[Ill.u.s.tration: Fig. 372. A B. Inverted sheet iron syphon. At C is seen the piece of tow moistened with alcohol, which, being set on fire, warms the tube B. D. A lighted torch of coloured spirit, the flame of which is dragged down the tube at A by the descending current, and is impelled upwards by the ascending current B.]
This plan of ventilation was proposed to be used in rooms in connexion with the chimney and chimney-piece, and in order to give it an ornamental appearance, the chimney-piece was supplied with two ornamental hollow columns, the ends of which were open at the mantel-shelf, and the tubes or columns were continued under the hearthstone, proceeding up the back of the grate and entering the chimney, in which there would be a constant current of heated air, and it was expected that the [Page 392] syphon arrangement would keep a current of air always in motion, and thus help to ventilate the room.
(Fig. 373.) This plan, however, does not appear to have been adopted, and wisely so, because half the time the syphon arrangement might invert itself, and vomit smoky air out of the chimney into the room; indeed it is surprising what odd and contradictory freaks are performed by currents of air. The author remembers a case where two rooms on the same floor, the one a dining-room and the other a drawing-room, were always exhibiting the most absurd phenomena of smoke. If the fire in one room was lit, then the other, in a few moments, began to smell exactly like the inside of a gas manufactory, and was, of course, more or less filled with smoke, whilst the room in which the fire was actually burning remained quite free from this annoyance. The smoke appeared to issue from the wainscot or moulding which runs round at the bottom of the wall, and was at first thought to be an escape from the chimney of the kitchen beneath, the inside of which was duly examined and thoroughly stopped with cement in every place likely to afford a channel to the smoke, and [Page 393] the crevice whence the smoke issued was also filled in neatly with cement. But it was all in vain; the smoke then made its way out from another part of the cornice, and at last the rooms exhibited a beautiful reciprocating action. If the drawing-room fire was lighted the dining-room was full of smoke, and if the latter was lighted the former had the agreeable visitation. At last the backs of the two grates were examined, and in each was discovered a hole about one inch in diameter; and it was also found that the s.p.a.ces at the back of the stoves had not been filled in properly, and, indeed, communicated with the hollow s.p.a.ce behind the cornice. When, therefore, the fire was lighted, and coals heaped on just above the hole, the gas and smoke distilled through the orifice and travelled on, where it found the most convenient exit; and the fact is sadly at variance (_apparently_) with theory, because it might be considered that cold air would rush towards a fire, and that the draught ought to have been from the cornice to the chimney instead of _vice versa_. The fact seems to be that the coal in all grates is, in the act of burning, distilling and giving off inflammable gas; when the coal was, therefore, heaped above the orifice, and was, possibly, caked hard at the top, the gas distilling from it escaped more easily from the little orifice than elsewhere, and chance determined that the channel or delivery pipe should be in the direction of the drawing-room when the fire was burning in the dining-room, and in the contrary direction when the fire was lighted in the latter chamber.
The nuisance was stopped by plugging the holes at the back of the grate with clay, and putting a sheet of iron over the orifice.
[Ill.u.s.tration: Fig. 373. A B. Chimney-piece supported on two hollow ornamental pillars corresponding with the short arm of a syphon. C C C.
The dotted line showing the pipes leading from each pillar under the hearth, and terminating in a long pipe pa.s.sing into the chimney. The arrows show the path of the air descending from the chimney-piece and ascending in the chimney.]
Before Dr. Faraday was appointed as a scientific counsellor to a.s.sist the deliberations of the Trinity Board in connexion with lighthouses, all the lamps were burnt in the lanterns with the smallest and most imperfect arrangement for carrying off the heated air and products of combustion; as a natural consequence, and particularly on cold nights, the windows of the lantern of the lighthouse were covered with ice derived from the condensation of the water produced by the combustion of the hydrogen of the oil, whilst the carbon generated such quant.i.ties of carbonic acid that the light-keepers were unable to stay in the lantern, and if obliged to visit the latter (whilst looking to improving the light of any single lamp that might be burning dimly), they were almost overpowered with the excess of carbonic acid, and stated, in their evidence, that it produced headache and sickness, and a tendency to insensibility. Faraday immediately established a system of ventilation; and by attaching a copper tube to the top of each lamp-chimney, and centering them all in one large funnel pa.s.sing to the top of the lighthouse, the whole of the water which previously condensed on the gla.s.s windows and impeded the light, besides injuring the bra.s.s and copper fittings, was carried off, as also the poisonous carbonic acid gas; and thus, as Dr. Faraday expressed himself, a complete system of sewage was applied to the lamps of the lighthouses.
If any one of the numerous stories of ships saved by the Eddystone Lighthouse could demonstrate more than another the value of this beacon [Page 394] in mid ocean, it must be the graphic account in the _Times_ of the gallant conduct of the British Admiral with his fleet whilst breasting the frightful storm of October, 1859, and endeavouring to reach Plymouth Sound:--
"It was on Sat.u.r.day, the 22nd October, that the _Hero_, the _Trafalgar_, the _Algiers_, and the _Aboukir_, accompanied by the _Mersey_, the _Emerald_, and the _Melpomene_, put to sea from Queenstown. Up to the afternoon of Monday the squadron met with no remarkable adventure, but about that time, just after the crews had been exercised at gunnery practice, heavy storms of hail and sleet began to set in. Still there was no immediate indication of the tempest at hand, and at sunset topsails were double-reefed and courses reefed for the night, with no particular character about the wind, except that of extreme variability.
As the morning broke on Tuesday--the day of the storm--the Land"s-end was sighted, and the rain and the wind continued to increase. About nine A.M. the advent of the gale was no longer doubtful; topgallantyards were sent on deck and topgallantmasts struck, and the signal was given from the flagship, "Form two columns; form line of battle; Admiral will endeavour to go to Plymouth." To Plymouth, accordingly, the course of the fleet was shaped, but so terrifically had the wind increased that it became very questionable whether the sternmost ships of the line could possibly succeed in entering the Sound. Upon this the Admiral determined to wear the fleet together, stand off, and face the storm, a manoeuvre which, under circ.u.mstances of great difficulty, was most gallantly executed. The ships were close upon the Eddystone Lighthouse, round which they "darted like dolphins" under the tremendous pressure of the gale, the _Trafalgar_ stopping in the midst of the storm to pick up a man who had fallen overboard. The whole squadron now stood off the land, the _Mersey_ and _Melpomene_ furling their sails, and the former vessel steaming along "like an ocean giant." Still the gale increased till about three P.M., when there occurred that remarkable phenomenon by which these rotatory tempests are characterized. The fleet had got into the very centre of the storm, the "eye" of the tornado, and, though the sea towered up and broke in tremendous billows all around, the wind suddenly ceased and the sun shone. When, however, the signal had been given and obeyed for setting sail again, the ships soon encountered the gale once more--not, as before, from the S.E., but the N.W.--and in greater force than ever. It was now a perfect hurricane; and for three hours the whole fury of the tempest was poured upon the squadron. When it began, at length, to abate a little, the four line-of-battle ships and one of the frigates were still in company, and all doing well. The _Mersey_ and the _Emerald_ had steamed into Plymouth, but the five remaining vessels kept in open order throughout that terrible night, wore in succession by night signal at about one A.M., made the land at daylight, formed line of battle, came grandly up Channel under sail at the rate of eleven knots an hour, steamed into Portland, and "took up their anchorage without the loss of a sail, a spar, or a ropeyarn.""
After making the important improvement in the ventilation of lighthouses, many letters were addressed to the learned philosopher by [Page 395] numerous light-keepers, one of which in plain but striking language related that "_the enemy_ (alluding to the water and carbonic acid) _was now driven out_."
[Ill.u.s.tration: The British fleet rounding the Eddystone Lighthouse during the great storm of October, 1859. _p. 394_]
The ingenious invention alluded to was succeeded by another and equally simple but philosophical arrangement, which Dr. Faraday presented to his brother, and it was duly patented. It consisted of an arrangement for ventilating gas burners, and it must be obvious that a necessity exists for such ventilation, because every cubic foot of coal gas when burnt produces a little more than a cubic foot of carbonic acid. A pound weight of ordinary coal gas contains about 3/10ths of its weight of hydrogen, which when burnt produces two pounds and 7/10ths of a pound of water. A pound of ordinary coal gas also contains about 7/10ths of its weight of charcoal, which produces when burnt rather more than two and a half pounds of carbonic acid gas--viz., 2.56. In order to burn this quant.i.ty of gas nineteen cubic feet and 3/10ths of a foot of atmospheric air, containing 4.26 cubic feet of oxygen, are required.
[Ill.u.s.tration: Fig. 374. A B. Gas pipe and argand burner; the air enters, as usual, up the centre of the argand. C C. The first gla.s.s chimney open at the top. D D. The second gla.s.s chimney closed at the top, with a disc of double talc, and fitting over C C, and leaving a s.p.a.ce between the two gla.s.ses, down which the air pa.s.ses, and into the ventilating tube, E E. H H. The ground-gla.s.s globe closed at the top, and surrounding the whole.[I]]
[Footnote I: Mr. Faraday, of Wardour-street, supplies this ventilating lamp.]
It is not therefore surprising that as common coal gas is sometimes purified carelessly, and contains a minute trace of sulphuretted hydrogen, with some bisulphide of carbon vapour, that it should produce the most prejudicial effects in badly ventilated rooms, and especially in some of those perched up gla.s.s boxes in large places of business, where clerks are obliged to sit for many consecutive hours, lighted by gas, and breathing their own breath and the products of combustion from the gas light, thereby rendering themselves liable to diseases of the lungs, and also to very troublesome throat attacks, when leaving their close gla.s.s boxes, and pa.s.sing into the cold night air. The dangerous product of the combustion of ordinary coal gas is sulphurous acid--viz., [Page 396] the same gas as that generated when a sulphur match is burnt; and if it will attack the bindings of books, and damage furniture, goods in shops, curtains, &c., in consequence of the large quant.i.ty of water with which it is accompanied, how much more is it not likely to injure the delicate organism of the breathing apparatus of the lungs? Dr. Faraday"s lamp is therefore a great boon, but, like a great many other clever things, it must be adapted to the currents of air and draught from the room; and means must be taken to prevent the draught becoming too powerful in Faraday"s lamp, or else the illuminating power is destroyed by the thorough combustion of the carbon of the coal gas, and the heat generated is so intense that the gla.s.ses soon crack, and of course become useless. The lamp will answer very well if (as has been already stated) the draught in the ventilating pipe is not too great.
[Ill.u.s.tration: Fig. 375. Section showing the two air-shafts. A. The downcast. B. The upcast. C C. One of the working galleries in connexion with the upcast and downcast. D. The furnace at the bottom of the _upcast_. In this sketch _one_ gallery only has been shown, to prevent confusion and to show the principle.]
The system already explained and ill.u.s.trated is likewise carried out on a much larger scale in the ventilation of coal pits, where a shaft is usually sunk into the ground for the admission of air, which, after circulating through the intricate windings and mazes of the coal pit workings, escapes at last from another shaft, at the bottom of which is placed a powerful furnace, and this is kept burning night and day, so [Page 397] that the movement of the air is maintained in one direction--viz., from the outer air down the shaft called _the downcast_, thence to the galleries, where the coal hewers are working, to the second shaft, near which the furnace is placed, and up this latter the air travels; the shaft, pit, or funnel being very appropriately termed the _upcast_.
Should the furnace at the bottom of the upcast be neglected, the ventilation may be just balanced, or set slightly towards the downcast; under these circ.u.mstances the carbonic acid from the fire will begin to circulate in the galleries, and poison those who are not aware of its presence and take the proper means to escape. Such accidents, amongst the host of others that occur in a coal pit, have actually been recorded; and the firemen, whose duty it might be to attend to the proper burning of the furnace, have had to pay the penalty of death for their own carelessness in falling asleep and neglecting to maintain the ventilation of the mine in one direction. (Fig. 375.)
These details are amply sufficient to demonstrate the manner in which heat is diffused through air, whilst the rarefication of the air by heat suggests the cause of those frightful storms of wind that rush from other and colder parts of the surface of the globe, to supply the void produced by the cooling and contraction of the enormous volumes of gaseous matter.
_The Radiation of Heat._
When rays of heat are emitted from incandescent matter, they are not necessarily visible, nay, they are generally invisible, and not accompanied with a manifestation of light, and pa.s.s with great velocity through a void or vacuum, also through air and certain other bodies.
From what has been stated respecting the manner in which air, by continually moving, and by convection, carries off heat, it might be thought that no proof existed that invisible rays of heat are really thrown off from a ball filled with boiling water. But this question is set at rest by the fact, that such a ball will cool rapidly when suspended by a string inside the receiver of an air pump from which the atmospheric air has been removed, so that no conduction of the particles of air could possibly remove the heat.
In the year 1786, Colonel Sir B. Thompson examined the relative conducting powers of air and a Torricellian vacuum--the latter being used because, as the experimenter stated, it was impossible to obtain a perfect vacuum, on account of the moist vapour which exhaled from the wet leather and the oil used in the machine, for at that time carefully _ground_ bra.s.s plates were not used in air-pumps, but plates only, with a circular piece of wet leather upon them. In a paper which Colonel Sir B. Thompson read before the Royal Society, he stated that "It appears that the Torricellian vacuum, which affords so ready a pa.s.sage to the electric fluid, so far from being a good conductor of heat, is a much worse one than common air, which of itself is reckoned among the worst; for when the bulb of the thermometer was surrounded with air, and the instrument was plunged into boiling water, the mercury rose from 18 to 27 [Page 398] in forty-five seconds; but in the former experiment, when it was surrounded by a Torricellian vacuum, it required to remain in the boiling water one minute thirty seconds to acquire that degree of heat. In the vacuum it required five minutes to rise to 48-2/10ths; but in air it rose to that height in two minutes forty seconds; and the proportion of the times in the other observation was nearly the same.
"It appears, from other experiments, that the conducting power of air to that of the Torricellian vacuum, under the circ.u.mstances described, is as 1000 to 702 nearly, for the quant.i.ties of heat communicated being equal, the intensity of the communication is as the times inversely. By others it appears that the conducting power of air is to that of the Torricellian vacuum as 1000 to 603."
[Ill.u.s.tration: Fig. 376. The air-pump and receiver, containing at A the electric light in the focus of a concave mirror, and at B a delicate thermometer, also in the focus of a concave mirror.]
It is therefore very interesting to discover that the attention of experimentalists was early directed to the fact that heat was independent of the air, and pa.s.sed either as waves of heat or molecules of heat through s.p.a.ce. The velocity with which heat moves through a vacuum is very great, and in an experiment performed by M. Pictet, no perceptible interval took place between the time at which caloric quitted a heated body and its reception by a thermometer at a distance of sixty-nine feet. It appears also, from the experiments of the same philosopher, to be thrown off or radiated in every direction, and not to be diverted (as shown at p. 369) by any strong current of air pa.s.sing it transversely. Sir Humphrey Davy ignited the charcoal points connected with a battery in a vacuum, taking care to place the charcoal points at the top of the jar, and a concave mirror, with a delicate thermometer in its focus, at the bottom of the vessel placed upon the air-pump plate.
The effect of radiation was [Page 399] ascertained first when the receiver was full of air, and next when it was exhausted to 1/120th (_i.e._, 199 parts pumped out, leaving only one part of air in the receiver). In the latter case, the effect of radiation was found to be three times greater than in an atmosphere of the common density. The greater rise of the thermometer _in vacuo_ than in air is to be ascribed to the conducting power of the latter; for this conducting power, by reducing the temperature of the heated body, has a constant tendency to diminish the activity of radiation, which is always proportional to the excess of the temperature of the heated body above that of the surrounding medium. (Fig. 376.)
Count Rumford"s experiments with a Torricellian vacuum gives the proportion of five _in vacuo_ to three in air for the quant.i.ties of heat lost by radiation, and by conduction or diffusion. It is not, perhaps, departing very far from the truth, if it be stated that one half of the heat lost by a heated body escapes by radiation, and that the rest is carried off by the convective power of currents of air.
[Ill.u.s.tration: Fig. 377. Negretti and Zambra"s terrestrial radiation thermometer. The bulb of this instrument is transparent, and the divisions engraved on its gla.s.s stem. In use it is placed with its bulb fully exposed to the sky, resting on gra.s.s, with its stem supported by little forks of wood, and protected from the wind.]
If the process of radiation was not constantly proceeding, it can easily be imagined that the temperature of our globe would become so elevated by the regular accession of heat from the sun"s rays, that the vegetation would be parched up and destroyed, and consequently all animals and the human race must become extinct. The best time to notice the radiation of heat from the earth is at night and after a hot summer"s day. If the sky is clear, it will be noticed (with the help of a thermometer,) that the ground is several degrees colder than the air a few feet above it. (Fig. 377.) It is this reduced temperature that causes the deposition of dew, and produces the earth-cloud which so nearly resembles a sheet of water as to have been occasionally mistaken for an inundation, the occurrence of the previous night. Mr. Luke Howard has called this cloud, which is the lowest form of these draperies of the sky, "The Stratus," or evening mist; but when permanent, and increased to a depth so as to rise above our heads, it is then called the morning fog, so peculiarly agreeable in London when incorporated with the black smoke, making a fine reddish-yellow ochreous mist. By placing a thermometer, standing at the ordinary temperature of the air, cased [Page 400] with a good radiating material, such as filaments of cotton, in the focus of a concave mirror, and by turning this arrangement towards a clear sky in the evening, it will be noticed that the temperature falls several degrees. Good radiators of heat are black and scratched surfaces, filaments of cotton, gra.s.s, twigs, boughs, and certain leaves, especially those with a rough surface.
Bad radiators of heat are bright and polished metallic surfaces, white woollen cloth or flannel, hard and dense substances, such as a gravel path and stone, or those leaves which have a polished surface, such as the common laurel. It is the frozen dew and mist which produce the beautiful effect of h.o.a.r-frost and icicles on the trees and bushes, the primary cause being the radiation of heat from the various objects on the surface of the earth, as well as from the latter itself. When the wind is high, dew does not deposit, as it is necessary that the air should be calm, in order to receive the cooling impression of the cold earth, and to deposit the moisture, which it holds in solution as invisible steam. When the wind blows, it mixes all parts of the air together, and prevents that difference of temperature which causes the deposit of dew. Hence the evening mist will be more generally observed in the bosom of a valley surrounded by hills and screened from the winds that may blow from either quarter. The continual presence of moisture in the air is well shown by the condensation of water on the outside of a gla.s.s of cold spring water, or especially on the outside of a jug containing iced water. The invisible steam is always ready to bathe the tender plants with dew, which would otherwise perish and be burnt up during a hot summer, if they did not radiate heat at night, and thus condense water upon themselves. The presence of watery vapour in the air becomes therefore a matter of great importance, and hence the construction of hygrometers or measurers of the moisture in the air.
Regnault"s condenser hygrometer consists of a tube made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being 1.8 in depth by 8.10 in diameter. This is fitted tightly to a bra.s.s stand, with a telescopic arrangement for adjusting when making an observation. The tube has a small lateral tubulure, to which is attached an India-rubber tube with ivory mouthpiece; this tubulure enters at right angles near the top, and traverses it to the bottom of largest part. A delicate thermometer is inserted in through a cork, or India-rubber washer, at the open end of the tube, the bulb of which descends to the centre of its largest part.
A thermometer is attached for taking the temperature of the air; also a bottle for containing ether.
To use the condenser hygrometer, a sufficient quant.i.ty of sulphuric ether is poured into the silver tube to cover the thermometer bulb. On allowing air to pa.s.s bubble by bubble through the ether, by breathing in the tube, an uniform temperature will be obtained; if the ether continues to be agitated by breathing briskly through the tube, a rapid reduction of temperature will be the result. At the moment the ether is cooled down to the dew-point temperature, the external surface of that portion [Page 401] of the silver tube containing the ether will become covered with a coating of moisture, and the degree shown by the thermometer at that instant will be the temperature of the dew-point.
The most simple form of the hygrometer was formerly a very favourite indicator of the state of the weather, and usually consisted of the figure of a monk with his hood, which is attached to a bit of catgut; this covering of paper, painted to represent the hood, falls over the head on the approach of damp weather, and inclines well back during the period that the air is dry or contains less moisture; and simple as it is, this hygrometer, in conjunction with the reading of the barometer, may a.s.sist _Paterfamilias_ in deciding the fate of a pet bonnet or velvet mantle, which is or is not to be worn on a doubtful day. (Fig.
378.)
[Ill.u.s.tration: Fig. 378. The monk hygroscope, in which the hood, A B, covers the head to dotted line C in wet weather, and takes various intermediate positions, being quite back and on the shoulders in dry states of the air. A thermometer, D, is usually attached.]
A decision on the possible changes of the weather requires considerable experience, and it has been said that one of the most celebrated marshals of France owed his invariable success in military combinations and attacks to his attention to the signs of the weather, as indicated by the state of the air during the phases of the moon. Inexperienced persons (and by that we mean young persons) may, however, take a certain position in the rank of "weather prophets" by consulting the weatherc.o.c.k, the barometer, and the hygrometer, before committing themselves to an opinion, if asked to say what the weather will be.
The dry and wet bulb hygrometer (as represented in the next engraving) consists of two parallel thermometers, as nearly identical as possible, mounted on a wooden bracket, one marked _dry_, the other _wet_. The bulb of the wet thermometer is covered with thin muslin, round the [Page 402] neck of which is twisted a conducting thread of lamp-wick, or common darning-cotton; this pa.s.ses into a vessel of water, placed at such a distance as to allow a length of conducting thread of about three inches; the cup or gla.s.s is placed on one side, and a little beneath, so that the water within may not affect the reading of the _dry bulb thermometer_. In observing, the eye should be placed on a level with the top of the mercury in the tube, and the observer should refrain from breathing whilst taking an observation. The temperature of the air and of evaporation is given by the readings of the _two thermometers_, from which can be calculated the dew-point, tables being furnished for that purpose with the instrument. (Fig. 379.)
[Ill.u.s.tration: Fig. 379. The dry and wet bulb hygrometer.]
The colour of the sky at particular times affords the most excellent guidance to doubting members of pic-nic or other out-of-door parties.
Not only does a rosy sunset presage fine weather, and a ruddy sunrise bad weather, but there are other tints which speak with equal clearness and accuracy. A bright yellow sky in the evening indicates wind; a pale yellow, wet; a neutral grey colour const.i.tutes a favourable sign in the evening, an unfavourable one in the morning. The clouds, again, are full of meaning in themselves. If their forms are soft, undefined, and feathery, the weather will be fine; if their edges are hard, sharp, and defined, it will be foul. Generally speaking, any deep, unusual hues betoken wind or rain, while the more quiet and delicate tints bespeak fine weather.
The principle of radiation of heat is employed by the Indian natives in the neighbourhood of Calcutta for the purpose of obtaining small [Page 403] quant.i.ties of ice. In that climate, the thermometer during the coldest nights does not indicate a lower temperature than about 40 Fahr. The sky, however, is perfectly cloudless, and as heat radiates with great rapidity from the surface of the ground, the Indian natives ingeniously place very shallow earthenware pans on straw, which is a bad conductor of heat, and hence insulates the pans from communication with the parched earth. In a few hours, the water in the pans is covered with a thin sheet of ice, and there can be no doubt of its production by an absolute loss of heat by radiation, because the plan does not succeed on a windy night, and succeeds best even when the pans are sunk in trenches dug in the earth. A windy night prevents that difference of temperature between one portion of the surface of the earth and another, which is so essential to a steady and uniform loss of heat, as it must be evident that the continual mixture of warmer portions of air with that which is colder would tend to prevent the desired lowness of temperature being attained.
The manner in which heat is observed to be radiated has suggested another theory to the fertile brain of philosophical observers, and it has been supposed that the conduction of heat may be nothing more than a radiation from one particle of matter to another, as through a bar of copper, in which the particles, though packed closely together, are not supposed to be in actual contact, so that it is possible to conceive each separate atom of copper receiving and radiating its heat to the neighbouring particle, and so on throughout the length and breadth of the metal. By this theory the radiation of heat through a vacuum is brought into close connexion with that of the radiation of heat through the air and other solid and liquid bodies.
Some of the most interesting phenomena of heat are those discovered by Leslie, who has proved in a very satisfactory manner that the rapidity with which a body cools, depends (like the reflection of light) more on the condition of the surface than on the nature of the material of which the surface is composed. With a globular and bright tin vessel it was observed that water of a certain heat contained in it, required 156 minutes to cool; but when the latter vessel was covered with a thin coating of lamp-black and size, the water fell to the same degree as that noticed in the first experiment in the s.p.a.ce of eighty-one minutes.
By very careful observations made with a differential air thermometer, Leslie determined that the power of radiating heat in various substances was as follows:--
Lamp-black 100 Writing paper 98 Sealing wax 95 Crown gla.s.s 90 Plumbago 75 Tarnished lead 45 Clean lead 19 Iron, polished 15 Tin plate 12 Gold 12 Silver 12 Copper 12
[Page 404]
As in the reflection of light, it was noticed that a piece of charcoal covered with gold leaf, partook of the nature of the precious metal so far as its power of throwing off or scattering the rays of light was concerned, so a piece of gla.s.s covered with gold-leaf appears to possess the same power of radiating heat as that of any brilliant metal.
Radiant heat, like light, can be propagated through a great variety of substances, but is stopped by the larger number; and it can be reflected, refracted, polarized, absorbed, or it may undergo a secondary radiation.
The intensity of radiant heat follows the same law as that of light, and decreases as the square of the distance from its source. The same law that governs the reflection of light, also prevails with that of heat; and it may be found by experiment that the angle of incidence is equal to the angle of reflection, so that the heat is disposed of in the same manner as light when it falls upon bright polished planes, convex and concave surfaces; hence the use of bright tin meat screens and Dutch ovens, and of all those simple pieces of culinary furniture which are employed in the kitchen for the purpose of arresting the cold currents of air that set towards burning matter, as also to reflect the heat upon whatever viands may be cooking before the fire. A bright silver teapot retains its heat better than a dirty one, and the fact is determined very readily by pouring boiling water into two teapots, the one being made of bright tin and the other of black j.a.panned tin. A thermometer inserted into each vessel will soon show that the latter radiates, and therefore loses its heat quicker than the former; the relative radiating powers of bright and blackened tin being as 15 to 100. Pipes for the conveyance of hot water or steam should be kept bright, if possible, although this trouble is avoided usually by packing them in bad conductors of heat, whilst the polish of the cylinder of a steam-engine is of great importance as a means of economizing heat.
[Ill.u.s.tration: Fig. 380. A B. The cone of paper, gilt inside. C. The red-hot ball. D. Stand with wood supporting a slice of phosphorus, which is brought into the focus of the rays of heat reflected through the cone.]
When the finger is approached within an inch or so of a red-hot ball, the heat radiated from the latter is so intense that it cannot be held there [Page 405] for more than a few seconds. If, however, the finger is coated with gold leaf it may be kept near the iron ball for some considerable time, because the radiant heat is reflected from the surface of the gold. If the word heat is written upon a sheet of paper and the letters afterwards gilt, the whole of the white surface is rapidly toasted and scorched when held before a fire, whilst the surface of the paper under the gold leaf remains perfectly white, which can be ascertained by turning the paper round and observing the other side. A sheet of paper gilt inside and turned round as a cone, being left open at both ends, may be employed as a reflecting surface; and if a bit of phosphorus, placed on paper, is held, say at two feet from a red-hot ball of about two inches diameter, the radial heat from the latter has not sufficient intensity at that distance to set it on fire quickly; if, however, the cone of gilt paper is used between the two, and the phosphorus brought into the focus of the rays of radial heat, it very quickly takes fire. (Fig. 380.)