296. Reflection of Heat from Concave Mirrors. A CONCAVE MIRROR is a polished spherical or parabolic surface, usually of metal, employed to concentrate rays of heat at a single point.

It is a property of such mirrors that all rays which before incidence are parallel to the axis, are after reflection converged to a single point, which point is the focus of the mirror. Conversely, if the rays proceed from the focus, they will be reflected in lines parallel to the axis.

A and B (Fig. 202) represent two reflectors, having their axes coincident, and their surfaces turned to each other. In the focus, n,

of the mirror, A, is placed a ball of hot iron, and in the focus, m, of the mirror, B, is placed an inflammable substance, as a piece of phosphorus. The heat radiating from the ball is reflected from A, parallel to the common axis of the mirror, and falling upon B, is again reflected to the focus m; the heat, concentrated at m, is sufficient to inflame the phosphorus, even when the mirrors are several yards distant from each other. If the mirror, A, alone is used, the phosphorus is not inflamed.

Parabolic reflectors brings parallel rays more accurately to a focus than spherical, but are more difficult to construct, and therefore are not used so much.

The property of concave mirrors, above explained, enables us to concentrate the heat of the sun's rays. In this case the reflector is called a burning mirror. It must be placed so that its axis is parallel to the rays of the sun, which, as they fall upon it, are reflected to the focus, where they produce heat enough to set inflammable substances on fire.

It is said that ARCHIMEDES was enabled by means of mirrors to set fire to the Roman ships in the harbor of the city of Syracuse. BUFFON showed the possibility of such an operation, by setting fire to a tarred plank, by means of burning mirrors, at a distance of more than 220 feet.

297. Reflecting Power of Different Substances. Those bodies which reflect a large portion of the incident heat are called good reflectors; those which reflect but little are called bad reflectors.

Fig. 203 shows the method of determining the relative reflecting powers of different bodies, adopted by LESLIE. He placed a cubical tin box, filled with water at the boilingpoint, in front of a parabolic reflector. The rays of heat, falling upon the reflector, are reflected and tend to come to a focus at F, but by interposing a square plate of some sub

stance between the mirror and its focus, the rays are again reflected, and come to a focus as far in front of the plate as F is behind it. The heat thus reflected is received upon one bulb of a differential thermometer, by means of which it is measured. By interposing plates of different substances in succession, their relative reflecting powers are determined.

In this way LESLIE showed that polished brass possessed the highest reflecting power; silver reflects only nine tenths, tin only eight tenths, and glass only one tenth as much as brass. Plates blackened by smoke do not reflect heat at all.

It has been stated that when radiant heat falls upon the surface of a body, some of it is reflected. There is some of it also that is

absorbed by the body, and some transmitted.

A substance that transmits heat is called diathermanous, and one that does not, athermanous.

Rock salt is the most diathermanous of all solids. Radiant heat, both luminous and obscure, will pass through it with about the same facility that light passes through glass. Glass is very transparent, that is, will let light through it readily, but is not specially diather

manous.

Incident rays not transmitted are either reflected or absorbed. It is only the rays absorbed that warm a body.

298. Absorbing Power. In order to determine the relative powers of absorption, LESLIE employed the apparatus shown in Fig. 204.

The source of heat and the reflector remaining as before, he placed the bulb of the differential thermometer in the focus of the reflector, covering it successively with layers of the substance to be experimented upon. In this way he

showed that those substances which reflect most heat absorb least, and the reverse.

When the bulb was blackened by smoke, the thermometer indicated the greatest change of temperature, and when covered with leaves of brass, it indicated the least change.

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299. Radiating Power. The RADIATING POWER of a body is its capacity to emit, or radiate the heat which it contains.

In determining the radiating power, LESLIE employed the apparatus shown in Fig. 204. In this case, instead of covering the bulb of the thermometer with layers of the substances to be experimented upon, he covered the different faces of the cubic box with layers of the different substances.

For example, let one face be made of tin, let a second be blackened by sinoke or lamp-black, let a third be covered by a layer of paper, and a fourth by a plate of glass. On turning these different faces towards the reflector, the thermometer indicates different degrees of temperature. If the blackened face be turned towards the reflector, the thermometer rises, showing that this face is a good radiator; if the paper-covered face be next turned towards the reflector, the thermometer falls, showing that paper is a poorer radiator than lamp-black; if the glass-covered face be turned towards the reflector, the thermometer falls still lower, indicating that glass is a poorer radiator than paper; finally, if the tinned face is turned towards the reflector, the thermometer falls still lower, indicating the fact that tin is a poorer radiator than glass.

LESLIE found, by this course of proceeding, that the radiating powers of bodies are the same as their absorbing powers; that is, a good radiator is also a good absorber but a bad reflector, and the

reverse.

It is commonly supposed that bodies of bright colors radiate heat to a less extent than those of a dull and dark color. This was disproved by Melloni, at least for obscure heat. He found that white lead and lamp-black radiated the same amount of heat.

300. Modifications of the Reflecting Powers of Bodies. The principal causes that modify the reflecting and absorbing powers of bodies are: polish, density, direction of the incident rays, nature of the source of heat, and color.

Other things being equal, polished bodies are better reflectors and worse absorbers than unpolished ones.

Other things being equal, dense bodies are better reflectors and worse absorbers than rare ones.

Other things being equal, the nearer the incident ray approaches the perpendicular, the less will be the portion reflected and the greater the portion absorbed.

The nature of the source of heat sometimes modifies the reflecting and absorbing powers. Thus, if a body is painted with white lead, it absorbs more heat from a cubical box of boiling water, than though the same heat were emitted by a lamp. But if a body is painted with lamp-black, the amount absorbed is the same, whatever may be its source.

Light-colored bodies absorb less and reflect more heat than darkcolored ones. This is found to be true in regard to luminous heat, such as that of the sun. But in the case of obscure heat, color does not seem to affect the absorption.

Whether a body is a good reflector, absorbent, or radiator, or whether it is the reverse, depends more upon the molecular condition of its surface than upon its color.

301. The Radiometer. This consists of a glass tube (Fig. 205) with a bulb blown in it, which rests on a wooden support. A fine steel point is fused on a small tube extend