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-203] Absorbing Power.
199 plate was made of polished brass, which metal is therefore the best reflector. The reflecting power of silver is only so that of brass; that of tin ; of glass . Water and lampblack were found to be destitute of reflecting power, for when the plates were coated with lampblack, or moistened with water, the thermometer indicated no increase in temperature, showing that it received no heat.
203. Absorbing power.-In order to compare the absorbent powers of various substances, Leslie arranged the experiment as shown in fig. 174. The source of heat and the reflector being the same as in the preceding experiment, the differential thermometer was placed in the focus where it received directly all the heat reflected by the mirror. The surface of the focal bulb was altered for each experiment by coating it successively with various materials, paper, tinfoil, gold, silver, copper, and leadfoil; it was also coated with a thin layer of lampblack; it was moistened, and so on. It
was thus found, that when the focal bulb was coated with lampblack, or with water, the thermometer indicated the highest temperatures ; whence it was concluded that lampblack and water hare the greatest absorbing power. The lowest temperature was exhibited when the bulb was coated with thin foil, more especially with brass ; thus indicating that these substances absorb the least heat. The result was arrived at which could indeed be foreseen, that those bodies which best reflect heat absorb it least ; and that, conversely, the best absorbents are the worst reflectors.
204. Emissive power.The emissive or radiating power is the property bodies have of emitting more or less easily the heat they contain ; it is the inverse of the absorbing power.
Leslie compared the emissive powers of various bodies by means of the apparatus represented in fig. 174. The focal bulb of the thermometer was left uncoated, and the various substances were applied successively to the sides of the tin cube. One of them, for instance, was left in its ordinary condition; the second was coated with lampblack; to the third a sheet of white paper was fixed, and to the fourth a glass plate.
Turning first of all the blackened face towards the reflector, the thermometer indicated a considerable increase of temperature, thus showing that the cube sent much heat towards the reflector. Turning then successively the other faces towards the reflector, it was found that the paper side emitted less heat than the blackened face, but more than the glass side, which in turn emitted more than the tin side.
Working in this manner, Leslie then found that lampblack has the greatest emissive power, then paper, then ordinary glass, then the metals. The order of their emissive powers is thus the same as that of their absorbing powers. It is thus concluded that bodies which best absorb heat, also radiate best ; and Dulong and Petit have proved that for each substance the emissive power is in all cases proportional to the absorbing power.
205. Causes which modify the reflecting, absorbing, and radiating powers.—As the radiating and absorbing powers are equal, any cause which affects the one affects the other also. And as the reflecting power varies in an inverse manner, whatever increases it diminishes the radiating and absorbing powers, and vice versâ.
It has been already stated that these different powers vary with different bodies, and that metals have the greatest reflecting power, and lampblack the feeblest. In the same body these powers are modified by the degree of polish, the density, the thickness of the radiating substance, the obliquity of the incident or emitted rays, and, lastly, by the nature of the source of heat.
It has been assumed usually that the reflecting power increases with the polish of the surface, and that the other powers diminish therewith. But Melloni showed, that by scratching a polished metallic surface its reflecting power was sometimes diminished and
Applications of Radiant Heat.
sometimes increased. This phenomenon he attributed to the greater or less density of the reflecting surface. If the plate had been originally hammered, its homogeneity would be destroyed by this process, the molecules would be closer together on the surface than in the interior, and the reflecting power would be increased. But if the surface is scratched the internal and less dense mass becomes exposed, and the reflecting power diminished. On the contrary, in a plate which has not been hammered and which is homogeneous, the reflecting power is increased when the plate is scratched, because the density at the surface is increased by the scratches.
The absorbing power varies with the inclination of the incident rays. It is greatest at right angles ; and it diminishes in proportion as the incident rays deviate from the perpendicular direction. This is one of the reasons why the sun is hotter in summer than in winter, because, in the former case, the solar rays are less oblique.
The radiating power of gaseous bodies in a state of combustion is very weak, as is seen by bringing the bulb of a thermometer near a hydrogen flame, the temperature of which is very high. But if a platinum spiral be placed in this flame, it assumes the temperature of the flame, and radiates a considerable quantity of heat, as is indicated by the thermometer. It is for an analogous reason, that the flames of oil and of gas lamps radiate more than a hydrogen flame, in consequence of the excess of carbon which they contain, and which, not being entirely burned, becomes incandescent in the flame.
The absorbing power of a body is also influenced by the nature of the source of heat. Thus, for the same quantity of heat emitted, a surface coated with white lead absorbs twice as much if the heat comes from a cube filled with hot water as it does if the heat is that of a lamp. Lampblack, on the contrary, absorbs the same amount of heat whatever be the source.
It is usual to discriminate between luminous heat, such as that emitted by a lamp, or by a platinum wire raised to redness, and obscure heat, such as the heat of boiling water, or that of a copper plate at a temperature of 400° C.
206. Applications.—The property which bodies possess of absorbing, emitting, and reflecting heat, meets with numerous applications in domestic economy and in the arts. Leslie stated in a general manner, that white bodies reflect heat very well, and absorb very little, and that the contrary is the case with black substances. This principle is not generally true, as Leslie supposed; for example white lead has as great an absorbing power for non-luminous rays
as lampblack. It applies to powerful absorbents like cloth, cotton, wool, and other organic substances when exposed to luminous heat. Accordingly, the most suitable coloured clothing for summer is just that which experience has taught us to use, namely, white, for it absorbs less of the sun's rays than black clothing, and hence feels cooler.
The polished fire-irons before a fire are cold, whilst the black. fender is often unbearably hot. If, on the contrary, a liquid is to be kept hot as long as possible, it must be placed in a brightly polished metallic vessel, for then, the emissive power being less, the cooling is slower. It is for this reason advantageous that the steam pipes, etc., of locomotives should be kept bright.
Snow is a powerful reflector, and, therefore, neither absorbs nor emits much heat. It is owing to its small emissive power that it protects from cold the ground and the plants which it covers; and owing to its small absorbing power it melts but slowly during a thaw. A branch of a tree, a bar of metal, a stone in the midst of a mass of snow, accelerate the fusion by the heat they absorb, and which they radiate about them.
In the Alps, the mountaineers accelerate the fusion of the snow by covering it with earth, which increases the absorbing power.
Metallic cooking vessels should be black and rough on the outside, for then their absorbing power is greater and they become heated more rapidly. Their bright and polished surface is purchased at the expense of combustible. This is what is seen in vessels of silver and of white porcelain. In common unglazed earthenware liquids are more rapidly heated, but also more rapidly cooled.
It is observed that the ripening of grape and other fruits is accelerated when they are placed in contact with a black wall (mortar mixed with lampblack). This arises from the fact, that from the great emissive power of the wall, as well as from its great absorbing power, it becomes more highly heated under the influence of the sun, and gives up more to the fruit.
CONDUCTING POWER OF BODIES.
207. Conductivity of solids.- In the phenomena of radiation which have been considered, heat is transmitted from one body to another through space, without raising the temperature of the spaces
-207] Conducting Power.
203 through which it passes. It may also be propagated through the mass of a body by an internal radiation from molecule to molecule. This internal propagation in the mass of a body is called conductivity; and good conductors are those bodies which readily transmit heat in their mass, while those through which it passes with difficulty are called bad conductors.
Organic substances conduct heat badly. De la Rive and De Candolle have shown that woods conduct better in the direction of their fibres than in a transverse direction; and have remarked upon the influence which this feeble conducting power, in a transverse direction, exerts in preserving a tree from sudden changes of temperature, enabling it to resist alike a sudden abstraction of heat from within, and the sudden accession of heat from without. Tyndall
has also shown that this tendency is aided by the low conducting power of the bark, which is in all cases less than that of the wood.
Cotton, wool, straw, bran, etc., are all bad conductors.
In order to compare the conducting power or conductivity of different solids, Ingenhousz constructed the apparatus which bears his name, and which is represented in fig. 175. It is a metallic trough, in which, by means of tubulures and corks, are fixed rods of the same dimensions, but of different materials ; for instance, iron, copper, wood, glass. These rods extend to a slight distance in the trough, and the parts outside are coated with wax, which melts at 61°. The box being filled with boiling water, it is observed that the wax melts to a certain distance on the metallic rods, while on the others there is no trace of fusion. The conducting power is