430

Radiation of Heat.

tances by suitable mirrors, affect a thermometer as quickly as the heat of the sun similarly reflected. The rapidity of its passage is probably as great as that of light, the rays of light and heat from the sun reaching the earth simultaneously. Although light and heat are united in the sun's rays, they are still separable, as by glass prisms or lenses, and by other means; and the focus of heat behind a burning glass is not precisely the focus of light. Heat in radiating through air and transparent liquids does not warm them, and its passage through air is not sensibly affected by winds or any other motion of the atmosphere. These resemblances in the phenomena of light and heat have led to the hypothesis that the two classes of appearances are only different modifications of action in the same subtle substance or ether.

All bodies radiate, whether they are above or below the temperature of the medium in which they are placed.

620. Thus, heat-rays are radiated by a flask of boiling water, by the living human body, by a ball of ice, or of red-hot iron; and this radiation goes on until an equilibrium of temperature is established The diffusion of heat by radiation, as it takes place in an instant to any distance, and is strongly manifested when there is any inequality of temperature between bodies exposed to each other, would produce a speedy balance of temperature throughout nature, but that heat leaves and enters bodies with a readiness depending on the condition of their surfaces, and on their internal conducting powers. A black stone-ware teapot, for instance, filled with boiling water, will radiate away 100 degrees of its heat in the same time that a similar vessel of polished metal will radiate only 12 degrees.

621. Professor Leslie was the first to investigate this subject and to discover many important facts. As common thermometers are not sufficiently delicate to determine very sudden changes of temperature, where the influence is so slight as in many cases of radiant heat, he contrived the beautiful differential thermometer, represented in fig. 170, p. 433, in conjunction with concave mirrors, to concentrate the heat and accumulate its energy.* Then taking, as the heated body, a cubical tin vessel filled with boiling water, and covering it successively with plates or layers of different substances and with different colours, and exposing the thermometer to it for

* A more delicate instrument of recent invention, called the thermopiie, will be described in the section on Electricity.

Radiation, Absorption, and Reflection.

431

a given time under all the changes, he noted the number of degrees which the thermometer rose (as seen in the table which here follows), and thus ascertained the radiating power of each sort of covering.

He next reversed the experiments by using his hot-water vessel always in the same state, and covering the thermometer bulb with the different substances and colours, and thus he ascertained that the comparative absorbing powers of the substances and colours were very nearly proportioned to their radiating powers: lampblack, for instance, absorbed or was heated 100°, while the polished metals absorbed, or were heated only 12°, and so for the others. Lastly, the absorbing powers being an indication of the weakness of the reflecting powers (for a body absorbing a given proportion of the heat which falls on it, can reflect only the remainder), he by the same experiments ascertained the radiating, absorbing, and reflective or mirror powers of the bodies, and, therefore, all the important points respecting radiant heat in its relation to different substances. The highly polished metals, owing to their lustre and smoothness, have the lowest radiating power, and it is found that this is not in any way affected by substances placed beneath them. A glass plate covered with gold or silver leaf possesses the radiating power of the bright metals.

It seems paradoxical that a clothing of a thin cotton or woollen fabric placed on a polished tin vessel, should cause the heat to be received by it or dissipated from it much more than if the vessel were naked and polished, but such is the fact. A metal with its surface scratched or roughened radiates or receives heat much more rapidly than highly polished metal.

622. The property of absorbing radiant heat was supposed to depend in some measure on the colour of the substance. As a general rule, the dark colours, i.e., those which absorb the most light, absorb also most heat, especially solar heat. Tyndall found .hat white in some cases exceeded black, black in some cases exceeded white, and the other colours were equally capricious, all evidently depending on the constitution of the substances. Radia.

432

Radiation. Effect of Coloured Surfaces.

tion and absorption were however in all cases found to go hand in hand-the substance which absorbed heat most powerfully radiated the same heat most copiously.

Franklin placed pieces of cloth of different colours on snow, and exposed them during a given period to the sun's rays—noting the different depths to which the cloths sank by the melting of the snow beneath; but it has been justly remarked of these experiments that the luminous rays of the sun are alike powerless to warm the cloth or to melt the snow. Whatever effect is produced is therefore owing to the dark solar rays which snow rapidly absorbs; these are also absorbed by dark-coloured cloth.

The more recent experiments of Dr. Bache lead to the inference that the radiating power of any surface is not materially affected by its colour, so that the colour of clothes worn during winter, has no marked influence in retaining warmth. The absorbent power is, however, entirely dependent on colour, so far as solar heat is concerned.

Many animals in the polar regions are remarkable for having a white fur, but the retention of heat in them is owing to other conditions, and not to the mere absence of colour. Animals with dark fur are also found in these regions.

623. Those surfaces which radiate heat freely also absorb it readily, and thus the best absorbers of heat are found at the top of the table, page 431. The bright metals at the lower part of the table absorb but little. They are, however, powerful reflectors, as a proof of which it may be stated that while reflecting to a focus heat sufficient to ignite phosphorus (113°), the surface will be found quite cold. If highly polished, they do not retain enough to convey to the hand the slightest sensation of warmth. Glass reflectors, owing to the metallic surface being behind a certain thickness of glass, retain a portion of the heat and become sensibly warm.

There is a difference among metals in the power of reflecting heat rays. According to Melloni, out of 100 rays, silver reflects 90, bright lead, 60, and glass, 10.

The rate of cooling in heated bodies is influenced by all the particulars noted above, viz., substance, surface, and colour, and by the excess of heat in the cooling body as compared with those around it.

624. The concentrating apparatus used for experiments on the radiation of heat consists of two concave highly polished tin mirrors, here represented at a and b (fig. 170), so formed and placed in relation to each other, that all the rays of light or heat issuing from

Reflection of Heat-rays.

433

the focus of one, as at c, shall, after a double reflection, be collected in the focus of the other, d. A stand under one focus, c, is intended to support the body giving out or receiving heat, and a stand under the other, d, supports the thermometer. For further explanation of the action of such mirrors we may refer to the section of Optics, on the concentration of light. The laws of heat reflection are precisely the same as those for the reflection of sound, already referred to in the Last section, Art. 500. Now the surface of a spherical concave acts so that every ray issuing from a point which is not the centre of the said concave, but half-way nearer the surface, shall, when reflected, become parallel to every other ray-as represented by the dotted lines in the figure; and it is the property of a similar mirror receiving parallel rays, to make them all meet in that focus :—thus, any influence radiating from c towards the mirror, a, will again,

a

Fig. 170.

after two reflections, be collected at d. (See also Art. 501.) To show their effect and mode of action, they may be placed exactly facing each other at any convenient distance, while a hot body of any kind, as a metallic ball or a canister of boiling water, is placed in the one focus, and a thermometer in the other, the thermometer will instantly rise; although if left in any intermediate situation nearer to the hot body, and therefore not in the focus, it will not be sensibly affected. If burning charcoal, or a red-hot copper ball, be placed in one focus, and a readily combustible substance, like phosphorus, in the other, the latter may be heated so as to melt and take tire at the distance of thirty feet or more.

If in one focus of the mirror-apparatus described above, there be placed, instead of the canister of hot water, a block of ice, the thermometer in the other focus immediately falls. This was formerly described as the radiation of cold, and persons were at one time disposed to think that it proved cold to be a substance of a different nature from that of heat. The case, however, is merely that the

434

Apparent Radiation of Cold.

thermometer happens then to be the hotter body, in one focus of the mirrors, brought into close relation with a colder body, the ice, placed in the other, and consequently, by the law of equable diffusion or exchanges, it must share its heat with the ice, and this will cause the thermometer to fall. The mirrors in any case exert their effect merely by preventing the spreading and dissipation of some radiant heat from either focus except towards the other, and of making two distant bodies act upon each other as if they were almost in contact, or very near. All the heat that seeks to radiate from the thermometer, d, in the direction of the surface of the mirror, b, if not met by an equal tension or force of temperature in the other mirror or focus to which they are directed at a and c, will radiate away to c, and become deficient at d, hence it is considered that an incessant interchange is taking place among bodies which are near to each other until an equalization of temperature is reached. This happens when every body radiates as much heat as it receives.

If a flask of water at 212° is placed in one focus, and a block of ice in the other, the ice will receive a larger portion of heat rays than it emits, and will show this by rapidly melting. If for the ice we substitute a red-hot copper ball at 1100°, the water will now appear to receive the heat from the ball, but it does not the less radiate heat. The difference is not so great between ice, at 32°, and water, at 212°, as between the latter and a metallic ball heated to redness.

625. Different substances allow a passage to rays of light more or less readily, and accordingly are said to have different degrees of transparency, as in the series from pure air, glass, water, &c., at one end, to paper, thin porcelain, stones, &c., at the other; so different substances influence very differently the passage of the rays of heat. But, what might not be expected, certain substances which transmit light freely are found to obstruct heat, as crystals of pure alum, while some which obstruct light, as a kind of black glass, used for the polarization of light by reflection (see section on Light), give free passage to heat. Another remarkable fact is, that heat from a source of great intensity, like the sun, passes readily through many substances,—as glass, water, and even ice, which absorb and arrest heat-rays from sources of lower temperature, as hot stones or liquids. Rock salt has the remarkable property of allowing the heat-rays of both kinds, solar and artificial, to pass through it with equal readiness.

It is important to remark here that because water or moisture in