Heat ray

“ We thus arrive at the fact that heat and light may be separated from each other; and if we examine the solar beam by that analysis which the prism affords, we shall find that there is no correspondence between intense light and ardent heat. By careful observation, it has been proved, when we have a temperature of 62° F. in the yellow ray, which ray has the greatest illuminating power ”

“ The rays that are emitted from the sun are partly absorbed by the atmosphere, which acts as a screen, and shields the earth's inhabitants from the full and perhaps destructive influence of the sun's heat. The quantity of heat received by us in one year is prodigious, for it has been calculated that it would suffice to melt a shell of ice forty-six feet thick, and covering every part of the globe. The heat-rays striking the earth, become dispersed in a variety of ways. ”

“ Although we do not fear that heat and light can be confounded in the mind, so different are their phenomena,—we have heat rays, as from dark hot iron, which give no light, while in the full flood of the lunar rays the heat is scarcely appreciable by the most delicate instruments;—yet it is important to show how far these two principles have—been separated from each other. Transparent bodies have varied powers of calorific transparency, or transcalescence: some obstructing the heat radiated from bodies of the highest temperatures almost entirely even in the thinnest layers ”

“ But by far the greater number of heat-rays penetrate the earth, and descend to a considerable depth. In winter this stored-up heat partly returns to the surface, and ultimately becomes dissipated into the air, and from the air into infinite space.
Heat, like light, is absorbed in different degrees by different substances. The colour and condition of surface seems to exert a great influence on its absorption; thus a black body absorbs more heat than a white one, and a rough surface more than a smooth one. ”

“ A blanket of air acts like the glass of a conservatory, letting the light rays in, but hindering the heat rays out. The light rays falling on the ground or the air are transformed into heat rays that, finding the return journey less easy, are consequently trapped. All substances are thus calorifiers, but water vapor is many times more potent than ordinary air to heat-ensnaring. ”

“ It has been demonstrated by laboratory experiments that all hot bodies emit rays of heat, whether we see the rays or not. When we see the rays the bodies are said to be red or white-hot. The process by which heat passes from one body to another without warming the intervening medium is called radiation. Radiation takes place only through transparent bodies. Rays of heat, like rays of light, pass through transparent bodies; whereas they are absorbed by, that is they make hot, opaque bodies. Heat rays travel in straight lines and are reflected from polished surfaces ”

“ A quivering beam of yellow light; then Elza saw that upon occasion, as Tarrano's finger slid a lever, the beam narrowed, intensified to a bright lavender. And now where it struck, the vegetation withered. Blackened, sometimes burst into tiny flame, and parted thus before them as they advanced.
The jungle was silent; yet, as Elza listened, beneath the crackle of the burning twigs she could hear the tiny myriad voices of insect life. Startled voices as the heat of Tarrano's beam struck them. Rustling leaves; breaking twigs; things scurrying and sliding away, unseen in the darkness. ”

“ Sir Humphrey Davy proved that radiant heat could traverse a vacuum, for by putting tin reflectors in an exhausted receiver he found that a hot substance in the focus of one reflector caused an increase in the heat of the other. If we put a red-hot or a hot substance in one reflector, and tinder in the other, the latter will take fire. The velocity of heat rays is equal to that of light, 186,000 miles in a [Pg 91] second, and indeed, radiant heat is identical with light. Heat is reflected as is light, and is refracted in the same way as sound. ”

“ Jimmy’s face was flushed; his eyes sparkled.
“Why—why, just that, Anne! I’m going to build a factory over where the coal is and burn the coal in the ground without bothering to mine it at all, and just pipe the heat up to the boilers. Don’t you see, Anne? Nobody ever thought of that before. They mine the coal now—dig it out and bring it up to the top and take it away on railroads to factories to be burned. And all you’ve got to do is leave it where it is, and put the factory overhead. Look at the work you save, Anne! ”

“ We can get you producer gas that is just as good as any you’re getting—from the coal we burn in the ground, if we control the air and steam right.”
Although this was clear to Mr. Wentworth, it may perhaps need explanation here. In modern furnaces, for the fusion of glass or other operations where great heat is necessary, the process of combustion of the fuel is carried on, not in one operation, as it is in the simple furnaces with which every one is familiar, but in two distinct, separate, progressive stages.
The first stage takes place in a subsidiary furnace known as a “gas producer.” ”

“ When the shortwave radiation strikes a solid object, such as the earth's surface or something on it, a large part of the radiant energy is converted to heat, warming the object. The heated object radiates energy too, but mostly in long waves because of the object's relatively low temperature. Water vapor and carbon dioxide in the air can absorb the longwave radiation, thereby raising the temperature of the air. ”

“ But the heat you don’t pipe up—that’s bunk. You pipe up the gases from the coal—the products of combustion that are only part used, and you burn them in the regenerator furnace. Maybe you could get very good producer gas. ”

“ This supplies the fire with an insufficient amount of oxygen, and although the fuel—paper, for instance—is entirely volatilized it is not entirely burned; there is smoke, which, if it could be mixed with more air, and at a sufficiently high temperature, would burst into flame.
This was the process Jimmy proposed to carry on in the ground; that is, only partly to consume the coal by supplying it with an insufficient amount of oxygen. And it was the unburned coal gases—the combustible smoke, in other words—that he proposed to pipe up to the furnace at the surface—not the actual heat. ”

“ The burning mine, hundreds of feet down in the ground, was in effect to be his subsidiary furnace—his gas-producer.
These unburned gases, from the producer, pass, hot, into the furnace proper; either directly or sometimes after being conveyed a considerable distance—as they would have to be according to Jimmy’s plan. In this latter event they cool off, but are heated up again by the waste heat of the furnace. ”

“ Anne objected. “This is burning underground—”
“It won’t.”
“Why won’t it?”
“Well, it won't because, you see, Anne”—Jimmy was thinking fast now—“because, don’t you see, a fire can’t burn without air. I won’t give it only just so much air. This one got started and ran away with itself before they could stop it. Mine will be ’way down very deep, where there ain’t any air, only just what I pump down to it.
“If I give it lots of air, it will burn hard, and there'll be lots of heat come up. Then if I don’t want so much heat, I won’t give it so much air. ”

“ Just think of it—the first steam-engine. And James Watt doped it out all by himself—just because he had noticed how the steam lifted the lid of that kettle. And he had seen it do that hundreds of times before—and so had everybody else—and never thought anything about it. Isn’t that wonderful, Anne?”
The girl’s eyes were very big and tender as she looked up into his face.
“Yes—it’s very wonderful, Jimmy. You know about so many wonderful things,” she said softly.
“I was just thinking, Anne—” He paused. “When coal burns underground, you can get the heat out of it just the same. ”

“ But because radiation intensity increases with temperature, the warmer object therefore loses heat, and its temperature decreases—part of its heat is transferred to the other object. Conversely, the cooler object receives more radiation than it is emitting and its amount of heat and temperature increases. Eventually, both objects attain the same temperature—their radiant energy gain and loss just balances. Thus, if a substance receives more radiation than it is losing, it gains heat and becomes warmer, but if it is radiating more energy than it is receiving, it loses heat and becomes cooler. ”

“ It is a matter of common observation that rays of the sun or of a fire falling on a body warm it, and it was in the first instance natural to suppose that heat itself somehow travelled across the intervening space from the sun or fire to the body warmed, in much the same way as heat may be carried by a current of hot air or water. But we now know that energy of radiation is not the same thing as heat, though it is converted into heat when the rays strike an absorbing substance. ”

“ About half is in the visible band, and the human eye is adapted to make most efficient use of this radiation. Generally, thermal radiation is not visible when the temperature of a substance is less than 1000°F. If we heat a substance, such as an iron rod, it appears dull red when it first becomes hot enough to emit visible radiation. Continued heating causes it to change in color to bright red, and then to orange, yellow, and white as its temperature increases and the maximum radiation intensity shifts toward the shorter wavelengths. ”

“ There is also a very sensible difference in the heating power of these colored rays. By passing the bulb of a delicate air thermometer through the different colored rays, it indicates the greatest heat in the red rays; next in the green, and so on diminishing to the violet. But the maximum of heat has been ascertained to be immediately beyond the red rays, and of course out of the Spectrum, in an unilluminated spot: thus indicating that there are invisible rays possessing a greater heating power than any of the seven colored rays. These are called calorific rays. ”

“ What is true of the sun's obscure rays is also true of calorific rays emanating from any obscure source,—from our own bodies, for example, or from the surface of a vessel containing boiling water. We must, in fact, figure a warm body also as having its particles in a state of vibration. When these motions are communicated from particle to particle of the body the heat is said to be conducted; when, on the contrary, the particles transmit their vibrations through the surrounding ether, the heat is said to be radiant. ”

“ Before this equality of temperatures, however, is reached, the hotter body will radiate more heat waves than it absorbs, while the colder body will absorb more heat waves than it emits. All bodies, whatever their temperature, are incessantly radiating heat waves. This may be proved experimentally with proper apparatus, as for example with an instrument known as the thermopile. When, however, the total heat waves radiated out by a body are less than it absorbs, the body gets gradually colder, and the temperature decreases. ”

“ The mean result of repeated trials, all differing but slightly, show that the energy of the direct solar rays acting on the polygonal reflector is reduced 0.235 before reaching the heater.
In accordance with the previous article, the investigation has been based on the assumption that the temperatures produced by radiant heat at given distances from its source are inversely as the diffusion of the rays at those distances. In other words, the temperature produced by solar radiation is as the density of the rays. ”

“ Now, if the sun were a cool, solid, or even liquid mass, the sudden accession of merely this quantity of heat would undoubtedly produce an enormous rise of temperature and a great increase of radiation. But, constituted as the sun is—mainly a mass of gas and vapor—the effect would be entirely different, the energy being principally expended in producing expansion and evaporation, with comparatively little increase of temperature or radiation. If one stirs up the fire under an open kettle, the water gets no hotter—it only boils faster. ”

“ Radiant energy travels outward in all directions from the emitting substance until it encounters something capable of absorbing it. The absorbed radiation increases the molecular activity in the substance, thereby increasing its temperature and the amount of heat it contains—this is why we frequently sense radiation as heat. The heated substance radiates energy too, and this energy can be absorbed by other substances and converted to heat. ”

“ Such air is said to be dynamically heated, and, if pure, it shows itself incompetent to radiate, even when a rock-salt window is provided for the passage of its rays. But if instead of being empty the tube contain a small quantity of vapour, the warmed air communicates its heat by contact to the vapour, the molecules of which convert into the radiant form the heat imparted to them by the atoms of the air. By this process also, which I have called Dynamic Radiation, the reciprocity of radiation and absorption has been conclusively proved. ”

“ On the other hand, if the air contain transparent vapour, and the sky appear clear, the counter-radiation will be less, the earth will cool rapidly, and the deposit of dew will be copious; provided the night be comparatively calm, for, when the wind blows, the circulating air supplies heat to the radiating substances, and prevents any considerable chilling. ”

“ There is a constant tendency to an equilibrium; and, during the night, the surface is robbed of more heat, by the colder air, than by day; as, when the earth is not receiving heat, it is constantly radiating it back into space. Even in these processes of convection and radiation, a similar law prevails to that which is discovered in examining into the rate of calorific absorption. ”