CHAPTER VI.

THE WONDERS OF HEAT: ITS SOURCES.

Humphry was so full of his project, that all the day long he could think of little else, and at night he lay awake in his bed for many hours, planning an infinity of rude schemes for accomplishing the object he had in view. He devised and fashioned a number of odd contrivances, too, for the purpose, but each in its turn was found to be of little or no avail.

Nevertheless, the idea was too great to be hastily abandoned, and the sense of the fame that success in such an undertaking would assuredly give to his name had entered so deep into the boy’s mind, that he got to crave more and more for worldly honours; and he would sit of an evening, alone among the rocks, dreaming of the time when he, a poor Cornish boy, was to be ranked among the intellectually noble, and reverenced throughout Europe for his genius and his benevolence.

Nor did the lad fail, when he visited his mother, to confide to her all his hopes of success and renown; and when the widow heard that he was bent on discovering the means of saving the lives of the poor miners—a class whom she had long learnt to pity, for she had been brought up in the midst of them, as it were—she felt prouder than ever of her darling boy, and shed many a tear of joy over him.

Mrs. Foxell, too, when she beheld Humphry’s vexation at the repeated failures of the models he constructed, encouraged the lad in every way, reminding him that _he_ himself had said the project would cost him years of hard study to accomplish.

Accordingly, after many disappointments, Humphry himself began to see that he could only hope to attain the result he desired by making himself acquainted with all that was already known upon the matter. He was ignorant even of the laws of combustion in general, and the rude experiments he had made had set his mind craving for knowledge on the subject. “Why did this thing burn, and that not?” he would inwardly inquire. “How came it that one body, as gunpowder for example, went alight all of an instant, and another, like tinder, took a long time to smoulder away? And why was phosphorous so easy to kindle, and wood, comparatively, so difficult, that the slightest friction would inflame the one, whereas it required a long time to light the other by such means? What mysterious process,” he would ask himself, “went on when any substance burst into flame, and whence came the light and heat that were then given out from materials that, a few moments before, had been dark and cold? Could the light and heat have been imprisoned in the substance, and were they set free during the combustion; or were these powers generated merely by the burning of the bodies?”

Then Humphry’s mind darted off to the deeper question, “What were the principles of light and heat themselves? Were they one and the same, or two distinct powers in nature? In a winter’s day,” he mused, “we have the same light from the sun, and but little heat; whilst in all cases of artificial illumination there is great heat and but little light, compared with that of the solar rays.”

These and many other such puzzling inquiries passed through Humphry’s brain, and left his mind in such a state of perplexity that he could not rest without a clearer insight into the subject. He soon saw, too, how silly he had been in setting to work before he had availed himself of the discoveries of those who had gone before him; for, he would say, how could he think of finding out, by himself, all that was known of the science of heat and light, when each of those sciences, as Mr. Tonkin had told him, had taken thousands of the wisest minds—ay, and thousands of years of intense study—to build up; for in them was contained the accumulated experience of all mankind from all time. Yet, because the truths were free to the world, he had refused to avail himself of them, and, like a proud fool, had though he could compass his end without any such knowledge at all.

* * * * *

It was not long before Humphry was hard at work making himself master of the laws of heat. He had borrowed of Mr. Tonkin the best book then extant upon the subject, and often when the streets of the little town in which he lived were silent as the tomb—and the distant bell of Madern Church, as it tolled the morning hours, was heard to boom upon the still air, almost like a moan—and the sound of the waves that rippled upon the neighbouring shore stole on the ear softly as the murmur of a sea-shell—the candle might be seen burning in the boy’s chamber, making the little diamond panes of the casement shine like plates of amber in the darkness (for every other window in the street looked black from the want of light), while the observant eye could trace on the white wall on one side of the room, the huge distorted shadow of the lad bending over his books.

As Humphry read on, and got to see clearer and deeper into the nature and properties of the subtle principle he was studying, he grew more and more enraptured with the wonders and the knowledge that were opened up to him at every step; and often when some new discovery burst upon his mind he would, in the fervour of his admiration, fall upon his knees, there alone in his chamber at night-time, and thank God that he had come to know so much of His goodness and glory; then he would rebuke himself, too, for having remained so long ignorant of the many beauties that lay concealed in the wisdom and exquisite fitness with which the phenomena of the universe are linked together. “What fairy tale of enchantment,” he would say to himself, “can display magic like this? What work of human invention can fill the mind with such amazement and delight at the subtlety of the art, as the mind feels when it first learns the wondrous story of Creation?”

When Humphry had read through the books that he had obtained of Mr. Tonkin and Mr. Borlase, he proceeded to repeat the most striking of the experiments in connexion with the subject, so as to impress the knowledge more firmly on his mind. But before doing this he reviewed the whole matter, and arranged it after his own manner—for he was not the boy to follow in a beaten track, and found no little delight in the exercise of his own genius.

* * * * *

“First,” said Humphry, as he pondered over the science of heat in general, “_the sources of it_ have to be considered; that is to say, whence is the heat of the earth derived? The universe is a vast reservoir of caloric, and it is capable of being evolved, by some means or other, from almost every substance that surrounds us; and though its production artificially is now so common that it has lost all wonder with us, there must have been a time when the elimination of it from substances on the earth must have been a matter of such amazement as to have produced a feeling of awe, on the part of the multitude, towards those who first discovered the art. This, perhaps,” the boy went on, “is the origin of the fable of Prometheus, who was, probably, the first man who found out the way to kindle a fire, and so was thought to have stolen the heat from the sun. Tradition says, the first artificial fire was produced by lightning striking a decayed tree. Our minds can, even now, almost conceive the terrible awe of the people who first witnessed the liberation of fire on the earth—who beheld, for the first time, the transparent red flames burst forth from the combustible and lick the air like burning tongues, while the smoke rolled upwards from them in dense leaden clouds. Then the intense pain felt on touching the fire must have made the populace almost believe that they had been stung by demon serpents, while the roar of the wind, as it rushed towards the blazing mass to supply the place of the lighter air that had been driven upwards by it, must have sounded to the people like spectral voices, and the ultimate dissipation of the huge solid substance into invisible gases must have appeared like the most marvellous magic to them. Thus it came that men at last got to worship the fire, for its wonders and its terror, though we kindle it nowadays almost without a thought or a fear.

“The sources of heat at present known to man,” continued the boy, as he wrote down the divisions of the subject in his note-book, “are many. First, there is the _heat of the sun, and, perhaps, that of the moon_; philosophers, however, have concentrated the moonbeams upwards of 300 times, by means of a burning-glass, nearly 3 feet in diameter, and yet the most delicate thermometers have shown not the least increase of temperature. This is said to arise from the feebleness of the light of the moon, as compared with that of the sun; for the lunar rays have been calculated to possess 300,000 times less illuminating power than the solar ones, whilst the light of the sun itself has been shown by experiment to have 12,000 times the intensity of the flame of a wax-candle, so that a little fragment of the great luminary the size of such a flame would possess the illuminating power of 12,000 wax-candles; and, since the diameter of the sun is nearly four times greater than the distance of the earth from the moon, this may give us some notion of the vast flood of light and (if the two are connected) of heat that are being continually streamed forth from the sun into the universe. Of the intensity of the _solar heat_, the law of the decrease of all radiant matter enables us to form some conception; for this teaches us that the heat of the sun’s rays, after travelling to the distance of the earth, must be diffused over at least 300,000 times a greater space than it is at the sun itself, and consequently that the intensity of the heat must be that number of times more highly concentrated at the sun’s surface than it is on reaching our atmosphere. Now, one of the largest burning mirrors that have ever been constructed, and which had the power of concentrating the sun’s rays rather more than 17,000 times, melted a piece of Pompey’s pillar in less than a minute, a piece of cast-iron in a quarter of a minute, a copper halfpenny in sixteen seconds, and fragments of slate and tile in three and four seconds. A lens that increased the intensity of the sun’s heat about 10,000 times fused pieces of platinum, gold, asbestos, quartz, &c., in three seconds; so that—as the solar fire must, at the sun itself, be 30 times more intense than the calorific power of its rays, even when thus concentrated, at the surface of the earth—it is evident that the fury of the sun’s heat must at its source be sufficient to dissipate the most obstinate metals in vapour, and to make the most infusible of the earths as liquid as glass.

“Then, again, there is the _heat of the stars_; for if each of these be suns, and they, like our own sun, give off heat, together with light, while the heat radiated by them decreases in the same proportion as their light, it is clear that the united beams of the starry host must give a certain general temperature to the realms of space. This temperature philosophers have calculated to be as low as that at which quicksilver freezes, and which degree of cold appears to be attained in the Arctic regions, during the long absence of the sun through a polar winter. According to the principles which regulate the radiation of light and heat, it is demonstrable that the starbeams can only maintain a temperature in infinite space which, when compared with the heat we derive from the sun, must be as much inferior to it as the light of a moonless midnight is to the light of mid-day at the equator; and it is plain, that the rate at which the earth cools down or radiates back into space the heat it receives from the sun must have its limit in the temperature of planetary space itself, so that, had this been higher or lower, the earth’s surface must have been hotter or colder than it is.

“But, besides the preceding _celestial_ sources of heat in nature, we must also (if we suppose that such things as give light to the earth radiate heat as well to it—though in ever so minute a degree) enumerate that peculiar cone or pyramid of luminous mist which is seen an hour or two after sunset, at certain months of the year, in the line of the ecliptic, and to which astronomers have given the name of ‘_the zodiacal light_.’ Travellers in tropical regions tell us that this is sometimes so brilliant that it seems a second sunset, lasting almost to midnight, and that the clouds which are scattered over the deep azure of the distant horizon appear to flit past the glowing nebulosity as before a golden curtain, while above these other clouds are seen reflecting from time to time brightly variegated colours.

“Then, again, there are the brilliant coruscations of the _aurora borealis_ (or ‘_northern dawn_,’ or ‘_polar light_,’ as it is sometimes called), though this appears to be rather an emanation from the earth itself than any celestial phenomenon. According to the best accounts, the light of the aurora exists almost within the bounds of our own atmosphere, and seems to stream from one of the poles of our globe, as if the earth had suddenly acquired the power of becoming self-luminous like the stars and sun.[30] This brilliant exhalation is rendered more interesting by the fact, that the great Herschel himself, from repeated observations of the spots on the sun, came to the conclusion that such spots are parts of the dark solid body of the sun itself, laid bare to our view by fluctuations in the solar atmosphere, and that from that atmosphere alone the light and heat proceed—the shining matter of the sun being, not a fluid, but a mass of brilliant or phosphoric clouds, glowing with the beams of the luminous strata of the solar atmosphere far above them—in the same manner as the aurora with us is said sometimes to illuminate a stratum of clouds below it.

“It is impossible to say what increase of heat our atmosphere may derive from the beams of the aurora and the zodiacal light, but as we have no reason for supposing the rays in these cases to be destitute of all heat, it is evident that, when enumerating the several sources of caloric in the universe, some mention should be made of them; for who can tell what would have been the effect upon the general stock of heat in nature without such phenomena, or how low the temperature of the earth’s surface might have been if the heat of the planetary space in which it moves had been less than it is?

“But a far more important source of caloric to the earth lies in its _subterranean heat_, or the increase of temperature which is found to ensue as we descend below the surface of our globe. Carefully conducted experiments have shown that, in our own climate, the temperature increases about 1° for every 50 feet that we go down. At 354 feet below the ground the heat is found to be 60°, in those parts where the surface of the earth itself has a mean temperature of but 50°. At 792 feet under the soil the thermometer rises to 70°; at 1434 feet it marks 80°; at 1872 feet it reaches 90°; whereas at 2556 feet it is as high as 100°. Now, if this rate of increase (viz. 1° in every 50 odd feet) continued uniformly as we descended, it would follow that, at a depth of rather less than 2 miles, water would be constantly at the boiling point, and at 9 miles below the surface everything would be red-hot, whilst at rather more than 20 miles the granite rocks themselves would be in a continual state of fusion. In the ‘United Mines’ of Cornwall one of the levels is so hot, that though a stream of cold water is allowed to flow through it, in order to reduce the temperature, the miners are compelled to work nearly naked, and will bathe in water at 80° to cool themselves. At the Tresavean Mine, in the same county, which is nearly 2000 feet deep, the temperature is greater than the intensest heat of summer in the dog-days.

“There is, however, no evidence to prove that the increase of temperature beneath the surface proceeds at a uniform rate when we descend to a greater depth than 1500 feet below the level of the sea. It will be seen by the rates of increase above given that the temperature underground rises at first 1° in every 35½ feet, whereas at great depths the increase amounts only to 1° in every 68 feet; and, as far as our observations have extended, the subterranean heat appears to bear a close relation to the thermic condition of the climate at the surface; for it is found, on descending to a depth of about 60 feet in our own climate, the heat of the earth no longer fluctuates with the different seasons, but remains always at the same point during winter and summer, whilst at the tropics the stratum of invariable temperature is situate at only 1 foot below the soil.

“But whether the subterranean heat be due to the absorption of the solar rays, or whether it arises (as some have supposed) from a vast body of central fire in the earth, it is certain that we have many indications of prodigious elevations of temperature beneath the soil. These appear to be due to great chemical changes taking place in the substances forming the crust of the earth. Every schoolboy knows that a mixture of sulphur and iron-filings buried under the ground will, in a short time, become so heated as to burst into flames. Now these two materials form the mineral called ‘iron pyrites,’ which prevails through every coal-field, and the moisture acting on these is known to generate heat enough to inflame neighbouring combustibles—many coal-mines having been set on fire spontaneously by such means. In the ‘United Mines,’ where the heat is found so oppressive, it is undoubtedly owing to the decomposition of immense quantities of pyrites that are known to exist at a short distance from the works.

“Of the prodigious subterranean heat existing in some parts of the earth we have the most unmistakeable evidence. In some places boiling _hot springs_—as the Geysers of Iceland—issue from the ground (in these eggs have been cooked in 4 minutes); and even in our own country the wells of Bath have a temperature of 115°, while at those of Carlsbad, in Bohemia, the heat of the water is as high as 167°. In other parts, ‘_fumaroles_,’ or eruptions of steam, burst from the soil; while in others there are ‘_solfataras_,’ or jets of sulphurous vapour; and in others, again—as in the valleys of the Eifel, at the lake of Laach in Germany—‘_mofettes_,’ or vast exhalations of carbonic acid gas, occur. Further, there are Artesian fire-springs—like those termed ‘_ho-tsing_’ in China, where a province has been lighted (by the gas issuing from them) for several thousand years. Moreover, eruptions of boiling acid mud, called ‘_salses_,’ are not unfrequent. The volcano at Carguaraizo, in Peru, threw up a torrent of hot mud in the year 1698, that covered nearly 80,000 acres of ground; and in 1797, an entire village near Rio Bambo was buried under a similar mass.

“All these phenomena are evidences of intense subterranean heat, many of them being simply the products of underground combustion: but lofty jets of flame have likewise been known to blaze up from the earth, to such a height that they could be seen at a distance of 24 miles from the eruption; as, for instance, at the village of Baklichli, near Baku, on the Caspian Sea. During _earthquakes_, too, the earth has been known to open and to vomit forth flames, and gases, and enormous fragments of rocks, accompanied with a noise of subterranean thunder; while in some places the heaving soil has been inflated by the force of the compressed vapours beneath, and expanded like a bladder filled with air. Such was the case among the plains of Malpais in Mexico, in the month of September, 1759, when a tract of ground, 3 to 4 miles in extent, rose up like a huge bubble—flames bursting from the earth the while over more than half a square league—and the volcano of Jorullo being formed at the summit.

“In _volcanoes_, or burning mountains, again, we have manifestations of the intense elevation of temperature existing at certain places within the crust of the earth. The explosions from the volcano of Guacamayo, in South America, are heard almost daily at a distance of 80 miles; whilst the noise of the detonations from one in the Sunda Islands, near Java, have been distinguished 970 miles from the spot. But the most remarkable instance on record of the fury and power of the subterranean fires is to be found in the eruption of one of the Icelandic volcanoes, called ‘Skaptaa Jokul,’ which occurred on the 8th of June, 1783. During this eruption the large river Skaptaa entirely disappeared, and the day after, a torrent of burning lava rushed down the sides of the mountain and not only filled, but overflowed the channel of the stream, though in many places it was 600 feet deep and 200 feet broad. Then pursuing the course of the river, the fiery current poured over a lofty cataract, and filled up in a few days an enormous cavity that the waters had been hollowing out for ages. A short while after this another large river, the Hverfisfliot, disappeared from its bed, and this was filled up by another fiery torrent, which overflowed the country in one night to the extent of more than 4 miles. It has been estimated that these two streams of burning molten rocks were together 90 miles in length by 20 odd miles in breadth, and, in some places, between 500 and 600 feet deep, while there were in the aggregate forty thousand million tons of red-hot rock poured out of the bowels of the earth in the short space of ten weeks, during which the eruption lasted.[31]

“Another of the natural sources of heat to the earth is to be found in the _electric discharges_ known under the names of forked and sheet lightning. Of lightning there appear to be three kinds: (1) The _zigzag_, which is linear and sharply defined at the edges; (2) the _sheet_, which illuminates whole clouds, that seem to open and reveal the light within them; and (3) the _globular_, which appears in the form of fire-balls. The two first of these kinds last but for the thousandth part of a second, while the globular form moves much more slowly. Of the amount of heat contained in each of the different species we have no precise knowledge, though, from the experiments by which we produce discharges of electricity artificially, on a small scale, we learn that it must be considerable. If a spark which is drawn from a small Leyden jar, and which has force enough to leap only some few inches through the air, has power to inflame combustibles, and even to fuse the metals in an attenuated form, what must be the heat evolved from an electric discharge where the insulating body consists of whole acres of clouds rather than a few square inches of tin-foil, and whence the fluid has power to leap through hundreds of feet of the atmosphere towards the nearest conductor? That the electric flash possesses great heating power, we have repeated proofs; for it melts all wires that are not sufficiently substantial to allow it a free passage, inflames decayed trees, overthrows buildings, and often fuses even the rocks themselves—the tubes called ‘_fulgurites_,’ which occur in beds of sandstone, and consist of fused sand (glass), are known to geologists to have been produced by such means—while at Funzie, in Fetlar, the lightning is recorded to have torn up a rock 105 feet long from its bed, and hurled the fragments to a considerable distance from the spot. Moreover, the electric heat is the greatest that we are enabled to produce artificially, and so much exceeds that of the strongest furnace, that platinum, which remains stubborn and infusible in a forge at a white heat, melts in the arc of flame produced by a powerful voltaic battery—like wax.

“These are the _natural_ sources of heat,” the youth wrote on; “the artificial methods of generating heat, however, are much more various. We can evolve heat by _mechanical_ means—as, for instance, by _percussion_ or _pressure_. The blacksmith hammers a nail until it becomes red-hot, and from it he lights his match; and in coining, the blank piece of metal becomes greatly heated by the sudden and violent action of the press. By the compression of air in a small tube, by means of a condensing syringe, a sufficient quantity of heat may be evolved to light German tinder; and it has been well said, ‘that, locked in a pint measure of air, there exists sufficient heat to make several square inches of metal red-hot.’ A piece of Indian-rubber, suddenly and forcibly drawn out, becomes warm in consequence of the extension, as may be easily perceived by applying it to the lip the moment it is stretched. Again, by the concussion of a flint and steel so much heat is produced that the sparks which fly off consist of small particles of iron that have been fused by it. Moreover, when a few grains of fulminating silver are struck by a hammer, the heat produced is sufficient to ignite gunpowder and to cause a violent explosion. Again, _friction_ is a prolific source of heat. The Indian ignites two pieces of wood by rubbing them together; and even two pieces of ice may be made to melt each other by the same means. It has been truly observed, too, that an unlimited supply of heat seems capable of being derived by friction from certain materials. Water has been made to boil in two hours and a half, merely by boring into a mass of metal that was surrounded by the fluid.

“But not only can we produce heat artificially by _mechanical_ means; we can do so far more plentifully and easily by _chemical_ action, for it has been found, that whenever two or more substances rapidly combine, heat is invariably produced. In _fermentation_ (which is nothing more than a decomposition of elements loosely united, and their reunion in a more perfect state of combination) considerable increase of temperature takes place. During the making of vinegar there is much heat evolved—the temperature rising, in some processes, from 60° to 85°. Again, in the process of respiration (which is merely the combination of the charcoal in the blood, with a certain portion of the air drawn into the lungs) the heat evolved is supposed to be the cause of our bodies remaining almost at a constant temperature. A little powder of the metal called antimony thrown into a jar of chlorine gas spontaneously ignites and burns with brilliancy, combining with the gas so readily that it takes fire and produces at first a liquid, and afterwards a soft solid called the ‘butter of antimony.’ Further, there is an oil-like fluid consisting of two gases (nitrogen and chlorine) that have been made to unite with each other, and this compound has such an affinity for combustible bodies, that even if a long rod, the extremity of which has been dipped in oil, be made to touch only a globule of it—the size of a mustard-seed—confined under water, it instantly explodes with a flash of light, and with such violence that it disperses the water in a shower, and breaks into atoms the vessel in which it is contained; so that experienced chemists always protect the face by a mask when making the experiment. Again, if oil of vitriol and spirits of wine, or if aquafortis and spirits of turpentine, be suddenly mixed, sufficient heat is set free to ignite the spirits. When caustic potash, too, dissolves in water, a considerable increase of temperature ensues. So if spirits of wine and water are mixed together, the mixture becomes much hotter and occupies a smaller space; whilst if four parts of strong oil of vitriol be mixed with one part of snow or pounded ice, the heat developed is sufficient to boil water, whereas if one part of the acid be added to four parts of snow, intense cold is produced. More than this, if a piece of clean platinum be immersed in a vessel containing a mixture of oxygen and hydrogen gases, such intense heat is evolved that the metal becomes suddenly red-hot, and the gases are made to combine so rapidly that a violent explosion ensues, and the two gases become water.

“But the most ordinary mode of obtaining heat artificially is _combustion_, though this is merely a process of rapid combination after all. It is by the heat evolved during the process of combustion that our houses are warmed in winter, our food cooked, the steam-engines of our factories and mines set in motion, our metals smelted, cast, and wrought, our glass made, our dishes hardened, and an infinity of useful services rendered to us; indeed, so much do we owe to combustion, that we are unable to comprehend the state which man must have been in before the method of producing heat artificially by such means was discovered.

“Lastly, there is the heat generated by _nervous energy_; for if the sole source of warmth to the animal frame lay in the chemical processes that are continually going on within it, why should a suddenly excited emotion (as in states of anger and blushing) have power to produce so considerable an increase of temperature in the human frame? Indeed we have personal knowledge, that almost every muscular movement produces sensible warmth, even as any violent excitement of the mind is attended with the same result. We know, too, that the injury of one little thread-like nerve can reduce a member of the body to a state of stony coldness; and that after death, when the chemical decompositions are proceeding as actively as during life, the body possesses no animal heat whatever.” Physiologists, moreover, have shown that, if the respiration be kept up artificially in an animal after its head has been cut off, the blood becomes arterialized, and the several chemical changes go on as during life—_but without the body being in the least warmed by them_.