To illustrate the gradual progress of heat by conduction, the lad took a square bar of iron, about 20 inches long, and he attached to the under side of it (by means of a little wax) 10 small wooden balls, so that they were about 2 inches apart from each other. Then he heated one end of the bar in the flame of a lamp, and found that the balls fell from under it one after another, as the heat found its way along the metal and melted the wax below. The arrangement of this simple and instructive experiment is here shown.

The next step was to learn the different conducting powers of different substances. For this purpose Humphry had several small metal cones made, all of the same size; one of these was of copper, another of iron, a third of zinc, a fourth of tin, a fifth of lead, a sixth of marble, and a seventh of brick. Then having tipped each of them with a small piece of wax, he stood them, all a short distance apart from each other, on the metal plate at the top of the iron stove by which Mr. Borlase’s surgery was heated. The result was, that the wax at the top of the copper cone was the first to melt. Some little time afterwards, that at the apex of the iron began to liquefy; and soon after the iron, that upon the zinc was rendered fluid; while, shortly following the zinc, the wax on the tin commenced trickling down the sides. A short interval elapsed, and then the cerate at the top of the lead became fluid. Again a lapse of a few moments occurred, after which the wax with which the marble cone was tipped began to flow; and, last of all, that upon the piece of brick was liquefied.

The different conducting powers for heat among the several substances employed were thus made evident. The metals were more capable than either marble or brick of diffusing the caloric from one part of them to another; while among the metallic substances themselves copper was proved to be a much better conductor than iron; iron, again, a little better conductor than zinc; and zinc, too, slightly better than tin. Lead, on the other hand, was the worst metallic conductor of all.

The limited means of the young experimentalist, however (for Humphry was obliged to seek Mr. Tonkin’s assistance for any particular apparatus he required), did not admit of his testing the conducting powers of either gold or silver. But had he done so, he would have found that the precious metals were much better conductors than any other—gold being the best of all, and silver only a little inferior to it. Platinum, however, was a striking exception, its heat-conducting power being only a little superior to that of iron.

Humphry after this sought to discover what would be the effect if he placed a good conducting metal in connexion with a bad one. For this purpose he employed a short curved bar of copper; and having heated it, he set it across the top of a small leaden pillar to cool, thus: when, to his utter astonishment, a series of musical sounds were given forth as the copper cooled, the tones now rising and now falling like those of an Æolian harp.

By the same means as Humphry had employed for testing the conducting powers of the metals, he ascertained that wood was a very bad conductor of heat, and that the lightest woods were the worst. Charcoal, too, he found to have but little power of diffusing the heat from one part of it to another. This explained to the boy the reason why a piece of charcoal, red-hot at one end, may be held—at a short distance even from the heated part—without burning the fingers.

Humphry now set to work to raise to a considerable temperature several pieces of such substances as he had ascertained to be good and bad conductors, so that he might learn what effect they respectively produced upon the touch when highly heated. As he had anticipated, the bad conductors—such as the wood and brick—could be handled without pain, whereas the good conductors—like the metals—burnt the fingers immediately they were brought into contact with them.

Pursuing this result, the lad, eager to display his knowledge to the servant of the house, took the boiling kettle from the kitchen fire, and, to the amazement of the maid, allowed the sooty bottom of it to rest upon his palm; for the crust of charcoal with which (by long usage) the vessel had become coated underneath—being a non-conductor—prevented the heat of the boiling water within being communicated to the hand.⁠[34]

On recounting to Mr. Borlase the experiments he had performed concerning the conduction of heat, Humphry was informed by that gentleman that it was painful to touch good conductors like the metals when they were heated about 120°. Air, however, he told the boy, might have its temperature raised even to 300°, without producing any sense of burning; adding, that some eminent sculptors had large ovens in connexion with their studios, for drying the moulds they employed in bronze castings; and though these places were often heated far above the boiling point of water, the workmen entered, and remained there for some minutes without much inconvenience; and even persons unused to such high temperatures might walk in and out of the ovens with impunity, though to such any attempt to remain occasioned a difficulty in breathing and a painful sensation about the eyes. It was found necessary, however, under such circumstances, to carefully avoid the contact of any good conductor; for if, while in the heated oven, a piece of metal were touched, it would inevitably burn—even the coins in the pocket were sufficient to produce intense pain. “A story is told,” he continued, “of a person who once, inadvertently, entered such a place with his spectacles on, and these, being mounted in silver, soon blistered the parts of his face with which they were in contact. On the other hand,” proceeded the surgeon, “it has been found that in high northern latitudes, where the cold is sometimes sufficiently intense to freeze mercury—though this requires the temperature to be 72° below that required to freeze water—yet even such excessive cold may be borne without uneasiness, provided the air be tranquil, and the persons well clothed in good non-conductors, such as wool and fur. If, however, metallic substances be touched at this low temperature, a sensation like that of burning is experienced, and the part quickly becomes blistered. The reason of this,” the doctor concluded, “is, that the heat, being as it were free to move in all those substances which are, like the metals, good conductors of it, is readily communicated to us by such substances when at a higher temperature than ourselves, while our heat is as readily abstracted by them when they are colder than we are. Hence good conductors, like metals, always feel colder to the touch than bad conductors, like wood or fur—even though these latter bodies can be shown by the thermometer to be of the same temperature as the others.”

Humphry’s conversation with the doctor induced him to try another experiment, illustrative of the conducting power of wood and metal. He took a small rod of polished brass, about a foot in length, and stretching a strip of writing-paper tightly over it at one end, he tried to burn the paper in the flame of a lamp, but discovered that it was impossible even to scorch it; for the heat, as soon as applied, was conducted away so rapidly along the metal, that it prevented the temperature of the paper being raised sufficiently to char it. On substituting, however, a smooth piece of wood for the brass rod, he found that the paper stretched over the end of it soon began to scorch in the flame, and that the wood itself shortly became ignited in consequence of its bad conducting power, which, opposing the diffusion of the heat along it, concentrated the effects upon the spot to which the flame was applied.

After this, the boy began to turn his attention to the conducting powers of liquids, rather than solids, with which he had previously dealt.

That liquids are very imperfect conductors of heat, Humphry made out in the following manner: He filled a tumbler with water, and in this he placed a piece of “fusible alloy,” which is a composition of metals melting at a temperature below boiling heat. Then a thin copper basin was made to float on the surface; and into this he put some pieces of red-hot charcoal; so that, after a time, the stratum of water at the top of the tumbler began to boil; but, even though the upper part of the liquid was at boiling-point, so slight was the power of the water to conduct the heat from one part of it to another, that the stick of alloy, which reached within an inch of the top, remained wholly unmelted by it.

The same effect was found to ensue with heated oil, though this the lad tried in a somewhat different manner. In a thin glass tube a small quantity of water was frozen by plunging it into a mixture of salt and snow. Then, upon the lump of ice at the bottom a small quantity of oil was poured; and, lastly, upon the oil some spirits of wine was made to float. The tube was now held over the chimney of a lamp, and the spirit made to boil until the whole was evaporated, when, on plunging a thermometer into the oil, it was found to be but slightly heated, while the ice itself had undergone no change, but remained still solid at the lower part of the tube.

Next, in due order, came the conduction of heat by gases and vapours; and of this Humphry obtained a remarkable illustration in a fact which he learnt of the engineer at the Wherry Mine, who told the lad that high-pressure steam did not burn, though its temperature was some hundred degrees above that of steam at a low pressure. The scalding effect of the vapour at a low pressure, the man informed the youth, arose from the small particles of hot water that were diffused throughout it, and which, indeed, rendered it visible in the air; whereas in high-pressure steam no such watery particles existed, and the vapour was consequently not only imperceptible to the sight, but, being a bad conductor of heat, it had no more power to burn than so much hot air.

Again, that gases, in a state of combustion, are bad conductors of heat, Humphry was aware, from having repeatedly passed his finger through the flame of a spirit-lamp without burning it, and yet the temperature of such a flame might be shown to be many hundred degrees beyond that of a piece of red-hot metal. Air, again, he knew to have little or no conducting power; and he had heard from Mr. Tonkin that in Russia and other cold countries double windows, with a stratum of air between them, were used to prevent the heat of the apartment being carried off. So, again, in furnaces, double walls with a stratum of confined air in the middle are employed to stop the egress of heat: even as in ice-houses the same means are adopted to stay the ingress of it.

The diffusion of heat by the process of conduction, however, generally occurs among solid bodies, in which the particles are more or less firmly united; but liquids and gases (where the particles, owing to the want of cohesion among them, are free to move) mostly became warmed by a very different process; that is to say, the heat applied to them is spread from one part to another—not by being propagated, as in solids, from one fixed particle to that which is next to it—but by the motion or circulation of the heated particles themselves, so that each in its turn receives a portion of the heat applied, and then giving place to another particle, the whole mass ultimately becomes raised to one uniform temperature by the direct agency of the radiant body, rather than by the indirect process of transference from atom to atom along the entire substance. The one process is termed the conduction of heat, the other the convection of it; and while the former prevails among the cohering particles of solid bodies, the latter generally obtains among fluids whose atoms are free to move.

In order to render visible this same circulation of the particles of fluids while in a heated state, Humphry bruised in a mortar a small piece of amber, and then having filled a glass tube with water, he threw in a few pinches of the powder, which, being nearly of the same specific gravity as the liquid, neither sank nor floated in it. Then applying a gentle heat to the centre of the bottom of the tube, the boy saw, by means of the amber-dust suspended in the fluid, that currents immediately began to ascend in the middle of the water, and to descend in it at the sides of the vessel—in the direction of the darts in the above engraving.

If, however, he heated the sides of the tube, the currents were found to take a contrary direction, going upwards at the sides and downwards in the centre.

On continuing the heat, Humphry perceived the currents to become more and more rapid, till the water boiled, and when the whole of the liquid had acquired an uniform temperature, he observed that they ceased altogether. He then endeavoured to ascertain if it were possible to produce these currents in a liquid by heating it at the top, but the boy discovered, on applying a spirit-lamp to the upper part of the tube, thus—that though the top of the water was made to boil, and the amber-dust there thrown into rapid circulation, the particles at the bottom remained unmoved, the fluid below being undisturbed and cold.

The reason of this was almost self-evident. The warm water was lighter than the cold, and therefore rose to the top immediately it became heated, while the cooler and heavier portions descended to occupy its place. Hence, in heating the tube at the bottom the current was observed to go upwards in the middle and downwards at the sides, these being kept comparatively cool by the action of the external air.

Pursuing this subject, Humphry took a large and a small Florence flask, and into the mouth of the large one he fitted two long bent glass tubes, by means of a perforated cork and cement. These, together with the large flask, were filled with water and then made to dip into the open mouth of the smaller flask, which was likewise filled with water, but tinged a deep blue with indigo. One of the tubes was arranged so as to dip only about half an inch below the surface of the blue liquid, while the other descended nearly to the bottom of it, and was slightly curved upwards at its extremity. The arrangement will be readily understood by reference to the annexed engraving. On applying the flame of a spirit-lamp to the lower flask, the blue liquid was seen to ascend by the tube on the left side; then reaching the large flask at the top, it there circulated through it, in the direction of the darts, and descended by the other tube, back again to the small flask at the bottom. Thus a perfect circulation was seen to be kept up, and the heat, by means of convection, carried from one flask to the other.

After this Humphry sought for some means of rendering the currents of heated air visible in the same manner. For such purpose he took a large glass jar, having a wide opening at the bottom and a narrow one at the top. Into the upper aperture he inserted a long lamp-glass, and down this he placed a diaphragm of card, so as to divide the glass chimney into two channels. Then the lad procured a shallow pan, and having poured a little water into it, he set a piece of lighted candle in it and covered it over with the jar and chimney, so that when the whole was duly arranged it appeared as here shown. Then having lighted a piece of brown paper and blown it out again, he held the smouldering end over the chimney, and saw, by the curling of the smoke from the paper, that the heated air from within was ascending the lamp-glass by one side of the diaphragm, and descending by the other, in the direction of the arrows in the illustration; whereas, when the card-board partition was removed from the chimney, the currents ceased, and the light was soon extinguished.

The boy applied the same simple means, likewise, to learn the direction of the currents of air on opening the door of a heated apartment, and found, by the smoke from a piece of smouldering paper, that at the upper part of the door the heated air from within was rushing outwards, and at the lower part the cold air from without was setting inwards, whilst at the middle scarcely any draught, one way or the other, was perceptible.

This naturally turned the boy’s attention to the subject of the wind, which appeared to him to be merely a vast current set up in the atmosphere by the heating power of the sun’s rays. He had noticed, too, that, shortly after sunrise, a breeze frequently sprang up at sea and blew towards the land, increasing as the day advanced, and declining and ultimately expiring at about sunset; whilst in the evening, after sundown, a wind often arose in the opposite direction—namely, from the land towards the sea—and lasted the whole of the night, ceasing only with the reappearance of the sun.

Humphry was therefore anxious to discover some experimental means of reproducing these effects on a small scale.

Having procured a large shallow milk-pan, he filled it with cold water, and then took a metal “hot-water plate,” and having poured some boiling water into this, he set it in the middle of the pan, saying to himself as he did so, “the cold water there, in the outer vessel, represents the ocean, while the heated metal plate in the centre stands for an island warmed by the rays of the sun; for the land, being a better absorber of heat than the sea, will have its temperature raised some degrees higher than the water in the course of the day.”

This done, the eager boy proceeded, by means of the smoke from a piece of smouldering paper as before, to discover the direction of the currents that would be set up in the air under such circumstances. As he held the smoking paper at the edge of the pan, Humphry was delighted to see the white fumes drawn towards the hot plate in the middle, or, in other words, from the miniature ocean towards the mimic island encompassed by it; and this he knew was precisely the current that was found to prevail throughout the day in tropical countries.

Then, to impress the phenomena firmly upon his mind, the boy drew in his note-book the annexed diagrams, illustrative of the currents produced in the atmosphere by the heating of the earth during the day, and the cooling of it during the night. “But if the inequality of the temperature between the land and the sea gives rise to such results, how much greater,” mused the boy to himself, “must be the effect produced by the difference between the heat of the earth at the equator—where the average temperature is said to be 80°—and at the poles, where it is calculated to be as low as 56° below zero, the difference being as much as 136°! What a vast aërial current must be set up by such means!”

Then the lad made another drawing, illustrative of the effect that would ensue under such conditions, and he set above it a series of arrows to show the direction of the currents that would be thus induced in the atmosphere. For the air, being heated by the vertical sun at the tropics, rises there, as it does up a chimney, while the colder air from the northern and southern hemispheres glides in from below, on both sides of the equator, to supply the place of that which has been made to ascend by the heat; precisely in the same manner as, when the fire burns, fresh air is continually rushing in under the door and windows. Then the heated air, after rising to a considerable height above the earth, at length flows over, as it were, and forms in the atmosphere an upper current from the equator to the poles, where it becomes cooled, and is then drawn down to supply the place of that which has been drafted from the colder to the warmer regions. “But,” said the boy, as he surveyed the drawing, “according to this the winds which are found to prevail in the tropics should blow north and south; whereas they are found to come from the north-east and south-east quarters.”

Humphry puzzled himself for a long time in endeavouring to explain the phenomenon, but it was more than his philosophy could accomplish; so he had to consult his old friend Mr. Tonkin again, and from him he learnt that the change in the direction of the currents is due to the motion of the globe and the unequal rates at which different parts of the earth’s surface revolve. Consequently, as the currents of air which set in towards the equator from the poles come from parts that revolve about the axis at a much slower rate than the equator itself, they hang back, or drag, upon the surface, in a contrary direction to the rotation of the earth itself; so that, while the globe turns eastward, they acquire somewhat of a westerly course, and, appearing to come from the opposite quarter, assume, therefore, the character of permanent north-easterly and south-easterly winds.

But to make the matter clearer, Mr. Tonkin exhibited the following illustration to the boy, in which the effect of the earth’s motion in changing the direction of the atmospheric currents is immediately apparent.

The old gentleman, however, informed Humphry that there are other terrestrial currents produced by the process of convection. In the ocean the same circulation of hot and cold streams is found to obtain; for the sea, warmed by the heated shores of the tropical regions, is made by convection to move from the equator like a vast river, while from the poles an immense current of colder liquid streams forward to supply its place. For the same reason as was before explained in connexion with the trade-winds, the polar current, having a slower rate of rotatory motion, assumes, on reaching the equator, a westerly direction, and so flows in one broad stream across the globe; then, striking against the vast continent of America, it divides into two large streams. One of these flows southward down the eastern coast of Southern America, and finally enters the Pacific Ocean through the Straits of Magellan. The other turns northward, enters the Gulf of Mexico, sweeps round the coast in a powerful current known as the Gulf Stream, and then proceeds along the Northern American shores to the coast of Newfoundland, where it crosses the world again, and occasionally extends even to the western shores of the British Isles.

The direction of these oceanic currents is indicated in the subjoined chart:

“There is, however,” continued Mr. Tonkin, “another great heat-stream traversing the earth, though this takes place within the crust itself, and is due more to conduction than convection, as in the other cases. For philosophers tell us, that the daily impressions of heat which the earth receives from the sun, follow each other into the interior of the mass, like the waves which start from the edge of a canal, and, like them, become more and more faint as they flow on, one after another, till they melt into the general level of the internal temperature. The parts of the earth near the equator,” added the old man, “are more heated by the sun than other parts, and on this account there is a perpetual internal conduction of heat from the equatorial to the northern and southern regions. Then, as all parts of the earth’s surface throw off heat into space by radiation, it is plain that at the poles, where the surface receives but little warmth from the sun, a constant waste of caloric is produced. There is thus a perpetual dispersion of heat from the polar parts into surrounding space, which is supplied by a perpetual internal flow of heat from the equator towards the poles. The radiation from the surface of the earth,” Mr. Tonkin concluded, “has its limit in the temperature of the planetary space in which it moves (for we may conceive our globe to be like a heated ball cooling down, in vacuo), and this has been calculated to be not more than 56° below zero,—which low temperature, indeed, appears to be attained in the long absence of the sun in a polar winter.”

The poetic boy was lost in wonder at the marvellous results to which his investigations had led him, and his mind was filled with a sense of sublimity at the thought of the enormous heat-tides that are continually flowing through the atmosphere, the ocean, and the solid crust of the earth itself.

“I’ll work it all out myself,” he cried; “that I will. I’ll not rest until I know all that is known of Nature and her wondrous ways.”