside the “empty” tube and in the “empty” flask; something which is material, because it occupies space and offers resistance. In fact the flask is full of that form of matter which is termed =air=, a thick coat of which surrounds the earth as the =atmosphere=. Air has weight, as you will learn more fully by and by; and that air in motion can transfer that motion to other bodies you are taught by the effects of the winds, which are merely air in motion.

Air therefore has all the characters of a material substance. Moreover it is a fluid, for it fits itself exactly to the shape of any vessel which contains it; its parts are very easily moved, or we should feel its resistance every time we move a limb; that it “flows” is seen in every breeze and every time you use a pair of bellows, when the air is driven in a stream out of the nozzle; and it presses on all sides anything contained in it.

But though air is a fluid it is not a liquid. In the first place it is very compressible. We saw that the water entered a little way into the tube or the neck of the flask in the preceding experiment. The reason of this is that the water compresses the air into a smaller volume. A bag full of air, such as a common air-cushion, can be squeezed till the air in its interior occupies a much smaller volume; and, if you treat a syringe full of air in the same way as the syringe full of water was treated, you will find, if the piston fits well, that it can be driven down some distance and then springs back again. Air in fact is not only a compressible, but it is an =elastic= fluid or =gas=. Heat expands air just as it expands water, but the expansion of air for the same degree of heat is much greater.

36. =Steam is an Elastic Fluid or Gas.=

In all the properties which have been mentioned water in the form of steam is an elastic fluid or gas like air.

If a little water is placed in the flask mentioned in the preceding section all the “empty” part of the space will contain air. If the flask is now made hot the water will at length boil, bubbles of steam forming in the water and breaking at its surface. By degrees, the air, which at first lay above the water, will be driven out; and if the whole flask is kept hot, the “empty” part of it will be full of the gaseous water, which is transparent and colourless like air. The steam flows out of the mouth of the flask still a clear and colourless gas; but it soon cools and becomes condensed as a cloud of small particles of fluid water.

Steam is lighter than air, and hence it rises in the air, just as bodies which are lighter than water rise in water.

37. =Gases and Vapours.=

Air is as much a gas in the coldest winter as it is in the hottest summer. But air can be liquefied by exposing it to a very low temperature, while, at the same time, it is subjected to an extremely great pressure. Thus, the difference between gases like air, which are condensed with extreme difficulty, and gases like steam, which are condensed easily, is only one of degree. Nevertheless there is a certain convenience in distinguishing those gases, which, like steam, are easily condensed as =vapours=. In what we ordinarily call steam, all the water of which it is composed remains gaseous only at and above the temperature of boiling water (212° Fahrenheit). Cooled ever so little below this point, most of it becomes condensed into hot liquid water. However, it must be recollected that though that particular form of gaseous water which we call steam exists only at and above the temperature of boiling water, yet water is capable of existing in the gaseous state down to the freezing-point.

Suppose that when our boiling flask contained nothing but water and steam, the mouth were stopped and the lamp removed. Then, so long as the temperature of the whole remained at that of boiling water, every cubic inch of steam above the water in the flask would weigh about ⅐th of a grain, since 100 cubic inches weigh about 15 grains. Suppose the capacity of the flask, exclusively of the fluid water in it, to be 100 cubic inches. Then, to begin with, the gaseous water which it contains will weigh 15 grains. If the flask is now allowed to cool, more and more of the gaseous water condenses into the fluid state; but, even down to the freezing-point, some water will remain in the gaseous state and will fill that part of the flask which is unoccupied by the fluid water. At blood-heat (98°) the gaseous water weighs only about a grain, though it still occupies 100 cubic inches; at the ordinary temperature of the air it weighs not more than ⅓rd of a grain; while, at the freezing-point, its weight is only ⅛th of a grain. But inasmuch as there is less and less actual weight of water in the same volume of gaseous water as the temperature falls, it follows that the density, or specific gravity, of the gaseous water must be less the lower the temperature. Moreover, while, at the boiling-point, gaseous water or steam resists compression with exactly the same force as air does, the lower the temperature the more easily compressible is the gaseous water.

Suppose an elastic bag were to be tied on to the nozzle of a kettle full of boiling water. If the bag were kept as hot as the boiling water it would become fully distended, and maintain its shape in spite of the pressure of the air upon all sides of it. If the bag were taken away it would retain its shape so long as it was kept as hot as boiling water; but, if it were allowed to cool, it would gradually become flattened by the outside air squeezing up the less and less resisting gaseous water of the lower temperatures. Hence, when the stopped flask has been allowed to cool, the air rushes in with great violence if it is opened.

38. =The Evaporation of Water at ordinary Temperatures.=

If some water is poured into a saucer and is allowed to stand even in a cool room or in the open air, you know that it sooner or later disappears. Wet clothes hung on a line soon dry—that is to say, the water clinging to them disappears or =evaporates=. The disappearance of the water under these circumstances results from the property just mentioned. In fact, it becomes gaseous water of the density appropriate to the temperature, and as such mixes with the air as any other gas would do. And as the sea, lakes, and rivers, are constantly giving off gaseous water into the air in proportion to the temperature, it is not wonderful that the atmosphere always contains gaseous water.

Air is said to be moist when the weight of water in a given quantity, say 100 cubic inches, is as much, or nearly as much, as can exist in the state of gas at the temperature. Under these circumstances, if the temperature is lowered even a very little, some of the gaseous water is converted into liquid water. We see this in hot moist weather, when the outside of a tumbler of fresh drawn cold spring water immediately becomes bedewed. The gaseous water in immediate contact with the tumbler, in fact, is cooled down below the point at which it can all exist as gas, and the superfluity is deposited as dew. In such days wet clothes do not dry well, because there is, already, nearly as much gaseous water in the atmosphere as the amount of heat marked by the thermometer can maintain in that state.

39. =When Hot Water is cooled, it Contracts to begin with, but after a time Expands.=

We have now seen what a wonderful change is brought about by heating water. At first, it expands gradually and slightly; but, when it reaches the boiling-point, it suddenly expands enormously, and is no longer a liquid, but a gas.

On the other hand, if warm water is allowed to cool, it gradually contracts till it reaches the ordinary temperature of the air in mild weather; but, if the weather is very cold, or if the water is cooled artificially, it goes on contracting only down to a certain temperature (39°), and then begins to expand again. In this peculiarity water is unlike all other bodies which are fluid at ordinary temperatures. Hence the temperature of 39° is that at which pure water has its greatest density or specific gravity, and water at this temperature is heavier, bulk for bulk, than the same water at any other temperature. Therefore if water at the top of a vessel is cooled down to this temperature, it falls to the bottom, and if the water at the bottom of a vessel is cooled below this temperature it rises to the top.

40. =Water cooled still further becomes the transparent brittle solid Ice.=