Radiant heat

“ It is well known that, when bodies are exposed to radiant heat, they are heated in proportion to their size; the smaller, then, may be moist, when the larger are dry by radiation. The effect of the sun’s penetrating heat through the cloud is to heat exposed objects above the temperature of the air; and if the objects are of any size they are considerably heated, and retain their heat more, while at the same time around them is a layer of warm air which is quite sufficient to force the water-vapour to leave the dust-particles in the fine rain. ”

“ 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. ”

“ With these the heating is accomplished entirely by convection, while with direct radiators the radiant heat rarely amounts to 40 per cent., generally not over 30, and often in practice considerably less. For a radiator in any particular location the radiant heat is constant for the same conditions of temperature of the radiator and surrounding objects, and is independent of currents of air. ”

“ But radiators are invariably built of clusters of pipes or surfaces, and as radiant heat travels only in straight lines and perpendicular to the surface of its source, a large proportion of the surface is wasted so far as radiant heat is concerned, due to what may be called the mutual interception of the rays. In the ordinary one-column cast-iron radiator, the proportion of surface from which no radiant heat takes place is nearly 20 per cent., in the two-column, 45 to 55 per cent. ”

“ 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. ”

“ Above all, we are concerned here with the determination of those quantities which at any place in a medium traversed by heat rays determine the state of the radiant heat. The state of radiation at a given place will not be represented by a vector which is determined by three components; for the energy flowing in a given direction is quite independent of that flowing in any other direction. In order to know the state of radiation, we must be able to specify, moreover, the energy which in the time flows through a surface element for every direction in space. ”

“ Fill each with hot water of the same temperature, and leave them equally exposed. Their rates of radiation will then be measurable by their rates of cooling. The314 black will cool the most rapidly, the rough canister next, and the bright one the slowest.
Radiant heat may be reflected like light from bright surfaces, the reflecting substance itself becoming heated in a proportion which diminishes just as its reflecting powers increase. ”

“ Thermotical Theories.—The phenomena of radiant heat, like those of radiant light, obviously admit of general explanation in two different ways;—by the emission of material particles, or by the propagation of undulations. Both these opinions have found supporters. Probably most persons, in adopting Prevost’s theory of exchanges, conceive the radiation of heat to be the radiation of matter. The undulation hypothesis, on the other hand, appears to be suggested by the production of heat by friction, and was accordingly maintained by Rumford and others. ”

“ Radiant heat passes instantaneously, and in greater or less quantities, through a certain class of bodies, solid as well as liquid. This class does not consist exactly of diaphanous substances, since opake plates or plates possessing but a feeble transparency are more diathermanous or permeable to radiant heat than other plates possessing perfect transparency.
There are different species of calorific rays. They are all emitted simultaneously and in different proportions by flame, but in the heat from other sources some of them are always wanting. ”

“ It also holds good for phosphorus burning in oxygen. Every gush of dazzling light has associated with it a gush of invisible radiant heat, which far transcends the light in energy. This condition of things applies to all bodies capable of being raised to a white heat, either in the solid or the molten condition. It would doubtless also apply to the luminous fogs formed by the condensation of incandescent vapours. ”

“ 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. ”

“ Proof that the radiant heat from a feebly luminous flame is also feeble, and that the increase of radiant light, by the introduction of a solid substance into the flame of the compound blow-pipe, is accompanied with an equivalent radiation of heat, and also that the increase of light, and radiant heat in a flame of hydrogen, by the introduction of a solid substance, is attended with a diminution in the heating power of the flame itself. ”

“ The radiation from a body whose temperature is lower than 500° C. is made up exclusively of energy whose wave length is greater than 7,600 tenth meters, and is therefore invisible to the eye, although a thermometer or even the human hand can often detect it as radiant heat. A brick wall in the summer sunshine gives off energy which can be felt as heat but can not be seen. ”

“ There's a band of radiant energy somewhere between ultrasonics and radiant heat that hits fast and goes deep, and comes out just as fast, and it gets triggered off by whatever this is that happens with near-absolute zero objects subjected to room temperature. ”

“ 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. ”

“ The intensity of radiant heat follows the same law as that of light, and decreases as the square of the distance from its source. The same law that governs the reflection of light, also prevails with that of heat; and it may be found by experiment that the angle of incidence is equal to the angle of reflection, so that the heat is disposed of in the same manner as light when it falls upon bright polished planes, convex and concave surfaces ”

“ Surface of radiators.—There are other considerations which alter the effectiveness of a radiator besides the design and setting, notably the condition of the surface. Radiators are rarely used with the natural iron surface, but are painted in all possible ways. The nature of the surface has practically no effect on the convected heat, but a very decided effect on the radiant heat; but as the latter is generally less than 35 per cent, of the whole, the effect on the total heat emitted is not so marked. Furthermore, usually only the top and outside surface of one side is painted at all. ”

“ 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. ”

“ All substances at temperatures above absolute zero (-469° F) , produce radiant energy. On earth, absolute zero can be approached only in the laboratory and with special apparatus. For all practical purposes, then, we can consider that all materials we ordinarily deal with are radiating energy—even cold ones. ”

“ Good reflectors are bad radiators and bad absorbers of heat, and the power of absorbing heat, or becoming superficially hot when exposed to radiant heat, is exactly proportionate to radiating efficiency. ”

“ Thus, in thermotics, the law of an intensity of radiation proportional to the sine of the angle of the ray with the surface, which is founded on direct experiments of radiation, is found to be necessary in order to explain the tendency of neighboring bodies to equality of temperature; and this leads to the higher generalization, that heat is radiant from points below the surface. But in the doctrine of the relation of heat to gases, as delivered by Laplace, there is none of this unexpected confirmation ”

“ He, before any other investigator, showed that radiant heat is refracted according to the laws governing the refraction of light by transparent media; that a portion of the radiation from the sun is incapable of exciting the sensation of vision, and that this portion is the less refrangible; that the different colors of the spectrum possess very unequal heating powers, which are not proportional to their luminosity; that substances differ very greatly in their power of transmitting radiant heat, and that this power does not depend solely upon their color ”

“ 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. ”

“ Radiation strikes the receiving object from all of the points—in greatest intensity from points perpendicular and closest to the object, and in lesser intensity from the other points. Because so many points are producing radiant energy, the decrease in intensity with distance from a flame source is less than that from a line source and much less than that from a point source. The number of points from which radiation can be received also increases rapidly with the area of the radiating surface. ”

“ The rate at which a substance emits or takes up radiant energy depends upon the nature of its surface. A rough, black surface, such as may be obtained by holding an object in the smoke from burning camphor, radiates and absorbs heat with greater freedom than any other ”

“ Through all these ages the sun must have been pouring out his radiant energy at at least his present rate. Radiation of heat from the earth may have been relatively less rapid from a denser carbon-laden atmosphere in times past than at present, but it never could have been more so. ”

“ Within the past six years we have become acquainted with changes incomparably more energetic than those of the most violent chemical action; many of them proceed with apparent spontaneity from a substance all by itself. In the case of radium neither extreme cold nor extreme heat has any perceptible effect upon the radiant stream. ”

“ 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. ”

“ A radiator for heating a room should therefore have a dull, dark surface, while a vessel which is designed to keep its contents from losing heat should have a highly polished exterior.
A perfectly transparent substance would radiate no energy, whatever the temperature to which it is {42} raised, for its absorbing power is zero and therefore its radiating power is also zero. No perfectly transparent substances exist, but some substances are a very near approach to it. ”

“ The presence of a considerable number of persons prevents radiation, and the excess of heat can be carried off only by currents of air, or by a more abundant transpiration. We fan ourselves to expedite the cooling by convection and evaporation, by bringing more air in contact with the skin; and if we leave the room when we are nearly smothered, to go out "to take a breath" in an empty room, we shall be astonished to find by the thermometer that the temperature of the two rooms is nearly the same; only that radiation is free in the empty one. ”