Dr. Bache has determined by experiments that the radiation of heat from a body is not affected by colour, so that in winter all coloured clothes are alike in that respect, and radiate heat without any appreciable difference. The power of _absorbing_ heat, however, is greatly dependent on colour; and as a general rule, good radiators of heat (such as a black cloth, or indeed any surface covered with lamp-black), are also excellent absorbents of heat. Dr. Hooke and Dr. Franklin placed pieces of cloth of similar texture and size on snow, allowing the sun"s rays to fall equally upon them. The dark specimen always absorbed more heat than the light ones, and the snow beneath them melted to a greater extent than under the others; and they both remarked that the effect was nearly in proportion to the depth of the shade, as in the following order:--After black, the maximum absorbent quality was possessed by, first, blue; second, green; third, purple; fourth, red; fifth, yellow.
The minimum absorbent power was observed to belong to white.
When radiant heat is allowed to pa.s.s through gla.s.s, the latter substance is not found to be transparent to heat rays as it is to those of light, but a considerable proportion of heat is arrested and stopped; consequently gla.s.s fire-screens are to be found in the mansions of the wealthy, because they obstruct the heat but do not exclude the cheerful light and blaze of the fireside.
Melloni"s researches on the nature of the rays of heat, and also on the media which affect them, would demand and merit a chapter to themselves; want of s.p.a.ce, however, obliges us to omit the consideration of thermo-electricity, and the refined and beautiful experiments of Melloni, whose labours are a model for the imitation of all original seekers after truth.
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CHAPTER XXVIII.
THE STEAM-ENGINE.
[Ill.u.s.tration: Fig. 381. Hanc.o.c.k"s steam omnibus, which ran on the common roads.]
It must be apparent to those who read popular works on science, that they possess, at all events, one point of utility--viz., that they are _indicative_ of the various subjects that may be selected in science for special, searching and exhaustive study. The subject of steam and the steam engine is not one that could be thoroughly treated of in the narrow s.p.a.ce allowed in this volume, but enough may be said to give some instruction and to impart common principles, whilst the minute details are better examined and learnt in the works of Bourne, Rankine, and other authors who devote themselves specially to the important commercial question of steam.
The first truth to be comprehended is, that all matter contains within its substance the power of creating heat--or as it may be expressed more plainly, solids, fluids, and gases contain what is termed _latent_ or insensible heat, in contradistinction to the heat which is apparent when we touch a vessel containing warm water or approach a cheerful fire; this latter is termed _sensible_ heat, and has formed the subject of the preceding chapters.
If a cold horse-shoe nail is applied to a thin dry slice of phosphorus laid on a sheet of paper, no combustion of the phosphorus ensues, because the temperature of the iron is not sufficiently high to affect that combustible substance; but if the horse-shoe nail is vigorously hammered on an anvil, the particles of the metal are brought closer together, and if it is applied to the phosphorus, so much heat has been generated, thrust or squeezed out by the hammering or _condensation_ of the iron, that it is now sufficiently warm to set fire to it.
[Page 407]
The reverse or ant.i.thesis to this experiment--viz., the production of cold--would be shown if it were possible to expand a ma.s.s of metal suddenly, and this can be effected by first melting together
207 parts by weight of lead.
118 " " tin.
284 " " bis.m.u.th.
When these metals are in the liquid state and perfectly mixed, they are poured from a sufficient height into a pail of cold water, for the purpose of _granulating_ or dividing them into small fragments.
If the granulated compound metal is now mixed with 1617 parts by weight of quicksilver, it becomes suddenly liquefied and expanded: liquefaction is the reverse of solidification, and hence cold is produced from the natural heat of the compound metals being rendered latent by the change from the solid to the liquid state; so that a small quant.i.ty of water placed in a gla.s.s tube, and surrounded with the metals whilst liquefying in the mercury, becomes rapidly converted into ice, the fall of the temperature, as shown by a thermometer, being from 60 Fahr. to 14, which is 18 degrees below the freezing point of water. In the former case, by hammering the iron the _latent heat_ is made _sensible_; whilst in the latter case, by the liquefaction of the compound metal in mercury, the _sensible_ heat is rendered _latent_. The heat rendered latent by melting different substances is not a constant quant.i.ty, but varies with every special body employed, and the Drs. Irvine have proved this fact by the following experiments:--
Ditto, reduced Heat to the of specific heat fluidity. of water.
Sulphur 143.68 Fahr. 27.14 Spermaceti 145 " -- Lead 163 " 5.6.
Bees"-wax 175 " -- Zinc 493 " 48.3.
Tin 500 " 33.
Bis.m.u.th 550 " 23.25.
Every one of these substances requires more heat to bring them into the liquid condition than ice, for which 140 of heat are sufficient, or are rendered latent during its conversion into water.
In coining at the Mint, the cold blank pieces of gold, silver, or copper become hot directly they have sustained the violent and sudden pressure of the coining press, and they must be heated again, or annealed, to restore the equilibrium of the heat disturbed by the violent blow, or else they remain hard and unfit to sustain the finishing process of milling.
The condensation of water when it a.s.sumes a smaller bulk by union with sulphuric acid, is easily proved by measuring a pint of water and a pint of acid, and mixing them together, when a very great increase of temperature may be perceived; and by placing into the mixture a cold copper wire that previously could not ignite phosphorus, it becomes [Page 408] very hot, and when removed and wiped it will cause phosphorus to fire directly it touches that substance. When the mixture of sulphuric acid and water is measured after it has cooled, it has no longer a bulk of two pints, but is found to have lost bulk equal to one or more ounces by measure. The heat evolved by a mixture of four parts of strong sulphuric acid and one part water is shown by the thermometer to be 300 Fahr., and this mode of obtaining heat has been used by aeronauts for the purpose of obtaining artificial warmth without the danger of setting fire to the gas in the balloon.
[Ill.u.s.tration: Fig. 382. Aeronauts in the car warming their hands by a bottle containing sulphuric acid and water.]
When alcohol and water are mixed a change of density occurs, and heat is produced; and if equal measures of alcohol of a specific gravity of .825, and water, each at 50 Fahr., are mixed, a temperature of 70 Fahr. is obtained; if the mixture is made in a gla.s.s vessel, as shown in the annexed cut, the combination is very apparent. To perform the experiment properly, water is poured into the lower tube and bulb, and alcohol into the top one; when this is done, the stopper is inserted, and the whole thoroughly shaken and mixed together; the warmth which is [Page 409] thus obtained is apparent to the hand, whilst the contraction is shown after the mixture is cold, as it no longer fills the two bulbs of the instrument. (Fig. 383.)
[Ill.u.s.tration: Fig. 383. Gla.s.s bulbs and tube to show the contraction in bulk of a mixture of alcohol and water.]
The latent heat of gases is easily shown by suddenly condensing air in a small syringe or pump, of which the piston contains a minute fragment of amadou (a species of fungus, _Polyporus igniarius_; this, according to Simmonds, after having been beaten with a mallet, and dipped in a solution of saltpetre, forms the s.p.u.n.k or German tinder of commerce; it is also used as a styptic, and made into razor strops), which takes fire, and before the invention of vesta and other matches, tobacco-smokers were in the habit of obtaining a light for their pipes and cigars in this manner--viz., by the latent heat obtained from the contraction or compression of air. Then, again, an instructive though opposite parallel is afforded by suddenly expanding or rarefying air in a gla.s.s receiver provided with a delicate thermometer. By pumping out some of the air, a considerable diminution of the temperature occurs, and equal to several degrees of the thermometer. Every child knows that steam direct from the kettle will scald, but if it issues from a high-pressure boiler, say at fifteen pounds on the square inch, the hand may be held with impunity in the escaping steam, as it merely feels gently warm, and not scalding. This is due partly to the loss of heat rendered latent by the expansion of the high-pressure steam directly it pa.s.ses into the air, and partly to the currents of air that are dragged into an escaping jet of steam. This tendency of the air to rush into a jet of steam was discovered by Faraday, and explains those curious experiments with a jet of steam by which b.a.l.l.s, empty flasks, and globular vessels are sustained and supported either perpendicularly or horizontally.
[Ill.u.s.tration: Fig. 384. A. Jet discharging high-pressure steam B B.
Lighted torch held round the escaping steam the flames from the former all rush into the latter.]
If steam at a pressure of about sixty pounds per inch is allowed to escape from a proper jet, and a large lighted circular torch composed of tow dipped in turpentine held over it, the course of the external air is shown, by the direction of [Page 410] the flames, which are forcibly pulled and blown into the jet of steam with a roaring noise, indicating the rapidity of the blast of air moving to the steam jet. (Fig. 384.)
Egg-sh.e.l.ls, empty flasks, india-rubber or light copper and bra.s.s b.a.l.l.s, are suspended in the most singular manner inside an escaping jet of high-pressure steam; and before the explanation of Faraday, reams of paper were used in the discussion of the possible theory to account for this effect; and what made the explanation still more difficult, was the fact that the jet of steam might be inclined at any angle between the horizontal and perpendicular, and still held the ball, egg-sh.e.l.l, or other spherical figure firmly in its vapory grasp. (Fig. 385.)
[Ill.u.s.tration: Fig. 385. A. Ball and socket jet at an angle, and discharging steam. The egg-sh.e.l.ls are supported by the enormous current of air moving into the jet in the direction of the arrows.]
In consequence of the great rush of air towards a jet of escaping high-pressure steam, Mr. Goldsmith Gurney has patented the application of this principle in his ventilating steam jet, which he has already successfully applied; in one case especially, where a coal-mine had been on fire for several years, and the whole working of the coal-measures in the pit was jeopardized by the spreading of the combustion to new workings; the fire was first extinguished by carbonic acid gas, pulled, as it were, into the coal-mine by a jet of steam blowing into the _downcast_, but placed in connexion with a furnace of burning c.o.ke; and the circulation of the carbonic acid, called _choke-damp_, through the pit workings was further a.s.sisted by a jet of high-pressure steam blowing upwards, and placed over the mouth of the _upcast_ shaft.
The experiment succeeded perfectly at the South Sauchie Colliery, near Alloa, about seven miles from Stirling, where a fire had raged for about thirty years over an area of twenty-six acres in the waste seam of coal nine feet thick. (Fig. 386.)
For the general purpose of ventilating the coalmine, Mr. Gurney"s plan was tried at the Ebbw Vale Colliery, and very economically, the waste steam alone being used. Experiments have also been satisfactorily made with it for blowing a cupola for smelting iron, and with dry steam--_i.e._, steam of a very high pressure--escaping through a warm tube, the results were perfectly successful.
[Page 411]
[Ill.u.s.tration: Fig. 386. Gurney"s steam jet. A. Furnace. B. Water tank.
C. Downcast stopping. D. Upcast stopping. E E E. Steam jets. F F.
Galleries from shaft to shaft.]
[Page 412]
With this digression from the subject of latent heat derived from the compression of air, we return again to the subject with another case in point, furnished by the Fountain of Hiero, as it is called, at Schemnitz, in Hungary, described by Professor Brande; and it may be observed that all the phenomena related would apply to the great pressure of the water from the water-towers at the Crystal Palace, if fitted with a similar air-vessel.
"A part of the machinery for working these mines is a perpendicular column of water 260 feet high (the Crystal Palace water-towers are each 284 feet high), which presses upon a quant.i.ty of air enclosed in a tight reservoir; the air is consequently condensed to an enormous degree by this height of water, which is equal to between eight and nine atmospheres; and when a pipe communicating with this reservoir of condensed air is suddenly opened, it rushes out with extreme velocity, instantly expands, and in so doing it absorbs so much heat as to precipitate the moisture it contains in a shower of snow, which may readily be gathered on a hat held in the blast. The force of this is so great, that the workman who holds the hat is obliged to lean his back against the wall to retain it in its position."
The best examples of latent heat are furnished by ice, water, and steam, and we are indebted chiefly to Dr. Black for the elegant and conclusive experiments demonstrating the important truths connected with the latent heat of these three conditions of matter. When various solids are heated, they frequently pa.s.s through certain intermediate conditions of softness, terminating in perfect liquidity; but ice and many other bodies change at once to the liquid state on the application of a sufficient quant.i.ty of heat. The process of melting ice is very slow, because every portion must absorb or render latent a certain quant.i.ty of heat before it can take the liquid state--hence the difficulty of melting blocks of ice when they are surrounded with non-conducting materials; and this fact the author has proposed to take advantage of in keeping water cool which is to be supplied to the ova of salmon whilst taking them to stock the rivers of Australia.
In order to prove that heat is rendered latent by the liquefaction of ice, it is only necessary to weigh a pound of finely-powdered ice and a pound of water at 212 Fahr. (_boiling water_), and mix them together; when the ice is all melted, the resulting temperature is only 52, therefore the boiling water has lost 160 of temperature, of which 20 can be accounted for, because the resulting temperature of the melted ice is 52; but in the liquefaction of the pound of ice, 140 have disappeared or become latent, or, as Dr. Black termed it, have become _combined_.
1 lb. of ice at 32 + 20 = 52, the resulting temperature.
1 lb. of water at 212 - 52 = 160 - 20 = 140, rendered latent.
140 represents the result obtained from innumerable experiments made by mixing equal parts of ice and boiling water, and it is this large quant.i.ty of latent heat required by ice and snow that prevents their sudden liquefaction, and the disastrous circ.u.mstances that would arise from the floods that must otherwise always be produced.
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To put the fact beyond all doubt, it is advisable to mix together equal weights of water at 32 and boiling water at 212, and the result is found by the thermometer to be the mean between the two, because half the extremes are always equal to the mean; and if the two temperatures are added together and divided by two, the result is a temperature of 122, as shown below:--
1 lb. of ice water at 32 + 1 lb. of water at 212 = 244 2 = 122.