270. Heat and Cold.—In common language we speak of heat and cold as two distinct and opposite things. That this is not strictly correct may be shown by the following experiment: Take three vessels, and fill the first with ice-cold water, the second with hot water, and the third with tepid water. If you place your right hand in the first and the left in the second, and let them remain a little time, on taking them out and plunging them together into the third vessel, the water in it will feel warm to the right hand and cold to the left. So the air of a cellar seems warm to you in winter and cold in summer in contrast with the air outside. For the same reason water of a temperature that would ordinarily be refreshingly cool to us seems warm when drank after eating ice-cream. It is manifest, then, that there is no fixed dividing-line between heat and cold. There is, in fact, no such thing as cold. Substances are cold from being deprived of heat; and no substance ever has all its heat taken from it. Sir Humphrey Davy proved that there is heat in ice by rubbing two pieces together in a very cold room. They were gradually melted. Now this was not done by the air, for that was at a temperature below the freezing point. The heat which melted the ice came from the ice itself by means of the rubbing.
271. Nature of Heat.—There are two theories in regard to the nature of heat. One is that heat is an imponderable (§ 16), and of course a very subtile substance, which pervades all matter. Its particles are supposed to repel each other strongly, and hence they have a tendency to diffuse themselves, and to separate the particles of matter from each other. It is in this way that they are supposed to occasion the expansion of substances. The other supposition, which is most commonly received, is that heat is a vibration of the particles of bodies, and that it passes from these to bodies less warm through a subtile fluid called ether, supposed to fill all space. You see that if this be the true theory, there is some analogy between heat and sound.
272. Sources of Heat.—The principal of the sources of heat on our earth is the sun, though that body is ninety-five millions of miles distant from us. As the heat, in traveling all this long journey, is becoming more and more diffused; or, in other words, as its rays are all the way separating from each other more and more, we can have no conception of the concentrated heat that exists in the sun itself. We can, however, approximate to the idea by observing the effects of heat when some of its separated rays are gathered to a point by a powerful lens, as represented in Fig. 191. A lens which concentrated the heat ten thousand times melted platinum, gold, quartz, etc., in a few seconds. And as the heat at the sun is supposed to be thirty times more concentrated than this, none of the most solid substances of our earth would remain solid if they were there, but would be some of them liquid, and others even in a state of vapor. The heat which the sun constantly radiates to the earth pervades all substances, producing motion, and awakening life every where, so that, in the expressive language of the Bible, "There is nothing hid from the heat thereof."
Another source of heat is within the earth itself. It has been found as we go down into the earth there is a constant increase of temperature the farther we go. This internal heat is attributed in part to subterranean fires and various chemical actions. We see here and there external evidences of the operation of these causes in the eruptions of volcanoes, the boiling springs, the jets of steam and sulphureous vapors, etc. But that the heat in our earth which comes from these subterranean sources is small compared with that which comes from the sun, is seen in the fact that the rate of increase of heat at great depths is much less than it is nearer the surface. This would seem to show that although fires within the earth may have considerable influence in heating its crust, on which we live, it derives the most of its heat from the sun, at least to a very great depth.
How great a source of heat electricity is we know not, but that considerable heat comes from this source is evident from the melting and burning effect which we often see resulting from the passage of the electric fluid.
Another very common source of heat is chemical action. We see it continually produced in chemical experiments. Combustion, which, as will be shown to you in the Second Part of this Series, is nothing but an example of chemical action, is the most common of all the chemical sources of heat. Animal heat is also, for the most part, a result of chemical action.
Mechanical action is a common source of heat. The rubbing of a match producing heat enough to occasion flame is a familiar example. The spark produced in what is called striking fire is the burning of a particle of steel set on fire by the blow. The Indian was accustomed to light his fire by the rubbing together of two dry sticks till he learned an easier way from civilized neighbors; and the blacksmith, previous to the invention of phosphorus matches, often lighted his fire by touching a sulphur match to a nail made red-hot by rapid and continued hammering. Machinery has sometimes been set on fire by friction, and the water around a mass of metal has been so heated by boring as even to boil. If you stretch a piece of India rubber several times in quick succession, and then apply it to your lips, you will perceive that the motion has warmed it.
273. Relations of Heat and Light.—Heat is sometimes alone, and is sometimes in intimate union with light. All substances have some amount of heat, and it passes from them to other bodies in their neighborhood that happen to have less heat in them. In doing this it may or may not have the company of light. In the radiation of heat from a stove, unless it be heated to redness, there is no light with the heat; but from an open, burning fire the light and heat come together. But the rays of the sun give us the best example of the union of light and heat. Traveling together at an equal pace they are most curiously mingled, as you will see when I come to speak particularly of light.
I will now proceed to notice the principal effects of heat; viz., expansion, liquefaction, and vaporization.
274. Expansion in Solids.—Heat, you have seen in § 23, acts in opposition to the attraction of cohesion, tending to separate the particles, and so produces an expansion of any substance. This may be exemplified in the experiment represented in Fig. 192, in which A B is an iron rod, which is of such a size that at the ordinary temperature it will fit into the space, C D, in a bar of iron, and easily pass through the hole, E. If the rod be heated it will be enlarged or expanded in all directions, so that it will neither fit into C D nor pass into the hole, E. When the wheel-wright puts a tire upon a wheel he uses the expansion of heat to make it fit tightly and firmly. The tire is made a little too small to have it fit upon the wheel as it is. But by being heated it is so expanded that it will readily go on to the wheel, and then in contracting as it cools it so compresses the fellies as to hold on very tightly. Water is poured on to cool the iron quickly, and thus prevent it from burning the wood. Iron hoops are put on barrels in a similar manner, the compression caused by their contraction binding the staves together very strongly. So in fastening the plates of boilers together, the rivets are put in red-hot, so that in their contraction they may press the plates closely together. If an iron gate just shuts into its place in cold weather, its expansion will prevent its shutting when warm weather comes. In order to avoid this difficulty, calculation must be made in fitting it for its place for the expansion to which it will be subjected by heat. So in laying the rails of a railroad in cold weather care must be taken not to put the ends too near together. In constructing iron bridges the expansion by heat must be calculated for in the arrangement. Nails often become loose after the lapse of years from the wear of the wood around them, occasioned by their alternate expansion and contraction. The leaking of gas-pipes in the earth is often undoubtedly caused by the loosening of the joints from contraction and expansion of the pipes by varying temperatures of the soil, especially where they are not laid very deep. If a stopper stick fast in a bottle it can be loosened by the application around the neck of a cloth dipped in hot water, because the neck becomes expanded at once by the heat. A similar expedient was once very ingeniously made use of in repairing the machinery of the steamer Persia at sea, and was perhaps the means of saving the vessel and the lives of all on board. The accident which occurred was the breaking of the port crank-pin of the engine. The problem to be solved was the removal of this pin, which weighed nearly a ton, and the substitution of a sound one which they had on hand in its place. But it was found impossible to start the broken pin from its socket with all the force which could be brought to bear upon it by a sort of battering-ram constructed extemporaneously for the purpose. It was determined now to try the expansive force of heat. An iron platform was built under the socket, and a brisk fire made upon it. The socket soon expanded, and the pin was now readily knocked out by the battering-ram, just as the stopper of the bottle is easily removed when the neck is heated. The walls of a very large building in Paris, which had bulged out and were in danger of falling, were restored to their upright position by the expansion of heat.
It was done in this way: Long rods of iron were run through the walls after the plan represented in Fig. 193 (p. 213), their ends being made with a screw-thread, with nuts fitted to them. The rods marked a were first heated, and as they lengthened the nuts were screwed up tight to the walls. On cooling, their contraction would of course draw the walls together. The other bars, b, were now heated and managed in the same way. The one set, you see, were made to hold on by their nuts to what had already been gained, while the other were expanding. By many repetitions of this process the walls were righted and the building saved. The same mode has been adopted successfully in other cases of a similar character.
275. Expansion in Liquids.—Liquids are expanded by heat more than solids are. But they are very unequally expanded by it. Thus water is expanded more than twice as much as mercury, and alcohol six times as much. We have a frequent example of the expansion of water by heat in our kitchens. If the tea-kettle be put over the fire filled to the brim, it will run over long before the water begins to boil. All liquids occupy more space in summer than in winter, and in the former case weigh less—that is, have less of real substance in them than in the latter. If, therefore, alcohol, or oil, or molasses be bought by the gallon in winter and sold in summer, there will be a profit afforded by the expansion. Twenty gallons of alcohol in winter becomes twenty-one in mid-summer.
The influence of the expansion of heat upon the specific gravity of liquids may be very prettily shown by the following experiment: Let some little bits of amber—a substance which is nearly of the same specific gravity with water—be thrown into water in a glass vessel, and let the water be heated, as represented in Fig. 194, by a spirit-lamp. That portion of the water which is heated passes upward because it is made specifically lighter, and colder water continually comes down to take its place. The upward and downward currents are as indicated by the arrows, the upward passing up in the middle, the downward coming down at the sides. This will be made manifest by the little bits of amber.
276. Thermometers.—It is the expansion of liquids by heat that in the thermometer gives us the measure of temperature. The liquid metal mercury is commonly used for this purpose, and answers well except in the extreme cold of the arctic regions. There, as mercury becomes solid at 39 degrees below zero, it is necessary to use a thermometer with alcohol in it, as this fluid can not be frozen by any degree of cold. The operation of the thermometer is simply this: Heat expands the fluid in the bulb, and the only way in which it can occupy more space as it expands is by rising in the tube. The abstraction of heat, on the other hand, causes contraction, and of course a proportionate falling of the fluid.
277. Fahrenheit's Thermometer.—The thermometer was invented in the beginning of the seventeenth century, but it is not decided who was the inventor. There may have been in this case, as in others, more inventors than one, the same ideas having, perhaps, entered several inquiring minds at the same time. Various fluids were used by different persons. Sir Isaac Newton used linseed oil. Fahrenheit, a native of Hamburg, who flourished in the first part of the last century, was the first to use mercury. Though various propositions were made by Newton and others in regard to the measurement of heat by thermometers, no thermometric scale seems to have met with general reception till that of Fahrenheit's, which was put forth about 1720. The plan of it is this: His zero is the point at which the mercury stood in the coldest freezing mixture that he could make; and he supposed that this was the greatest possible degree of cold, as it was the greatest that he knew. He next found the point at which the mercury stood in melting ice. This he called the freezing point, because the temperature is the same in water passing into the solid from the fluid state as in water passing into the fluid state from the solid. In other words, this point in the scale marks the transition line between the two states. From this point Fahrenheit marked off 32 equal spaces or degrees down to zero. He now found the point at which the mercury stands in boiling water, and called this the boiling point. Marking off the space on the scale between this, and the freezing point in the same manner, there are 180 degrees—that is, the boiling point is 212 degrees above zero. The degrees above zero are commonly designated by the mark +, plus; and those below by the mark -, minus. Thus, +32° signifies 32 degrees above zero, and -32° signifies 32 degrees below.
278. Other Thermometers.—Fahrenheit's thermometer is the one commonly used in this country. But there are several other thermometers on different scales, as the Centigrade, Reaumur's, and De Lisle's. In Fig. 195 you see the plans of the scales of these thermometers placed side by side. In the Centigrade thermometer, which is in use in France, and indeed in a large part of Europe, the zero, you see, is placed at the freezing point; and the space between this and the boiling point is divided into 100 degrees, which gives it the name Centigrade. Reaumur's, which is in use in Russia, has the same zero, but he has only 80 degrees from this to the boiling point. De Lisle's, which has gone entirely out of use, has its zero at the boiling point. The arrangement of Fahrenheit, although its zero is a mere arbitrary point, is, on the whole, the best, because its degrees are of such a size that they mark differences of temperature with sufficient minuteness for all practical purposes of an ordinary character without resorting to fractional parts.
279. Expansion in Aeriform Substances.—Heat produces a vastly greater expansive effect in air, the gases, and vapors, than it does in liquids. The expansion of air by heat may be shown very prettily in this way: Take a glass tube that has a bulb on one end, and, placing the other open end in water (as represented in Fig. 196), apply the palm of your hand to the bulb. The heat of the hand being communicated to the bulb will expand the air, and so, as you see, bubbles of air will escape through the water. On removing the hand, and allowing the bulb to cool, the air in it will be condensed, and water will pass up in the tube in proportion to the amount of air which has escaped. A bladder partly filled with air will be made to swell out to plumpness if it be heated sufficiently, and a full one may be so heated as to burst from the expansion of the air. Porous wood, as chestnut, snaps very much when burned, because the heat expands the air contained in the pores.
280. Balloons.—The first balloons that were used were filled with heated air. You have already seen, in § 149, why it is that balloons rise. Now in the hot-air balloon it is the expansion of the air by heat that makes it lighter than the surrounding air. Of course such a balloon is not as effective as the gas balloon, for the air within it loses its comparative lightness as it becomes cooled; while the gas which is used, being very much lighter than air at the same temperature, does not lose its lightness as the balloon goes up. You learned in § 152 that the atmosphere becomes thinner as we go upward. The gas balloon, therefore, rises until it arrives at that point where the air is of about the same specific gravity with the gas, and there it stops. It is made to descend by letting out some of the gas from a valve. Gas was not used for balloons till 1782. Hydrogen gas was employed at first, being over fourteen times lighter than air. Of late the common burning gas, carbureted hydrogen, has been generally used, because it can be so readily obtained where there are gas-works.
281. Currents in the Air from Heat.—Heat is the grand mover of the atmosphere. Any portion of it that becomes warmer than surrounding portions rises, or rather is pushed up, for the same reason that a hot-air balloon rises, the only difference between the two cases being that in the one the air is confined, and in the other is left free, and so becomes diffused. And it is this rising of the air from expansion that causes nearly all the movements that we witness in the air. We see this exemplified in various ways wherever there is a fire. The air that is heated by the fire is forced upward by the colder air, which, on the principles of specific gravity, seeks to get below the warmer and lighter air. The hot air that comes through the registers of a furnace is pushed up by colder air below. For the same reason the heated air around a stove-pipe is constantly going upward. This is very prettily shown by the toy represented in Fig. 197 (p. 218), which is a paper cut spirally, and suspended, as you see, upon the point of a wire. The upward current makes the paper revolve rapidly around the wire. It is from the rising of warm air that the galleries of a church are warmer than the space below. In a common room the disposition of the air is continually to have its warmest portions above and the colder below. It is for this reason that we have our arrangements for producing or introducing heat at as low a point as possible.
282. Chimneys.—We speak of the draught of a chimney, and we say of one that does not smoke that it draws well, as if the smoke were in some way actually drawn up. But the same principles apply here as are developed in § 281. The smoke, which is a combination of heated air and gases with some solid matters in a fine state, is forced up the chimney. When a chimney does not draw well we open a door or a window for a little while until the fire gets thoroughly agoing. Why is this? It is that we may have denser air than there is in the room, so that the smoke may be pushed up more forcibly. When the chimney becomes well heated there is ordinarily no difficulty, because then the smoke in it is not obliged to part with much of its heat to the walls of the chimney, and therefore is so much lighter than the air in the room that it is very easily forced upward. The principal reason that a stove-pipe generally draws better than a chimney is that there is much less heat expended in establishing and maintaining the upward current. Especially is this true if the chimney be a large one. In such a case there are both a great extent of brick and a large body of air to be heated to establish the upward current, and these must be kept warm in order to maintain it.[3]
283. Winds.—If you open a door of a heated room a candle held near the floor will have its flame blown inward, while one held near the top of the door will have its flame blown toward the cold entry. Here you have a good illustration of the manner in which winds are produced. Wherever the wind blows it is air pushing out of the way other air that is warmer, in order that it may, in obedience to gravitation, get as near the earth as possible. Take, for example, the land and sea breezes, as they are called. During a hot summer's day the sun heats the earth powerfully, while the ocean receives but little of its heat. The heated land heats the air above it; and as the air over the ocean is cooler, and therefore heavier, it pushes upward the air of the land, for the same reason that water pushes up oil; and as this goes on continuously a regular current is established. The wind blows in upon the land, as represented in Fig. 198, while the warmer air passes upward into the higher regions of the atmosphere, and turns toward the sea. The arrows show the course of the currents. The resemblance of all this to the effect upon the candle held near the open door is very obvious, the cold air from the entry blowing in below representing the breeze from the ocean, and the warm air of the room blowing out above representing the passage of the warm air of the land out toward the ocean. At night this is apt to be reversed. The earth becomes cooled, and with it the air that is over it. The result is that the cooled air of the land now pushes upward the warmer air of the sea, as seen in Fig. 199.
284. Winds as Affected by the Rotation of the Earth.—The heat of the vertical sun upon the tropics causes a rise of heated air into the upper regions, while there is a rush of colder air toward the equator from both north and south. This effect is represented in Fig. 200 (p. 221), E being the sun, N the north pole, and S the south pole. An effect similar to that represented in Figs. 198 and 199 is produced here, but it is on a much larger scale. But the diagram does not present the matter in its true light in all respects. The prevailing winds in the equatorial regions are not north and south winds, as would appear from this diagram; but they are from the northeast and southeast. I will explain this by Fig. 201. As the earth turns on its axis it is plain that there is no part of the surface of the earth that moves so rapidly as the equator, E W, for that moves in a larger circle than any other part. And the nearer you go to either pole, N or S, the less is the rapidity of the revolution. Now the atmosphere, as stated in § 188, partakes of the motion of the earth. The air, therefore, at the equator is moving from west to east with the rotation of the earth faster than it is any where else, and the nearer you go to either pole the slower is its motion. It follows from this that any portion of air blowing from the north or the south toward the equator, as it comes from where it was moving east slower than air at the equator is, would from its lesser momentum lag behind the air of the equator, the wind would be curved toward the westward, as indicated by the arrows. The result would be that the northern wind would be converted into a northeaster, and the southern into a southeaster. All this can be made more clear with a globe, or, indeed, with any round object.
285. Liquefaction.—The change of solids into liquids is one of the most observable effects of heat. This change requires different degrees of heat in different substances. Thus while iron melts at the high heat of 2786°, lead melts at 633°, sulphur at 239°, ice at 32°, and mercury at 39° below zero. Mercury is never found in a solid state, but it sometimes becomes solid in the arctic regions when carried there and exposed in the open air. We are apt to think of water as being in a more natural state when liquid than when it is solid, just as we think of iron as being naturally solid and mercury as naturally liquid. But in all these cases the state of the substance depends on its temperature, and this is varied by circumstances. Water at the equator is always liquid, and the idea of ice there is exceedingly unnatural; while near the poles it is the reverse, ice and snow reigning every where throughout the whole year.
286. Evaporation.—There are two ways in which the change of a liquid into a vapor occurs. One is a rapid change when heat is so applied as to raise the liquid to its boiling point. This is commonly termed vaporization. The other mode is the ordinary gradual evaporation which goes on from the surface of the liquid. This process is going on continuously, not requiring any particular degree of heat, but occurring under all degrees of the temperature of a liquid. Its rapidity, however, is in proportion to the degree of heat, as may be seen by the rise of vapor from water that is being heated, long before it begins to boil. The same thing can also be seen in a bright summer's morning, when the heat of the sun causes the moisture gathered from rain or dew to rise so abundantly from fences, and boards, and roofs as to be visible like smoke.
287. Solution of Water in Air.—Evaporation is constantly going on from every wet surface, except when the air is so loaded with moisture that it can take up no more. The vapor is not ordinarily visible, the particles of water passing quietly upward among those of the air, being dissolved in the air just as some solids are dissolved in water. It becomes visible only when so much of it rises that the solution of the water in the air is not readily effected. The readiness with which the solution takes place depends much upon the temperature of the atmosphere. Some very common phenomena illustrate this. In a very cold day the breath of animals, as it comes out of the mouth, seems to be loaded with moisture. Why? It is not because there is more moisture in it than in warm weather, but because cold air can not hold in solution so much water as warm air can. The same explanation applies to the smoking of wet fences and roofs in the sun of a summer's morning. The moisture is heated by the sun, but the air, not having become very warm as yet, can not readily dissolve all the moisture that rises. The phenomenon is not apt to occur when the hot sun shines after a shower at mid-day or in the afternoon, because then the air is warm enough to take up all the moisture that is sent up into it.
How water, being heavier than air, rises in the atmosphere is a mystery. It has been supposed by some that it was owing to a kind of affinity existing between water and air. But in opposition to this is the fact that evaporation takes place more rapidly under the exhausted receiver of an air-pump, where there is almost no air, than it does where it is freely exposed to the atmosphere.
288. Clouds.—The water which goes up in the air in evaporation is variously disposed of. Some of it is deposited as dew or frost. Some of it forms fog. Some of it also mounts far upward and forms the clouds, which are really collections of fog made high up in the air. In fog and in clouds the water which in its evaporation is invisible becomes visible. Let us see how this is. There is always more or less of water in clear air, but the particles are so minutely divided and so thoroughly mingled with the particles of the air that they can not be seen. But in a fog or cloud the particles of water are gathered together in little companies, as we may express it. And it is supposed, some think ascertained, that each of these companies of particles is globular and hollow. If so, then we may regard every cloud as a vast collection of minute bubbles or balloons careering through the air.
289. Shapes of Clouds.—Clouds have a very great variety of shape, the causes of which are for the most part not understood. They are generally divided into four classes: Cirrus, Cumulus, Stratus, and Nimbus. The Cirrus is represented in Fig. 202 (p. 225). It is a light, fleecy cloud, having graceful turns like curls, and hence its name, which is the Latin word for curl. Such clouds are commonly very high up in the air. The Cumulus (Latin for heap) you see in Fig. 203 (p. 225). Clouds taking this form appear as heaps rounded upward, and often appear like mountains of snow when they are illuminated by the sun. We see such clouds mostly in summer. The Stratus (Latin for covering) is seen in the same figure under the Cumulus. Clouds of this form lie low in the horizon, stretched along like a sheet. They often form in the latter part of the day, and increase in the night, but the rising sun dissipates them. The Nimbus, or rain-cloud, is represented in Fig. 204 (p. 226). It has a uniform gray or dark color. We often have two forms of cloud mingled together. Thus in Fig. 205 (p. 226) we have a mixture of the Stratus and the Cirrus, termed Cirro-Stratus. This is commonly called the mackerel-sky, and is quite a sure prognostic of rain. Then we have the Cirro-Cumulus, Fig. 206 (p. 227), and the Cumulo-Stratus, Fig. 207 (p. 227).
Water is gathered into clouds undoubtedly, in part at least, from the influence of attraction. But what the circumstances are that give them all these various shapes we know not. Whatever they are, they sometimes operate very extensively, giving a similar shape to all the clouds that cover the whole arch of the heavens; and at other times they operate variously in different localities, producing different shapes, sometimes even in near neighborhood to each other. Sometimes the edge of a cloud is irregular, or curved, or feathery; and at others it is a well-defined line, stretching along over a large portion of the horizon. In all these cases we have only divers arrangements of the same thing—a collection of vesicles of water containing air, which is made lighter than the air outside of the cloud by means which I shall speak of in another part of this chapter.
290. Rain, Snow, and Hail.—When it rains the vesicles or minute bubbles of which the clouds are composed are broken up, and each drop of rain contains the water which came from a multitude of these vesicles. But let us see exactly how this result is produced. Rain comes from the contraction of the clouds by cold. A cold current of air coming in contact with a cloud will condense its bubbles into drops, and these of course will fall. The same result occurs if a cloud passes into a cold stratum of air. But let us look at the process more minutely. Let us see what the effect of cold is upon the bubbles. The first effect may be made clear by Fig. 208. If a bubble be contracted by the influence of cold, the water of its wall being made thicker, there will be a gathering of it from gravitation at the lower part, as represented by the dotted line. You often see a similar effect in the soap-bubble. It rises filled with the warm air from your lungs, and as it goes up it is contracted by the colder air which is around it. This contraction makes the water hang downward from the bottom of it. And as the soap-bubble at length perhaps bursts in the air from the weight of this water, so it is with the vesicles in the cloud. And many of these, united together by attraction, form a drop. When the cold is sufficiently severe it makes the water of the ruptured vesicles of the cloud arrange itself in snow-crystals instead of drops. And when the cold acts with great rapidity upon a cloud it presses the particles of the water together so suddenly that there is not time for the crystalline arrangement, and hail is formed.
291. Vaporization.—The production of vapor by boiling differs in some respects from quiet evaporation. Here the liquid is raised in temperature to its boiling point, and the formation of vapor is not confined to the surface. In water the boiling point is 212°, but it varies more or less from this in other liquids. Thus the boiling point of alcohol is 173°, of ether 95°, oil of turpentine 568°, and mercury 652°.
292. Influence of Pressure upon the Formation of Vapor.—Pressure restrains the production of vapor, whether it be formed by evaporation or vaporization. We know by experiments with the air-pump that the less pressure of air there is upon the surface of a liquid the more rapidly will evaporation from it go on. I have already spoken of the influence of pressure upon the boiling of liquids in § 171. I will give here a few additional illustrations. Ether boils when it is heated to 95°, three degrees below the heat of the blood in our bodies. If we place some of it in a vessel under the receiver of an air-pump, by exhausting the air we can so take off the pressure that the ether will boil at the ordinary temperature of the air in a room. The restraint of pressure upon boiling is very strikingly shown in the digester, Fig. 209. This is a strong boiler, a, partly filled with water. A thermometer, d, is fastened into it so as to indicate the heat of the water. There is also a tube, c, extending to near the bottom of the boiler into a small quantity of mercury which is there. Let, now, the boiler be heated till the water boils, the air being left to escape by the stop-cock, b. If the stop-cock be shut, and we continue to apply the heat, we can raise the water to a very high temperature without having it boil at all, because of the pressure of the condensed steam upon its surface. An apparatus somewhat after this plan, called Papin's digester, has been used sometimes in cooking. The great heat to which water can thus be raised causes it to extract the nutritious matter from bones and cartilages, affording material for soup from what is commonly thrown away. To guard against the danger of explosion a safety-valve is provided, having a weight upon it which will keep it shut until a certain amount of pressure accumulates, and then it is forced open, letting out some of the steam.
293. Steam.—The cloud of steam, so called, which you so often see escaping from a locomotive is not really steam. Steam is transparent and invisible. You can see that it is so if you observe it issuing from the spout of a tea-kettle. It is only after it gets an inch or more from the spout that it becomes visible, and then it is really changed from steam into water by the condensing influence of the cold air. And the water in the cloud thus formed is probably in the same condition with the water in the clouds above, as described in § 288.
294. The Steam-Engine.—As compressed or condensed air has great power by its elasticity, as seen in the air-gun, § 164, so also has condensed steam. It is steam condensed, and endeavoring, therefore, in proportion to its condensation, to expand itself, which constitutes the moving force of the steam-engine. The steam is generated in a boiler, having, like the boiler of Papin's digester, a valve with a weight attached to it. This valve is called a safety-valve, because when the steam has reached a certain degree of condensation it lifts the valve, and, as some of the steam escapes, such an increase of pressure as would occasion an explosion is prevented. The expansive force of steam in a boiler is estimated in pounds by the weight on the valve, and hence the common expression that there are so many pounds of steam on. But the boiler is only the generator of steam, and it remains to show how the steam is used in moving machinery. This is done by allowing the steam to pass from the boiler into a cylinder, and then move a piston back and forth by its expansive force. The manner in which it does this may be made clear by the diagram, Fig. 210 (p. 231). Let e be a piston in a cylinder, f, which has four openings, a, b, c, and d. These all have valves. The steam is supplied from the boiler to the cylinder through a and c, and makes its escape from b and d. Suppose, now, the piston is near the bottom of the cylinder, as represented. The valve at a is now opened that steam may enter to push up the piston, and the valve at b shuts that the steam may not escape. At the same time, that pressure may be taken off from the upper surface of the piston, d opens that the steam may escape, and c shuts that none may enter. When the piston is to be forced downward all this is reversed—c opens to admit the steam, d shuts to prevent its escaping; and below, b is opened to let the steam escape, and a is shut to prevent any from entering. This is the plan of what is called the high-pressure engine. The low-pressure engine differs from it in having the steam, as it escapes from the cylinder, pass into water to be condensed. The latter requires less pressure of steam to work it, and therefore is the safest. The manner in which the motion of the piston is made to work various kinds of machinery I need not stop to explain, especially as exemplifications of it may be seen in every quarter.
295. Communication of Heat.—Heat has a constant tendency to an equilibrium. If therefore any warm substance be in the neighborhood of one which has less heat, a flow of heat from the former toward the latter takes place. Now this communication of heat occurs in three different ways, called Convection, Conduction, and Radiation. I will speak of each of these separately.
296. Convection.—This mode of diffusion of heat is in operation in those substances whose particles are movable among each other—viz., liquids and aeriform substances. I have already alluded to examples of this mode in speaking of the movements which heat causes in these substances. The heat goes along with the particles which are moved, or is conveyed along with them, and hence the term convection. In this movement the heated particles always ascend, for the reason given in § 275. Of the multitude of examples of convection I will present but a few.
In the upward current about a stove-pipe you have an example of convection, the heat generated being carried upward by the particles of this current. This being so, the heat of a stove has no effect upon the air below it by convection, though it does have by radiation, as you will soon see. Any hot fluid becomes cool chiefly by convection. The air coming in contact with it taking some of its heat rises, and other air comes in its turn to be also heated, and so on till the fluid becomes of the same temperature with the air, and then the currents of air cease. The liquid cools more rapidly by stirring it, because the air is brought into contact with a greater extent of surface, and so the heat is conveyed away more rapidly. The result is the same whether we disturb the surface by stirring it or by blowing upon it. In the latter case, however, the effect is increased by making the air to come more rapidly upon the disturbed surface. So in fanning, it is the bringing of the air faster upon the surface of the body that causes the more rapid, convection of heat from it. Every one must have observed the fact that a buckwheat cake cools much more quickly than a flour or rice cake. It is because it has so many pores and little projections, and so presents a much larger amount of surface to the heat-conveying air than the smoother and more solid cakes. Viscid fluids, as molasses, oil, etc., when heated do not cool as readily as water, because their particles are not as movable, and therefore heat is not conveyed as rapidly upward to be given off to the air.
297. Conduction.—In this mode of diffusion the heat goes through or among the particles of substances. For example, if one end of a bar of iron be held in the fire, it travels through or among the particles to the other end. The gradual progress of the heat may be seen by the following simple experiment: Take a rod of iron and attach to it, as seen in Fig. 211, some little balls of wood by means of wax. By heating one end with a lamp the balls will drop one after another as the heat passing along melts the wax which holds them.
298. Conductors and Non-Conductors.—Heat is conducted more rapidly through some substances than through others. There is great variety in this respect. There is considerable among those which are reckoned as good conductors, as is shown by the experiment represented in Fig. 212. Here are cones of the same size of seven different substances—copper, iron, zinc, tin, lead, marble, and brick—all tipped with a little wax, and placed on a stove. The wax will melt on the copper cone first, showing that this is the best conductor of them all; and on the brick one last, showing that this is the poorest conductor. The conducting powers of the rest are according to the order in which I have mentioned them.
Those substances which allow heat to pass through them very slowly are called non-conductors. The term, though convenient, is not a strictly correct one, for there are no substances which do not conduct heat in some degree. Wood is one of these poor conductors, and hence wooden handles are put upon various instruments and vessels that are used about fires, as the soldering irons of the tinman, the metallic tea-pot, etc. As cloth is a non-conductor, the holder is used in taking off the tea-kettle and in using the flat-iron. Glass is so poor a conductor that if you hold a rod or tube of it across the flame of a spirit lamp or gas burner, and heat it even to redness, you can place your fingers very near to the heated portion with impunity. I had occasion to-day to bend a small glass tube in this way, and I observed some water in it quite near to the heated part which remained undisturbed through the process. It is the non-conducting quality of glass that makes it so liable to break, when it is thick, if it be exposed to any sudden change of temperature. For example, if hot water be poured into a thick glass vessel, the inner surface is quickly expanded; but the outer surface not expanding with it, because the heat is not readily conducted through, this irregularity in expansion causes a fracture. It is for this reason that the flasks, retorts, etc., used by the chemist are made very thin, especially where the heat is to be applied.