The fairy tales of science/Water Bewitched
Water Bewitched.
“Fire burn, and cauldron bubble!”—Macbeth.
The vapour that escapes from the spout of an ordinary tea-kettle, is a much more wonderful emanation than any of those flimsy spirits which the wierd sisters summoned from their magic cauldron. Those deluded old ladies, who wasted so much time in collecting disgusting ingredients for their infernal broth, in dancing wildly around their cooking utensils, and in breaking-in and training broomsticks, have happily disappeared from the face of this beautiful earth, As we cannot look into their magic cauldron, let us peep into the homely kettle.
Science has revealed so many beautiful truths concerning boiling water, that we deem it advisable to devote an entire chapter to their consideration. The reader must not think that we have chosen a trivial subject. It has been well said, that there is no great and no small in nature, and that the force which shapes the world gives form to the dewdrop. To this remark we may add a similar one—namely, that some of the grandest phenomena in nature are represented on a small scale in a kettle of boiling water.
“Mary, bring the kettle!”
Heat, by entering bodies, expands them through a range which includes, as three successive stages, the forms of solid, liquid, and air, or gas; becoming thus in nature the grand antagonist and modifier of that attraction which holds corporeal particles together, and which, if acting alone, would reduce the whole material universe to one solid, lifeless mass.
The influence of heat on the dimensions of material substances affords a convenient method of estimating the relative quantity of heat which will produce a given effect; for since it appears that a certain increase of temperature will invariably be accompanied by a certain degree of expansion of bulk, it follows that, if we can estimate the degree of expansion in any given case, we may thence infer the amount of temperature. Upon this principle depends the utility of those philosophical instruments called thermometers, or heat-measures. As we shall frequently have to refer to the indications of the thermometer, we will describe the construction of this beautiful little instrument.
The mercurial thermometer consists essentially of a fine glass tube with a bulb at one extremity, and which, having been filled with hot mercury or quicksilver, introduced through the open extremity, has been hermetically sealed while full, so that no air can possibly enter. As the tube and mercury in it gradually cool, the enclosed fluid contracts and consequently sinks, leaving above it a vacant space or vacuum, through which it may again expand on the application of heat.
To such a tube it is necessary to add a scale, showing at what height the mercury will stand at any given temperature,—for a tube of mercury without a scale would be just as useless as a balance without weights. Now, to form a scale that shall agree with other scales we must find two fixed points, and then divide the intervening space into a given number of equal parts, or degrees. These fixed points are the temperatures of melting snow or ice, called the freezing-point, and of pure boiling water, named the boiling-point. The first is found by plunging the instrument into melting ice, and then, after the temperature of the bath is attained, marking the position of the mercury upon the tube; it is now placed in a deep metallic vessel nearly filled with water, which is heated until rapid ebullition ensues, and in this manner the position of the boiling-point is ascertained. Fahrenheit's scale being the standard generally adopted in England, it is usual to divide the space between the two points into 180 degrees, the freezing-point being marked 32°, and the boiling-point 112°. In the Centigrade thermometer, which is used on the Continent, the space is divided into 100 equal parts, the two points being marked respectively 0° and 100°. The reader will understand that a degree of heat is a mere arbitrary division, and that 212° Fahr. and 100° Cent. indicate the same temperature. We shall adopt the unphilosophical but convenient scale of Fahrenheit throughout this chapter.
No indication is afforded by the thermometer of the absolute quantity of heat contained in any substance, but merely of the amount of free or sensible heat capable of producing a certain degree of expansion in a column of mercury. If a quantity of ice, at the temperature of zero, or 0°, be placed in a warm room, it will immediately begin to melt, and a thermometer plunged into it will soon indicate 32°, though at first the column of mercury stood at zero. But, strange to say, the mercury will remain stationary at the freezing-point until the whole of the ice has passed into the liquid form. Thus we see that a large quantity of heat is absorbed by the ice in the act of thawing, so as to be no longer appreciable by the thermometer.
Again, if an open vessel containing ice-cold water be placed upon a fire, the temperature of the liquid will rapidly rise to 212°, but at this point it will remain stationary until the whole of the water is converted into steam. The heat thus lost or absorbed during liquefaction and vaporization is called hidden or latent heat, in contradistinction to the heat of temperature.
But we must not forget our kettle. The stream of vapour now issuing from the spout reminds us of the Arabian fable of the genie, who escaped from the fisherman's bottle in the form of a column of smoke. But the genie of the tea-kettle is infinitely more powerful than the genie of the bottle, who was, moreover, a stupid, blustering fellow, quite unlike our faithful servant, Steam. Let us see how our mighty genie may be evoked; in other words, let us ascertain the conditions under which vaporization takes place. Vapours, of which steam is the most familiar to us, are light, expansible, and generally invisible gases, resembling air completely in their mechanical properties while they exist, but subject to be condensed into liquids or solids by cold. Steam is perfectly invisible; but as soon as it comes into contact with the cold air, it is condensed into a white cloud, which consists of minute liquid particles.
When converted into steam, water undergoes a great expansion, a cubic inch becoming under ordinary circumstances a cubic foot of steam; or, to be exact, one cubic inch of water expands, when sufficiently heated, into 1694 cubic inches of steam. We have already shown that this change, like the liquefaction of solids, is effected by the addition of heat to the water. But a much larger quantity of heat enters into vapours than into liquids—into steam than into water. If over a steady fire a certain quantity of ice-cold water requires one hour to bring it to the boiling point, it will require a continuance of the same heat for five hours more to boil it off entirely. Yet liquids do not become hotter after they begin to boil, however long or with whatever violence the boiling is continued. This fact is of importance in domestic economy, particularly in cookery, and attention to it would save much fuel. Soups made to boil in a gentle way by the application of a moderate heat, are just as hot as when they are made to boil on a strong fire with the greatest violence. Again, when water in a copper is once brought to the boiling point, the fire may be reduced, as having no further effect in raising its temperature.[1]
If a thermometer be plunged into the steam that fills the upper part of the kettle, it will indicate 212. The steam is thus found to be no hotter than the water itself. What then becomes of all the heat that passes into the kettle, since it is neither discovered in the water nor in the steam? It becomes latent—that is to say, it enters into the water and converts it into steam without raising its temperature. As much heat disappears in the vaporization of a single pint of water as would suffice to raise the temperature of 1000 pints by one degree! But the reader will be able to form a more adequate conception of the latent heat of steam, from the fact that one gallon of water converted into steam will, by condensation, raise five gallons and a half of ice-cold water to the boiling point!
Could we see through the sides of the kettle we should observe so many strange movements in the liquid that we might easily persuade ourselves that we were peering into some magic cauldron. By substituting a thin glass flask for the kettle, the whole process of boiling may be seen to perfection. On gradually heating water in such a vessel, we first observe the formation of tiny air-bubbles, which dart through the liquid with marvellous rapidity. As the temperature increases these "beaded bubbles winking at the brim" give place to much larger bubbles, which are formed at the bottom of the vessel, and which rise a little way in the liquid, and then contract and disappear in a most mysterious manner, producing a hissing or simmering sound. But as the heating goes on, these bubbles, which consist of steam, rise higher and higher in the liquid, till at last they reach the surface and escape, producing a bubbling agitation, or the phenomena of ebullition. It may now be remarked that steam itself is invisible, as the upper part of the flask appears quite empty; but when it escapes into the cold air it is condensed into a white cloud of minute drops of water.
It was first remarked by Gay-Lussac, an illustrious French chemist, that liquids are converted more easily into vapour when in contact with angular and uneven surfaces, than when the surfaces which they touch are smooth and polished. He also remarked that water boils at a temperature two degrees higher in glass than in metal; so that if into water in a glass flask which has ceased to boil, a twisted piece of cold iron wire be dropped, the boiling is instantly resumed.
Solid bodies having different temperatures will, if kept in contact, gradually change until they all acquire the same temperature. But this diffusion does not take place instantaneously, or there would be no such thing as difference of temperature. The rapidity with which heat is conducted varies in different substances; for example, if we place a silver spoon and a wooden one in boiling water, the handle of the former will become too hot to be held before that of the wooden one is sensibly warmed. Silver is, therefore, a good conductor and wood a bad conductor of heat.
Liquids conduct heat very slowly and imperfectly. If mercury be poured into a jar, and boiling water be poured over it, the metallic fluid will receive heat but slowly from the water. A thermometer let down a few feet below the surface of a pond or of the sea, would, on being drawn up, indicate a lower temperature than that of the surface water; for the latter, heated by the rays of the sun, communicates little or no heat to the water below. Indeed, it may be questioned whether water has any conducting power.
It may be reasonably inquired how it happens that water is made to boil so readily by the application of heat. A little consideration will show that the effect, in a great measure, depends on the manner in which the liquid is heated, by placing it above the source of heat. If we require boiling water we must place the kettle on the fire, and not in the ash-hole. When heat is applied to a vessel of water, in the ordinary way, the fluid particles near the bottom of the vessel, being heated first and expanding, become specifically lighter and ascend; colder particles occupy their place, and ascend in their turn; and thus a current is established, the heated particles rising up through the centre, and colder particles descending at the sides. This is evidently a very different process from conduction. In the case of a solid the heat is conducted from particle to particle; but in liquids there can be no change of temperature without a displacement of particles. Each particle, as soon as it receives a fresh accession of heat, starts off with it, and conveys it to a distance, displacing other and colder particles in its progress. This process has received the name of convection.
The more a liquid is expanded by a given change of temperature, the greater will be the difference of specific gravity between the part which is heated and the rest of the mass, and the more rapid, therefore, will be the circulation from the change. Any tenacity or viscosity in the liquid will impede its motion, and when water is thickened with flour, or other farinaceous substances, it parts with its acquired heat very slowly. Many a person has burned his mouth with hot porridge and expressed his surprise at the slowness with which it cools, without being able to assign the philosophical reason of the phenomenon.[2]
The currents that exist in the ocean are produced by convection, and are quite as easily accounted for as the currents in the heated water of our tea-kettle. The oceanic currents are of great constancy and regularity, but they are modified in their direction by the general distribution of land and water on the earth's surface. That part of the ocean which is immediately under the tropics, and between the eastern and western hemispheres, for example, becomes highly heated. The water being greatly expanded, flows off on either side towards the poles, acquiring a westerly direction as it passes south of the coast of Guinea, and striking the promontory of Cape St. Roque, on the South American coast, is split into two streams. The smaller one continues southwards towards Cape Horn; while the larger current maintains a north-westerly course into the Gulf of Mexico, where it receives further accessions of heat, and is gradually changed in its direction. It now passes along the southern shores of North America, and finally emerges northward in the narrow channel between the peninsula of Florida and the Bahama Islands,where it assumes the name of the Gulf Stream. The temperature of this current is found to be nine or ten degrees higher than that of the neighbouring ocean. This current passes on, gradually widening and becoming less marked, till it is lost on the western shores of Europe. A less accurately defined under-current, from the poles, is constantly setting in towards the Equator, to supply the place of the heated water which takes the course already described. Besides rendering important aid to the navigator, these oceanic currents assist in mantaining an equilibrium of temperature on the earth, moderating the severity of the polar frosts, and tempering the sultry heats of the tropics.[3]
Among the circumstances which materially affect the vaporization of liquids, one of the most important is atmospheric pressure. We have said that water boils at 212°, but this statement requires some qualification, as the boiling point of water will vary according to the pressure of the atmosphere as indicated by the barometer. The aërial ocean which envelopes this planet presses upon the surface of the liquid ocean with a force equal to nearly fifteen pounds on every square inch; in other words, a column of air an inch square, extending from the level of the sea to the top of the atmosphere, weighs between fourteen and fifteen pounds. The elastic force of air is necessarily equal to its pressure. Let us try to make this point intelligible to the reader. If the mercury of a barometer stands at a height of about thirty inches in the open air, indicating a pressure of fifteen pounds, it will stand at exactly the same height in a close room from which all communication with the external air has been cut off. The lowest stratum of the atmosphere is pressed upon by the strata above it, and being highly elastic, it assumes the condition of a bent spring. The confined air of the room is therefore able to support thirty inches of mercury by the elasticity which it acquired before the doors and windows were closed.
We shall now be able to understand the relation that subsists between the phenomenon of ebullition and atmospheric pressure. Water evaporates, or is converted into steam at all temperatures, until the whole space above it is filled with watery vapour of a certain elasticity. This is a wise provision of nature, for if water obstinately retained its liquid form at all temperatures below 212°, the moisture that descended to the earth in the form of rain would never be evaporated during the hottest summers. But there is a difference between evaporation at low temperatures and ebullition or boiling. Water must be heated until its vapour acquires an elasticity equal to that of the atmosphere before ebullition can take place. At 212° the elastic force of steam will support a column of mercury thirty inches high, and at this temperature the steam-bubbles acquire the power of breaking through the surface of the heated water, provided the barometer stands at thirty inches.
Were we to carry our kettle to the summit of a high mountain, we should find that the water would boil at a very low temperature, and never become hot enough to make a decent cup of tea. Thus at the town of Potosi, on the Andes, where the super-incumbent pressure of air will only support some eighteen inches of mercury, water boils at 188°. Again, were we to carry our kettle to the bottom of a deep mine, we should have to heat the water to a point considerably higher than 212° before it would boil, owing to the increased height of the column of air pressing upon its surface.
We now turn to the examination of another interesting point connected with the boiling of water. The reader will doubtless imagine that the hotter a vessel is into which water is poured the sooner the liquid will boil. This is far from being the case, as may be proved by pouring a small quantity of water into a silver basin heated to redness. Instead of flashing into steam, as might be expected, the water will gather itself into a globule and dance about on the hot surface as if bewitched. The liquid is in a state of incessant motion: sometimes it elongates itself into an oval in one direction; then, drawing itself up, it stretches out in a cross direction, and these changes take place so rapidly that a star-shaped figure or rosette is often the result. While the drop is in this spheroidal condition, as it has been called, let the lamp which heats it be withdrawn; the basin gradually cools, and after a short time the drop loses its spheroidal form, spreads out on the metallic surface, and is instantly thrown into violent ebullition. This striking phenomenon is generally known as Leidenfrost's experiment.
All volatizable liquids under similar circumstances behave as water does. Liquid sulphurous acid, for instance, when poured into a red-hot silver or platinum crucible, retains its spheroidal state; its temperature never rising beyond its boiling point. Now, as the boiling point of this liquid is 18°, and therefore much below the freezing point of water, we can actually freeze water in a red-hot crucible by pouring it into the sulphurous acid! The same thing occurs with a mixture of ether and solid carbonic acid when introduced into a red-hot metallic vessel. The mixture requires for its conversion into gas as much time as it would in the air at the ordinary temperature. If we introduce into this mixture a small tube containing a little mercury, the liquid metal instantly congeals into a solid![4] Again, in the place of a metallic basin or crucible, water near its boiling point may be made use of to support a drop of ether. Instead of mixing with the hot water, the ether gathers itself up into a globule and rolls about upon the surface of the other liquid.
Let us confine our attention to the original experiment, to the dancing drop in the red-hot basin. By a series of beautiful experiments it has been satisfactorily proved that the spheroidal drop never touches the heated surface, but is separated from it by a considerable interval. To what, then, is this interval due? Let us quote the words of a clever writer to whom we are indebted for many of the facts contained in this chapter.
"At an early period of railway history, it was proposed by that original genius George Stephenson to substitute for ordinary steel springs, in the case of locomotives, springs of elastic steam. It was proposed to convey the steam into cylinders in which pistons should move steam-tight; these pistons supported by the steam beneath them, were to bear the weight of the locomotive. Now what the great engineer proposed for the locomotive, the spheroidal drop effects for itself—it is borne upon a cushion of its own steam. The surface must be hot enough to generate steam of sufficient tension to lift the drop. The body which bears the drop must be of such a nature as to yield up readily a supply of heat; for the drop evaporates and becomes gradually smaller, and to make good the heat absorbed by the vapour, the substance on which the drop rests must yield heat freely; in other words, it must be a good conductor of heat.
"It is to the escape of steam in regular pulses from beneath the drop that the beautiful figures which it sometimes exhibits are to be referred. By using a very flat basin over which the spheroidal drop spreads itself widely, we render it difficult for the vapour to escape from the centre to the edges of the drop; and this resistance may be increased till the vapour finds it easier to break in bubbles through the middle of the drop than to escape laterally.
"All these facts are in perfect harmony with the explanation, that it is the development and incessant removal of a steam-spring at the lower surface of the drop which keeps the liquid from contact with the metal and shields it from the communication of heat by contact. Owing to this, indeed, the liquid in the spheroidal condition never reaches its boiling temperature. If you plunge a thermometer into a spheroid of water in a red-hot vessel, its temperature will be found to be several degrees under 212°. When the lamp is withdrawn and the basin cools, the tension of the steam underneath the drop becomes gradually feebler. The spring loses its force, the drop sinks and finally comes in contact with the metal. Heat is then suddenly imparted to the liquid, which immediately bursts into ebullition."[5]
It is well known that we may introduce the hand, if moist, into melted lead, nay, into white-hot melted copper or iron, and move it slowly about in thee liquids, not only without burning the hand, but without even feeling the intense heat of the melted metals; whereas iron or copper at a heat far below redness, instantly causes a blister or burn. This apparent anomaly is easily explained. The intense heat of the melted metal instantly vaporizes the moisture of the hand, and the experimentalist receives no injury, as his hand is protected by a thick glove of non-conducting steam.
It is highly probable that the priests of old were acquainted with this fact, and made good use of it in the ordeal of fire. When a person was accused of some crime which could not be proved against him, he was subjected to the fiery ordeal, that is to say, he had to plunge his arm into molten lead or walk barefooted over red-hot ploughshares. If he passed through the ordeal scathless, his innocence was held to be satisfactorily established. Now the reader need not be told that the safety of the suspected person did not depend on his freedom from guilt but on the moisture of his arm or feet and the heat of the metal. The greatest criminal might walk over hot ploughshares, provided they were hot enough to give him sandals of vapour.
Truly the humble tea-kettle is wonderfully suggestive. We had almost forgotten that it forms the text of the present chapter, but just now the water boiled over and reminded us that we had not touched upon those grand kettles of nature, the Geysers, or intermittent boiling fountains of Iceland.
The Geysers, of which there are a considerable number, are springs of hot water holding a large quantity of silex or flint in solution, which issue from the beds of lava of which the wonderful volcanic island is chiefly composed. A jet of boiling water, accompanied with a great evolution of vapour, first appears, and is ejected to a considerable height; a dense volume of steam succeeds, and is thrown up with prodigious force, and a terrific noise like that produced by the escape of vapour from the boiler of a steam-engine. Nature's cauldron boils over! This operation sometimes lasts for more than an hour, and after an interval of repose of uncertain duration, the same phenomena are repeated.
The Great Geyser is the most celebrated of these boiling fountains. Sir George Mackenzie, who was the first to describe it, states that its eruptions were preceded by a sound resembling the distant discharge of heavy ordnance, and the ground shook sensibly; the sound was rapidly repeated, when the water in the basin, after heaving several times, suddenly rose in a large column, accompanied by clouds of steam, to the height of ten or twelve feet. The column then seemed to burst, and sinking down produced a wave, which caused the water to overflow the basin. A succession of eighteen or twenty jets now took place, some of which rose from a height of from fifty to ninety feet. The last eruption was the most violent; this being over, the water suddenly disappeared from the basin, and sunk down a pipe in the centre to a depth of ten feet; but in the course of a few hours the phenomena were repeated with increased energy. The basin of the Great Geyser is an irregular oval, about fifty-six feet by forty-six, formed of a mound of flinty deposits about seven feet high. The channel through which the water is ejected is about sixteen feet in diameter at the opening, but it contracts to ten feet lower down; its depth is estimated at sixty feet.
From experiments made by the Chevalier Bunsen, in 1846, it appears that the Geysers are irregular tubes fed with rain and snow-water, and that their peculiar form favours the heating of the lower portions of the contained water, by the subterranean fires, to a degree far above the boiling point. The eruption of one of these Geysers is explained by supposing that when the whole of the contained water is sufficiently heated to allow of ebullition towards the upper part of the tube, portion after portion of the highly heated water successively bursts into steam as the pressure is diminished by the removal of the upper portion of the aqueous column.
That this is the true explanation of the phenomena is highly probable, since artificial Geysers have been constructed of iron tubes, which being filled with water, and heated near the lower extremity by burning charcoal, eject little columns of boiling water, and mimic all the phenomena presented by the natural Geysers.
Let us now ring the bell, and tell Mary to take away the tea-kettle, for there is no knowing what abstruse subjects it may suggest, as it sits on the hob, singing its peculiar version of "Home, sweet home!" The reader must admit that the title we have chosen for this chapter is the only term that would embrace all the wonderful facts we have related. The bubbles and currents of boiling water, the dancing and ever-changing globule, and the huge cauldrons of Iceland, fall quite naturally under the indefinite heading of "Water Bewitched."
- ↑ Professor Graham.
- ↑ Professor Daniell.
- ↑ Professor Miller
- ↑ Liebig.
- ↑ Westminster Review.