When Humphry had written thus far concerning the sources of heat (for the boy was delighted to note down his thoughts on the various subjects he was studying),⁠[32] he began to ponder over the several modes in which heat was communicated to bodies removed from the different sources of it; for, said he to himself, “if there had been no means of propagating heat from one part of space to another, the fires could not have warmed us, and the sun would have been only a moon to our globe, while we should have been deprived of some of our most agreeable sensations. Hence, in the consideration of such a subject, it becomes necessary to attend to, and distinguish between, the several ways in which a body at an elevated temperature communicates its heat to others that are either in contact with it or at a distance from it, as well as the several conditions which determine the reception or absorption of the heat by different substances.

“Now, heat may be communicated from one body to another in three different ways—

1. By emission of rays of heat from a distance;

2. By conduction along the particles of a solid body;

3. By convection or circulation among the particles of a fluid.

“The propagation of heat by the emission of heat-rays from a warm or burning body at a distance, is the one that first demands attention. This mode of communication is generally styled ‘radiation of heat,’ and it is evident that the heat-rays emanating from one body may be communicated to another, either directly, by the process of transmission through the intervening substances, or indirectly, by reflexion from the surfaces of those opposing them; for the heat-rays, like those of light, always proceed in a straight line, and are susceptible, likewise, of being reflected or driven off at an equal angle from polished surfaces.”

Having settled thus much in his own mind, and arranged the subject with that logical precision which was a marked feature in the genius of the youth, he proceeded to test experimentally the emissive energies, or, in other words, the radiating powers of different substances.

For this purpose he provided himself with a square tin canister: one of the four sides of this he brought to as high a polish as he possibly could; the second side he coated with a mixture of lamp-black and gum-water; over the third side he pasted a piece of paper, and the fourth he covered with glass. Then, having provided himself with a thermometer from the surgery below, he proceeded to arrange the canister at some distance from the thermometer, but on a level with it. After this he filled the canister with boiling-hot water, and then proceeded to note how the thermometer was affected when the canister was turned round, and each of its sides successively brought before the instrument. The boy soon ascertained, to his great joy, that the heat was thrown off most rapidly from the blackened side of the canister; next to that, he found the surface covered with paper to radiate heat more rapidly than the other two; then the glass side was discovered to possess more emissive power than the polished surface, while the polished surface itself had the least radiating power of all.

Delighted with the result of the experiment, and pleased with the knowledge it gave him as to the emissive energies of different substances for heat, the boy, to assure himself that he was not mistaken, held his hand at a short distance from the canister, and caused the differently-coated sides to pass successively before it. As he did so, he could feel the heat increase gradually as the polished side passed from before his hand and the blackened one came round in front of it; so that, had he not been aware of the fact, he would hardly have believed that the water in the canister was as hot at that part where the bright tin had been left as it was where the side had been blackened over.

Next the lad tried another modification of the same experiment. Having blackened one canister entirely over, and brightly polished the outside of another which was of the same size, he filled the two vessels with boiling water, and putting a thermometer into each he placed them upon a table at opposite corners of an empty room, and then found that the thermometer in the blackened vessel fell much quicker than that in the polished tin one; so he now saw that the reason why the black side of the canister in the first experiment felt hotter than the polished surface did to his hand was, that the water there was parting with its heat at a more rapid rate, so that in the entirely blackened vessel it necessarily cooled down sooner than in the bright tin one.

Humphry was so delighted with the truths he had thus discovered, that he tried a number of other experiments as to the radiating power of different substances, and at last came to the conclusion that lamp-black, sealing-wax, wool, paper, glass, and black-lead, were much better radiators of heat than the metals; their power of giving off heat being in the order in which they are here mentioned—a surface of lamp-black cooling quicker than one of sealing-wax, and sealing-wax again more rapidly than writing-paper, and so on down to the metals, which cooled the slowest of all.

Then, having dealt with different substances, the youth set to work to ascertain what effect an alteration in the arrangement of the surface produced in the radiating power of the same substance. Accordingly he tarnished, by means of acid, one of the sides of the polished tin canister he had previously employed, and found, on filling the vessel again with hot water, that the dulled surface parted with its heat quicker than the bright one. After this, he proceeded to roughen another of the sides with some emery paper, and then, on re-filling the vessel, he discovered that the scratched surface cooled at a greater rate than the smooth polished one.

“So, then,” said young Humphry to himself, “not only have different substances various radiating powers for heat, but also a difference in the arrangement of the surface of the same substance is attended with a like effect.”

Still the lad had to examine the result produced by bodies of different densities, and this he did by means of a vessel of cast-iron and one of wrought-iron, when he found that the cast metal parted with its heat quicker than the hammered or wrought metal; so that it was evident a lighter material was a better radiator of caloric than a heavier one—for the particles of the iron in being wrought had been brought closer together, and the metal thus rendered of greater density.

This done, Humphry made an entry in his note-book, “that not only did rough or dull surfaces part with their heat quicker than smooth or bright surfaces, but that light bodies were better radiators than heavy ones.”

The young experimentalist was overjoyed with the progress he had made, and he would have rambled off into a number of speculations as to the effect which the principles he had discovered must produce in nature (for he saw that the different surfaces of different countries must yield a like result); but he was too intent on pursuing the investigations he had undertaken to allow himself, yet awhile, to apply them to the explanation of terrestrial phenomena. Moreover, he had still to learn the different rates of cooling among bodies in the air and in a vacuum. To do this, however, an air-pump was necessary, and how he was to obtain such an apparatus puzzled his ingenuity for a considerable time.

At length the youth remembered to have seen a large syringe among Mr. Tonkin’s instruments, and having obtained the loan of this, he applied it to a stand, and used it as the pump for extracting the air from the receiver. When the instrument was complete, Humphry found that bodies which took between two and three minutes to cool in the air were as long as five minutes in parting with the same quantity of heat in a vessel from which all the air had been exhausted. So he now perceived, that the same substances gave off their heat twice as quick in the open air as they did in vacuo.

The next step was to ascertain the different amounts of radiation among different bodies on the earth. For this purpose the boy borrowed as many thermometers as he could procure among his friends in the town; and early one evening, after the sun had declined, and when the soil was parting with the heat it had received in the course of the day, he proceeded to test, by means of the instruments, the several rates at which the various substances upon the earth were being cooled down. One of the thermometers he suspended in the air four feet above the grass-plat in the garden at the back of the doctor’s house; another he placed on some wool which he had spread on a raised board; another he deposited on the surface of the raised board itself; and a fourth he rested on the grass-plat. Shortly afterwards he proceeded to note the temperatures indicated by the several thermometers in the different situations, when he found that the one in the air stood at rather more than 60°, while that resting on the wool was at 54½°, and that lying on the board at 57°, whereas the one on the grass-plat marked only 51°. In an hour or two after this, the boy noticed that the blades of grass were suffused with dew, and that the fibres of the wool also were beaded over with little drops of moisture, but to a less extent than the grass, while the surface of the board remained almost dry.

“So then,” he said to himself, “wool is a better radiator than wood, and, cooling quicker, condenses the moisture of the air more rapidly upon it; but grass again, as the thermometer showed, cooled quicker even than the wool, and therefore collects more dew than either.”

This induced young Humphry to try another experiment, in order to ascertain whether those bodies which cooled most rapidly collected the most dew on their surfaces. Accordingly he placed a piece of bright polished metal and a piece of glass (for the surfaces of these substances were nearly the same) on the gravel-path, and was delighted to perceive that in a short time the glass was covered with moisture while the metal remained perfectly dry. A strip of flannel was then put beside the other two, and, being a good radiator, it soon became spotted with dew-drops. After this the boy coated the piece of polished metal with lamp-black, and found it then, like the others, capable of condensing the moisture of the air upon its surface.

“It is as I expected,” cried the lad; “the dew which the ancients imagined to be shed from the stars is simply the condensation of the vapour in the atmosphere upon cold surfaces; and, consequently, those bodies which have the greatest radiating power, and so become cold the quickest, are found to have the largest deposition of dew formed upon them, while those which, like polished metals, part with their heat but slowly, and so remain for a long time at the same temperature, have but little moisture condensed upon their surface. The deposition of dew,” he went on musing, “is precisely similar to the condensation of moisture that occurs on the outside of a bottle of very cold water when brought into a warm room. The cold surface of the glass abstracts heat from the vapour in the air of the apartment, and so causes it to be condensed in the form of little watery globules on the surface. In the same manner the earth, parting at night with the heat it has received during the day from the sun, becomes cooler than the atmosphere above it—for thermometers show, that when the grass is at 51° in the evening, the air only four feet above it is more than 60°—and accordingly the cold surface of the blades acts upon the vapour in the atmosphere, precisely the same as the outside of the cold bottle does upon the air in a warm room.”

So pleased was the lad with the insight that his investigations had given him into some of the mysteries of nature, that he continued his experiments on this subject for many nights; and in the course of these he found, that not only had different bodies different dew-collecting powers, but that different colours even possessed the same property; for on exposing a piece of yellow, of green, of red, and of black glass to the night air, he perceived that the moisture appeared first on the yellow glass, then on the green, but that none at all showed itself on either the red or black glasses.

To his astonishment, however, he at length discovered, that when the evenings were cloudy, and there seemed to be a greater quantity of moisture in the atmosphere, the pieces of flannel and glass, and little piles of swan’s-down with which he had studded the gravel-walk, remained unmoistened with dew; whereas, when the nights were clear and apparently dry, they, one and all, with the exception of the polished metals, became rapidly suffused with moisture. This for awhile entirely baffled the boy’s comprehension. “How came it that more dew was deposited on dry clear nights than on dull damp ones?” Surely, such being the case, the dew cannot be said to proceed from the vapour in the atmosphere; for if it does, reasoned Humphry, it is evident that when there is more moisture in the air there should be more dew deposited on the earth.

At length it struck the boy that, perhaps, the clouds themselves might interfere in some way or other with the result; so the next fine clear night he strewed the gravel-walk, as before, with fragments of such substances as he had already found to be the best collectors of dew; and then, at the other end of the path, he placed pieces of the same substances under a small awning, which he made out of his pocket-handkerchief, fastened at each corner to a short stick. This he did in order to see what effect would be produced by screening bodies from the sky—since the clouds, he fancied, might act in some such manner.

On returning to the garden after a short interval, Humphry was rejoiced to find that there was a copious deposition of dew on the pieces of glass and wool that he had left exposed to the sky, while the surfaces of those which were screened by the little awning above them remained perfectly dry.

“Yes,” cried the lad, “the clouds do act as screens. They give back, perhaps, some of the heat that the earth at night is radiating into space, and so prevent bodies cooling down as rapidly as they otherwise would.”

However, to satisfy himself that the clouds really did interfere with the radiation of bodies on the earth, Humphry arranged an apparatus for testing the point. This consisted of a thermometer, the bulb of which was first incased in wool (for that substance he knew to part readily with heat) and afterwards fixed in the focus of a small concave mirror. Then on the next windy night, when the clouds were drifting swiftly across the sky—leaving the heavens occasionally clear, and occasionally hiding the light of the stars—the anxious lad turned the mirror towards the blue vault above, and, on doing so, he could hardly repress his glee as he beheld the quicksilver in the tube of the thermometer descend and ascend, each time the sky became clear or clouded. Though, by means of another thermometer, he knew the temperature of the surrounding atmosphere to be 60°, Humphry nevertheless found that, when the sky was unclouded, the mercury in the one attached to the mirror indicated only 45°, whilst immediately that a cloud passed over the firmament, and so prevented the bulb from parting with its heat, the quicksilver rose rapidly again to the temperature of the air around. So intensely did Humphry exult in the result of this experiment, that he remained long, watching the thermometer rise and fall, as the clouds swept one after another across the sky.

The next day, Humphry, now that he had made himself acquainted with the circumstances that regulated the emission or radiation of heat from bodies, began to turn his attention to the reflexion of it from such substances as impeded the progress of the rays; “for,” said he, “if bodies at an elevated temperature have the power of sending out rays of heat in all directions—in the same manner as luminous bodies emit rays of light—it follows that substances opposing the passage of the heat-rays must either absorb them, and so become heated themselves—or they must transmit them and so allow the rays to proceed in their original direction—or else they must reflect them and so bend them into another course.”

For the study of the reflexion of heat the lad procured two concave mirrors, made of tin-plate and about 1 foot in diameter. These he arranged so as to slide up and down a pillar, to which they were respectively attached. Thus provided, Humphry proceeded to place a small “Florence flask,” filled with hot water, in the focus of one of the mirrors, while in the other focus he arranged a thermometer after this fashion:

Now, though the mirrors were some feet apart, the mercury in the tube, to the boy’s great delight, rose almost to the heat of the boiling water in the flask.

After a few moments’ reflection, the lad fancied the effect might perhaps be due to the radiation of the heat from the flask itself, rather than to the reflexion of it from the mirrors. So, to satisfy himself whether or not such were the case, he placed a sheet of pasteboard immediately in front of the mirror near the thermometer, and thus prevented any rays being reflected from the one to the other. No sooner, however, had he done so than the mercury was seen to fall in the tube—even though the source of heat was as near to the thermometer as before; but directly he removed the pasteboard from between the mirror and the thermometer, the quicksilver rose rapidly again, and stood at the same number of degrees as it previously did.

Having convinced himself upon this point, he then drew the thermometer away from the focus and nearer to the heated flask, so that, if the effect were due to radiation, the mercury, as it approached the source of heat, should rise higher in the tube. The contrary result, however, was found to ensue; and it will be seen on reference to the preceding engraving, that by radiation only a few of the heat-rays (which are indicated by the diverging unbroken lines) would fall upon the thermometer, whereas by reflexion a much larger number of such rays become concentrated upon the bulb in the focus of the opposite mirror—as shown by the dotted lines in the diagram.

Humphry was now anxious to see whether, by reflexion of the heat-rays, he could ignite combustible bodies at a distance; but for this purpose he changed the situation of the mirrors, arranging them vertically one above the other, instead of horizontally, or each on a level with the other, as before. Then he made a small basket of iron wire, and having filled this with burning charcoal, he suspended it below the upper mirror, so that it hung exactly in its focus, whilst above the lower mirror he fixed a small piece of phosphorus, and this was exactly in the focus also. Thus, on the completion of the arrangement, the boy was as astonished as he was delighted to perceive that the phosphorus was immediately inflamed by the reflected rays of heat. Some fulminating silver was then exploded in like manner. After this Humphry boiled some water in a flask that he substituted for the piece of phosphorus in the focus of the lower mirror, and finally cooked a chop, by the same means, at some considerable distance from the fire.

Next, instead of the two mirrors, he rolled up a sheet of bright gilt paper, with the metallic side inwards, into the form of a long cone or funnel, so that the opening was larger at one end than at the other; then holding the larger end towards a clear fire, he found the rays of heat were concentrated into a focus at a little distance beyond the smaller end, and there he caused a bit of phosphorus again to inflame, by means of the reflected heat. The subjoined diagram exhibits the arrangement.⁠[33]

Humphry now began to wonder what effect would be produced by a piece of ice placed in the focus of one of the mirrors; and he thought for a long time whether the rays of cold would be reflected from the ice, as those of heat had been from the hot water and the burning charcoal. As the winter had long set in, he found no difficulty in obtaining such a piece as he required from one of the neighbouring ponds, and then arranging the mirrors as before, he placed it in the focus of one of them, while in that of the other he fixed the thermometer which he had previously employed.

To the lad’s astonishment he discovered that the mercury immediately began to fall, and at length stood at 32°, or the freezing point. “So then!” he cried, “it is possible to reflect rays of cold as well as those of heat. And yet,” said he to himself, after musing for a while, “is it the ice, after all, that is radiating cold to the thermometer, or the thermometer itself, which, being warmer than the frozen water, is really and truly radiating heat to the ice?” If, instead of the thermometer, he had placed a red-hot body in the one focus, while the ice remained in the other, Humphry knew well enough that the warmer body, as it became cool, would be giving off heat to the colder one. “Why then,” he asked himself, “should he fancy that the thermometer itself—because it was only a few degrees warmer than the ice—lacked the power of parting with its heat to the colder body, in the same manner as the red-hot charcoal?”

The lad was soon convinced of his previous fallacy; and when he saw that the apparently contradictory effect was no anomaly after all, he could hardly refrain from smiling at the simplicity which had led him to believe at first that rays of cold were reflected from the ice to the thermometer, instead of the rays of heat being given off by the thermometer to the ice.

As yet, however, Humphry had experimented concerning the reflexion of heat with mirrors only of polished metal; and one day, when he was recounting to Mr. Tonkin the curious effects he had produced, the old gentleman asked the lad what he imagined would have been the effect if, instead of metal mirrors, he had used glass ones.

Humphry answered confidently, that, as the results were due only to the reflexion of the rays from the concave surface, a glass mirror, of course, would have given precisely the same effects as the metal ones.

“Try it,” was all the old man said, as he smiled at the positiveness of the boy’s reply.

Nor was the young experimentalist long in doing so, for he saw by Mr. Tonkin’s manner that some strange difference in the effect would ensue—though for the life of him he could not divine what it was to be.

Accordingly, at the earliest opportunity, the boy substituted the glass concave mirror, which Mr. Tonkin had lent him for the purpose, for one of the metal ones which he had previously employed; then filling the little wire basket with red-hot charcoal, as before, and hanging it in the focus of the upper mirror, he once more suspended a piece of phosphorus in the focus of the lower mirror, which was now of glass instead of metal. To his utter amazement, however, the phosphorus was no longer capable of being inflamed in such a manner.

It was but the work of a moment to remove the combustible from the focus of the glass mirror, and to place a thermometer there in its stead; and this soon showed that there was now little, if any, heat reflected.

“How wonderful!” cried the startled boy. “What can be the cause of it? I’ll arrange the mirrors differently,” he added, “and see if I can find it out.”

But no sooner did Humphry put his finger on the glass than he drew it suddenly back, as he exclaimed, “How hot the lower mirror has become! and I remember when I used the metal one, that I was surprised to find, on removing it, though the heat was sufficient to boil water and ignite bodies in its focus, the metal surface of the mirror itself was scarcely warmed. But now that glass is used,” he went on, “the mirror itself is rendered hot, while in the focus of it there is scarcely any perceptible increase of temperature. So, then,” he added, “the glass absorbs the heat-rays, and therefore does not reflect them, while the metal on the other hand reflects, because it does not absorb them. Still it’s very strange,” mused Humphry, as he proceeded to blacken a small card, “for the glass mirror must reflect the light of the fire, though it absorbs the heat from it. I’ll try whether such is the case or not.”

The card was then placed in the focus, and a bright spot of light was seen shining like silver in the centre of the blackened surface.

“Yes,” cried the lad, “it reflects the light, but not the heat of the fire. How strange! I wonder whether the same effect would be produced by the sun’s rays!”

Accordingly the next day, when the sun was shining brightly, Humphry arranged the mirror in the garden, so that the beams might be concentrated in its focus; and then, to his greater astonishment, he found that he could inflame combustibles by the solar heat with the glass mirror, in the same manner as he had previously done by artificial heat with the metal ones.

“The light and heat of the sun, then,” said Humphry, as he stood watching the white fumes of the burning phosphorus rise in the air, “are capable of being reflected by glass, whereas the light only of an artificial fire can be concentrated into a focus by it—the heat in the latter case being absorbed.”

The metal mirror, likewise, was found to possess the power of reflecting both the solar light and heat, in the same manner as it had been before made to reflect both the light and heat of an artificial fire.

The experiment with the glass mirror, however, clearly showed that solar heat differed in some way or other from terrestrial heat; but how, was a source of continual wonder to the lad.

From the reflexion of heat, Humphry proceeded to the transmission of it.

Light passes readily through certain substances, which are therefore said to be transparent, while others impede the progress of the beams, and are consequently called opaque. “Is there, then,” mused the boy, “such a property as transparence and opacity for heat, as well as light, among bodies? Are some substances pellucid, as it were, to heat like they are to light? and are some as impermeable to the one as they are to the other?”

The lad knew well that the heat of the sun was capable of being transmitted through glass as well as its light, for he had often concentrated the solar beams by means of a magnifying or “burning” lens, as it is called: glass, therefore, was transparent to the solar heat as well as light; but was it so to the rays of artificial heat?

To ascertain this, Humphry borrowed old Dr. Tonkin’s large reading lens, and held it before the fire so that the focus fell upon the bulb of a thermometer. But though the light of the burning coals was seen concentrated into a bright spot upon the bulb, still the mercury in the tube gave no indications of any increase of temperature. The lens, however, which was scarcely warmed when the sun’s rays passed through it, became greatly heated when the rays of the artificial fire were made to fall upon it—thus showing, that while it transmitted the solar heat it absorbed the terrestrial.

It was evident, therefore, that though the heat of the sun has the power of passing freely through glass, artificial heat, on the other hand, is completely stopped by it.

Humphry then thought he should like to try the effect of a piece of black glass, for this would be perfectly opaque to light, and he longed much to see whether it would be equally impermeable to heat. On holding a square piece before the fire, the boy was surprised to perceive the thermometer he had arranged behind it rise rapidly, thus showing that though black glass was in-transparent to light, it was by no means opaque to heat. That the quicksilver was made to mount in the tube solely by the influence of the heat-rays which traversed the black glass—and not by any indirect radiation from the fire—Humphry assured himself, by placing a piece of white glass, of the same size and thickness as the black one, before the thermometer: the quicksilver, however, was immediately seen to fall. “How marvellous is this!” he exclaimed. “Light and heat, then, are capable of being separated one from the other; and there are bodies in nature which, like white glass, are transparent to light, but opaque to heat; while there are others, like black glass, that allow the heat-rays to pass through them, though they are incapable of being traversed by the luminous ones.”

The boy was so full of the new truth that had thus become impressed upon his mind, that he hurried off to Mr. Tonkin to confer with him on the result. From him Humphry learnt that there were other substances, besides glass, that gave equally curious effects—the most striking of these, the old gentleman told the boy, were alum and rock-salt, for though both were transparent to light, they had by no means the same power of transmitting heat; for it would be found that while a small plate of rock-salt allowed the rays of heat to pass almost freely through it, a similar plate of alum was nearly impermeable to them.

The young philosopher was not long in trying the experiment. Having procured two such plates as Mr. Tonkin had advised, he used them as small screens in front of the fire, and found that a thermometer behind the rock-salt rose rapidly; whereas, behind the alum, it was scarcely affected, for the heat was nearly all stopped by it.

The possibility of separating heat from light made a powerful impression upon the ardent boy, and he wondered whether he could arrive at the same result with the solar beams as he had with the rays of an artificial fire. For a long time he pondered over the matter, and conceived and tried a number of fruitless experiments in connexion with it.

At length, however, he remembered to have read somewhere, that by means of a glass prism the beam of white light proceeding from the sun might be separated into all the colours of the rainbow.

Accordingly he set to work to repeat the experiment. Having darkened his room he made a hole in the window-shutter, and placed behind it a glass prism, with one of the sharp edges downwards and one of the flat sides uppermost, as shown in the annexed illustration:

Immediately that the arrangement was complete, and the beam from without fell on the glass within, the wall on the opposite side was iridescent with a strip of variegated light, as if a slice of a bright rainbow were clinging to it. The lower end of the luminous band was a rich warm red, and this passed, by a tint of orange, into a bright yellow, which again died away, by deepening hues of green, into a narrow strip of dark blue, while, at the upper end, the indigo tint became warmed into a brilliant edging of violet.

When the rapture of the boy on first beholding the sight had, in a measure, subsided, he proceeded, by means of a thermometer, to ascertain the temperature of the several rays. First he tried the upper end of the spectrum, and found that in the blue ray the mercury marked 56°. Then passing downwards Humphry was overjoyed to see the quicksilver mount as he proceeded towards the middle, where, in the yellow ray, the instrument indicated a temperature of 62°, i.e. 6° higher than in the blue; while at the lower end—at the extremity of the red ray—the temperature was found to be as high as 79°, i.e. 17° higher than it was in the yellow.

There was then, altogether, as much as 23° difference between the heat at the extreme ends of the luminous band—the red ray being upwards of half as hot again as the blue one—so that light and heat were capable of being separated even in the solar beams themselves; for the yellow contained the most light of all, and yet it was 17° colder than the extreme verge of the red ray, where there was only a faint luminous blush to be perceived.

The next step was to ascertain the circumstances regulating the reception or absorption of heat.

Humphry had now investigated the laws which governed the radiation or emission of the rays of heat from bodies at an elevated temperature. He had ascertained that these rays not only emanated from heated substances at different rates, and so caused them to cool down more or less rapidly, but that—though their tendency was to proceed, like the rays of light, in a straight line—they were capable of being reflected or bent back by certain bodies opposing their progress, and that in such cases the reflecting bodies themselves did not become heated by them. Other bodies, again, he had found to have the power of transmitting the rays of heat, that is to say, of allowing them to pass through their substance rather than reflecting or driving them back from their surface; and such transmitting bodies, moreover, were likewise scarcely warmed by the heat that traversed them. Now he was about to investigate the conditions that determined the absorption of the heat-rays, by bodies upon which they fell after being given off by radiation from others of a higher temperature.

The lad’s first experiment upon this subject was to blacken the surface of one of the metal mirrors that he had previously found to reflect the heat, without being itself warmed in so doing.

The result proved to be as Humphry had anticipated. The mirror no longer had the power of concentrating the heat in a focus, at a short distance in front of it: for now, instead of reflecting the rays and remaining cool as before, it absorbed all the heat that fell upon it, and became itself warmed by the neighbouring radiator.

The same effect ensued when the surface of the mirror was whitened with chalk, and the same again when it was roughened, or scratched, with emery paper: so that rough and dull bodies proved to be better absorbers of heat—even as they were better radiators—than bright or polished ones.

Hence there appeared to be some connexion between the radiating and absorbing powers of different substances—those which cool the quickest seeming to be capable, also, of being heated in the shortest time.

To test this the lad placed a blackened and a bright-polished vessel in front of the fire, and found that the thermometer in the black vessel rose much more rapidly than did that in the bright one.

Humphry then availed himself of these two vessels as a means of testing the relation between the absorbing and radiating powers of black and bright-polished surfaces. Into the mouths of the black and the bright tin vessel he inserted a thermometer, and then placed between them one of the square canisters he had previously employed, and which, it will be remembered, had one of its sides bright, while the opposite one was coated with lamp-black.

Having filled the middle canister with boiling-hot water, he proceeded first to note the radiating and absorbing effects when the different surfaces were opposed to each other. On arranging the middle canister so that its black side was turned towards the polished vessel at one end, and its polished side to the blackened vessel at the other end, there was no effect produced upon either of the thermometers; for then the opposite powers of the different surfaces exactly balanced each other. When, however, the apparatus was so adjusted that similar surfaces were opposed—that is to say, so that the blackened side of the canister in the middle was turned towards the black vessel, and the bright-polished side to the bright-polished vessel—the thermometer in the black vessel immediately indicated a great excess of heat; for then not only was there a good radiator opposed to a good absorber, but, on the other side, the two bright surfaces were facing each other—that is, the bad radiator was turned towards the bad absorber—so that even the little heat which was given off from the polished side of the canister was driven back again to it by the surface of the neighbouring bright vessel. Hence everything favoured the radiation and absorption of the heat on the one side, where black was opposed to black, and prevented it on the other, where metal was facing metal; and thus the great elevation of the thermometer was accounted for.

As it was now winter time and the snow lay thick upon the earth, Humphry availed himself of the circumstance to test the absorbing powers of different colours. For this purpose he took a number of pieces of different coloured cloths, and placing them at mid-day upon the snow, so that the sun’s rays could fall directly upon them, he found that the dark colours sank the deepest into the frozen mass beneath, while the lighter hues produced scarcely any thawing effect, and the white remained utterly inactive.

The same result was obtained by means of coloured glasses; for against a window-pane that was covered with hoar-frost the lad placed some pieces of black, red, green, and yellow glass, and the consequence was, that the ice opposite to the black and red pieces was melted long before any thawing effect was visible upon the frozen film screened by the other colours.

When the weather grew warm Humphry obtained another very curious illustration of the power of black substances to absorb the heat of the sun’s rays. Having filled a glass tube with spirits of wine, he placed it in the focus of a lens, and found that the solar heat traversed the transparent liquid without warming it. On immersing a small piece of charcoal, however, in the alcohol, so great was the absorptive power of the black surface that the fluid immediately began to boil. By the same means, too, he succeeded in raising the temperature of water to the boiling point. This showed that water, as well as spirits of wine, was a good transmitter and bad absorber of heat; that is to say, that the rays passed freely through each without warming either, unless some substance were immersed in the liquid in order to detain and absorb their heat.

Air, on the other hand, the boy knew to have little or no heat-absorbing power; for the rays emitted by a distant hot body traversed the atmosphere without sensibly raising its temperature. He had read, too, that philosophers had calculated that only one-fifth of the solar heat was absorbed in passing through 1000 feet of the air, and that but one-third of the entire heat of the sun was taken up by the passage of the beams through the whole atmosphere.

Humphry, moreover, sought to discover whether the sun’s heat, reflected from a mirror, would produce the same effect as the direct solar beams. Accordingly, before the winter passed away, he placed two pieces of blackened card upon the snow, at a considerable distance apart. One of these he left exposed to the direct rays of the sun, while upon the other he caused the sunbeams to fall indirectly, by reflecting them from a polished metal surface. The black card that was submitted to the direct solar beams sank, after a little time, deep into the snow, while the frozen mass around—though the beams fell full upon it—was but slightly thawed. With the black card, however, upon which the sun’s rays were reflected, a precisely opposite result ensued. In that case the surrounding snow itself was the first to melt, while the blackened surface seemed to have been deprived of its power of absorbing heat, and remained high on the unthawed pile beneath it.

Further, Humphry noticed that the snow which lay near the trunks of trees, or wooden posts, melted much sooner than that which was at a distance from them, and that the thawing always commenced at the side facing the sun. Hence it was evident that the solar heat, after being either reflected or radiated from bodies on the earth, and so made to fall indirectly upon other bodies, was rendered capable of being absorbed by substances which, like snow, had but little or no power of being warmed by it directly.

Why this should be, or what alteration the solar rays underwent in impinging upon terrestrial bodies, so that substances which before absorbed the sun’s heat with difficulty became afterwards more easily warmed by it, was more than Humphry’s philosophy could explain—though it cost him many a day’s hard thinking in trying to account for the result.

Having now investigated the conditions which governed the diffusion of heat from a distant point, Humphry next proceeded to inquire into the circumstances which regulated the communication of heat to bodies in contact with others at an elevated temperature.

This constitutes what is called the conduction of caloric, and occurs between different bodies, or parts of the same body, immediately adjoining each other. The communication of heat by conduction is a slow process compared with that of radiation, which is, probably, as rapid as the diffusion of light itself. Philosophers have calculated, that even if the crust of the earth were made of cast-iron (which is a much better conductor than rocks and stones), it would take myriads of years to transfer the heat from a depth of 150 miles below the surface to the surface itself; whereas by radiation the solar heat travels from the sun to us in 8½ minutes.

The laws which regulate the communication of caloric to distant objects are similar to those which would ensue if the heat really consisted of so many hot particles darted out from the heated body in all directions; and colder bodies placed in the neighbourhood of heated ones, either become hot in the same manner as they would if such particles were positively absorbed by them, and entered into their substance; or they reflect the heat to other bodies, while they themselves are unwarmed by it—as if (according to the hypothesis) the caloric particles were elastic, and had the power of bounding off from smooth surfaces interfering with their progress; or else they transmit the heat rays, as if the imaginary particles of caloric were capable of freely traversing certain substances, and that, also, without sensibly raising the temperature of the permeated mass.

But all bodies, or parts of bodies, which are in immediate contact with some other at a higher temperature, become themselves warmed; not by rays thrown out from the heated mass, but by conducting or diffusing the heat from one point to another, and so disseminating it ultimately throughout their whole substance.

Humphry was thus particular in impressing upon his mind the precise difference between the radiation and conduction of caloric before entering upon the study of the latter process; for he knew that without clear and distinct views upon the subject it was impossible for him to arrive at any absolute knowledge.