158.] ABSORPTION AND RADIATION. 329 covered with lamp-black, and a second side with writing-paper, let a third be scratched in various directions, and let the fourth remain polished. On placing the canister, filled with hot water, in the focus of one mirror, and a thermoscope in the focus of the other, it will be found, on presenting each side in succession to the mirror, that a different temperature is indicated. The heat radiated will be found to be greatest from the lamp-black, less from the paper, still less from the scratched face, and least of all from the polished surface. In consequence of the more rapid radiation from blackened than from polished surfaces of the same metal, a given quantity of a hot liquid placed in a blackened vessel will reach the temperature of the surrounding air sooner than if it be placed in a vessel similar in size and shape, but with a polished surface. The amount of heat radiated from the surface of an object destitute of reflecting power varies with the angle; the intensity being proportioned to the cosine of the angle which the issuing. rays form with the perpendicular to the surface. This fact was proved experimentally by Leslie. FIG. 130. B B Opposite to a concave mirror he placed a vessel with flat sides filled with boiling water, the side of the vessel next the mirror being coated with lampblack. Two screens, A A, B B, pierced with apertures of equal size, were interposed between the mirror and the heated vessel, and in the focus of the mirror was placed the blackened ball, f, of a thermoscope; no alteration in the indication of the thermometer was produced by varying the inclination of the vessel. The dotted line a b represents the extent of the radiating surface when the vessel is vertical. And it is obvious that as the obliquity of the surface a c is increased a larger extent of radiating surface is exposed to the thermometer, so that the intensity of the emitted ray must be inversely as the extent of surface in the two cases. In the economic applications of heat, constant scope is afforded for the employment of the powers of reflection, radiation, and absorption. The meat-screen and the Dutch oven, when kept bright, afford instances of the application of the reflection of heat to beneficial purposes, in directing the heat upon the objects 330 FORMATION OF DEW. [158. between them and the fire. Tea made in a silver teapot, which owing to its polished surface long retains its high temperature, is superior in flavour to that made in black earthenware, which rapidly loses its heat by radiation. Pipes for the conveyance of steam should be kept bright until they reach the apartment where the heat is to be distributed, and there the surface should be blackened, in order to favour the process of radiation. (159) Formation of Dew.-The distribution of heat by radiation is not confined to bodies highly heated. All substances, whatever be their temperature, are constantly radiating a certain portion of heat, the amount of which depends upon their temperature. If the different bodies are all at the same temperature, each absorbs from surrounding objects in a given time exactly as much heat as it radiates towards them in the same time. But suppose the bulb of a thermometer to be placed in the focus of a small parabolic mirror, which is turned towards a perfectly cloudless sky, in such a direction that the sun's rays shall not fall upon the mirror, the temperature will sink several degrees; at night, frequently as much as 8° or 10° C. The thermometer, like all other objects, is constantly radiating heat the mirror cuts it off from the rays proceeding from surrounding objects, and the portion of space towards which it is presented not returning the heat radiated towards it from the instrument, the FIG. 131. temperature of the thermometer necessarily falls. A similar experiment is easily made with the conjugate mirrors. If in the focus of one mirror, a cage filled with ice (c, fig. 131) be supported, and in the focus of the opposite mirror, the bulb, B, of the differential thermoscope, which has been blackened to favour radiation, and which is screened from the radiation of surrounding objects by a second small mirror, placed as at A, the liquid will soon rise in the stem connected with the blackened bulb, because the bulb radiates towards the ice, which only partially returns the rays that it receives; and the radiation from sur 159.] FORMATION OF DEW. 331 rounding bodies upon the thermoscope being prevented, its temperature falls.* The principles of radiation were happily applied by Wells to the explanation of the phenomenon of dew. Dew is formed most copiously during a calm, clear night succeeding a hot day it is deposited in exposed situations and upon the leaves of plants and on filamentous objects in general. As soon as the sun dips below the horizon, and in shady places even before sunset, radiation from the earth is no longer compensated by the solar rays: the temperature of the surface is, therefore, speedily reduced below that of the stratum of air in contact with it; this stratum being charged with moisture, is no longer able to support so much water in the gaseous form, but deposits it (just as when a glass of cold spring water is brought into a warm and moist room, it becomes bedewed with moisture on its outside); and the cohesion collects the water into the pearly drops that stud the herbage and sparkle in the sloping rays of the sun. On cloudy nights little or no dew is deposited, because the masses of suspended water intercept the rays from the earth, and return them to its surface. Overhanging buildings, or the projecting branches of trees, in a similar way, return the heat to the objects beneath them, and prevent the reduction of temperature which necessarily precedes the deposition of dew. On windy nights the equilibrium is rapidly restored by the contact of fresh surfaces of air with the radiating crust of the earth, and little or no dew is formed. Upon metallic bodies, which are bad radiators, and upon the hard-beaten path or road, where the heat is conducted rapidly from the strata beneath, scarcely any dew is deposited; while upon the branching shrub, the tufted grass, and the downy leaf, abundance of moisture is collected, these being precisely the objects which derive most benefit from its presence. In India, near the town of Hoogly, about forty miles from Calcutta, the principle of radiation is applied to the artificial production of ice. Flat shallow excavations, from one to two feet in depth, are loosely lined with rice straw, or some similar bad conductor of heat, and upon the surface of this layer are placed shallow pans of porous earthenware, filled with water to the depth of one or two inches. Radiation rapidly reduces the temperature below the freezing point, and ice is formed in thin crusts, which are removed as they are produced, and stored away in suitable * For a discussion of the theory of exchanges the reader may consult Stewart's Elementary Treatise on Heat, 2nd ed. 1871, 189, et seq. 332 ICE BY RADIATION-LAW OF COOLING. [159. ice-houses until night, when the ice is conveyed in boats to Calcutta. Winter is the ice-making season-viz., from the end of November to the middle of February.* The fundamental fact of cooling by radiation of the bodies on which dew is being formed, is easily verified. If a thermometer be laid upon a grass-plat, on a clear night, it will be found to indicate a temperature several degrees below that shown by a second thermometer, suspended a metre or more from the surface. (160) Law of Cooling by Radiation.-The rapidity of the cooling of any body by radiation depends upon the excess of its temperature over that of the external air. The hotter the body, the more rapidly does it cool; and as it approaches the temperature of the air, the more slowly does it lose its excess of heat. Newton assumed that the quantity of heat lost by a hot body for equal intervals of time, was proportioned to the excess of its temperature above that of the surrounding air; so that if a body heated to 100° in an atmosphere of o°, lose 10° in one minute, the same body heated to 50° would lose 5° per minute, the air being also at o°. Later experiments, however, have shown that this assumption is not exact, even for low temperatures, and that it becomes very inaccurate at high ones. An admirable series of researches upon the rate of cooling by radiation was made by Dulong and Petit (Ann. Chim. Phys. 1817 [2], vii. 337). They employed a hollow sphere of thin brass, blackened in the interior, and furnished with arrangements for exhausting it of air. For the heated body they used a thermometer with a large bulb, heated to a determinate degree, and supported in the centre of the hollow sphere. They then placed the apparatus in water which was maintained at a constant temperature, and they observed that the rate of cooling differed with A curious formation of ice at the bottom of some rapid, clear, and rocky streams is occasionally seen under the influence of radiation, during the preva lence of bright frosty weather. Ice thus formed is termed ground-ice. The water cools down to 39°2 F. (4° C.) as usual, but below this point the colder water no longer forms a protecting layer, as in still sheets or gently moving streams; the agitation produced by the passage of the water through its precipitous and irregular channel makes the temperature uniform throughout, till it arrives at the freezing point. Angularities and points, under all circumstances, favour the deposition of crystals, and to the irregular surfaces of the rocky fragments in the bed of the stream the ice attaches itself in silvery, cauliflower-shaped, spongy masses, sometimes accumulating in quantity sufficient to dam up the stream, and cause it to overflow; at others, as the ice increases in volume and buoyancy, it rises in large flakes, raising to the surface portions of rock, and even iron itself. 161.] LAW OF COOLING-ABSORBABILITY OF HEAT. 333 the nature of the gaseous medium contained in the globe. If the temperature of the sphere continued constant whilst the experiments were made in vacuo upon the heated body at temperatures ascending according to the terms of an arithmetic progression, the rapidity of cooling increased according to the terms of a geometric progression, diminished by a constant quantity; this constant quantity being the heat radiated back upon the cooling body, from the inner surface of the sphere. If the temperature of the sphere and that of the heated body were both raised according to the terms of an arithmetic progression, so that the difference between the two was always constant, it was found that the rate of cooling increased as the temperature rose, according to the terms of a geometric progression. (161) Relative Absorbability of Different Kinds of Heat.-The transmission of radiant heat takes place more freely in vacuo than in air. The absorption of heat is, moreover, influenced by an important cause, to which no allusion has yet been made, and which was first placed in its true light by the experiments of Melloni (Ann. Chim. Phys. 1831 [2], xlviii. 385; 1833, liii. 5, and Iv. 337). It may be illustrated in the following manner :— If a number of sources of heat be employed, each different in kind and intensity, such as the naked flame of an oil lamp, a platinum wire heated to redness in the flame of a spirit lamp, a sheet of copper heated to about 400° C. in a current of heated air which is rising from a lamp placed beneath it, and a copper canister filled with boiling water,-the face of a thermoscope covered with lamp-black may be placed at such a distance from each of these sources of heat that the liquid shall stand in each case at the same point; that is, the temperature to which the thermoscope is exposed shall be equal in each case. Now, if these distances be noted, and if the face of the thermoscope be covered with a variety of other substances in succession, instead of with lamp-black, the thermoscope when exposed to each of the different sources of heat in succession, will appear to receive different quantities of heat, although placed at the distances at which, when it was coated with lamp-black, the heat appeared to be equal. Thus, suppose that the heat absorbed, when the lampblack was used, in each case were equal to 100: if the thermoscope were coated with white-lead, it was found that, at the same distance from the naked flame as before, it indicated a heat of only 53; opposite to the red-hot platinum the heat was 56, instead of 100 as with the lamp-black with the copper at 400° C., a heat of 89 instead of 100 was indicated; while opposite the |