If radiant heat passes from one medium to another, however, the rays of heat are bent from a rectilinear course. This bending is called refraction, and is entirely analogous to the refraction of light, which is yet to be explained.

3. The intensity of radiant heat varies directly as the temperature of the radiating body, and inversely as the square of the distance to which it is transmitted.

The first part of this law is verified by exposing one of the bulbs of a differential thermometer to a blackened cubical box, filled with hot water, the other bulb being protected by a screen. If the water is in the first instance of a given temperature, and then falls to a half, or a third of that temperature, the differential thermometer will manifest a half, or a third of its original indication, and so on for any temperature.

The second part of the law may also be verified by means of the differential thermometer. In this case the heated body is kept always at the same temperature, and one bulb of the differential thermometer is placed at different distances from it. It will be found that at a double distance the indication is only a fourth of the original indication, at a triple distance only a ninth, and so on.

These laws are rigorously true in a vacuum; in the air they may be approximatively verified, but not absolutely, on account of the action of the atmosphere upon radiant heat, as will be explained hereafter.

Mutual Exchange of Heat between bodies.

195. The process of radiation of heat between bodies is mutual and continuous. According to the laws given in the preceding article, those bodies which are most heated give off most heat; hence, the hottest bodies of a group give off more heat than they receive, and the coldest ones receive more than they give off. The consequence is that there is a continual tendency towards equalization of tem

What is the third law? How is the first part of the law verified? The second part? Are these laws rigorously true in the air? (195.) Explain the action of radiation to produce uniformity of temperature.

perature. If all the bodies are of the same temperature, each will give off as much as it receives, and no further change of temperature can occur. The process of radiation, however, goes on as before.

All the bodies in a room, for example, tend to come to a uniform temperature. We say, tend to come to a uniform temperature, because this condition is never fully realized. Bodies nearest the walls are continually exchanging heat with the walls, and as these are in communication, either with the outer air, or with other rooms, their temperature will be influenced thereby, and will in turn exert an influence upon the remaining bodies in the room.

V.-REFLECTION, ABSORPTION, EMISSION, AND CONDUCTIBILITY.

Reflection of Heat.

196. When a ray of heat falls upon the surface of a body, it is divided into two parts, one of which enters the body and is absorbed, whilst the other is deflected or bent from its course. This bending is called reflection.

The point at which the bending takes place, is called the point of incidence. The ray before incidence is called the incident ray; after incidence it is called the reflected ray. If a perpendicular be drawn to the surface at the point of incidence, it is called a normal to the surface at that point. The plane of the incident ray, and the normal at the point of incidence, is called the plane of incidence. The plane of the normal and the reflected ray is called the plane of reflection. The angles which the incident and reflected rays make with the normal, are called, respectively, angles of incidence and reflection.

Mustrate by the example of articles in a room? (196) What is reflection of heat? What is the point of incidence? The incident ray? The reflected ray? The plane of incidence? The plane of reflection? The angles of incidence and refleotion?

Laws which govern the Reflection of Heat.

197. The following laws, indicated by theory, have been confirmed by experiment:

1. The planes of incidence and reflection coincide.

2. The angles of incidence and reflection are equal.

The apparatus, employed in establishing these laws, is shown in Fig. 137. A is a tin box with its faces blackened, in which hot water is placed. B is a reflecting surface, and D is a differential thermometer. BC is a normal to the reflecting surface.

The surface, A, radiates heat in all directions, but only a single ray is permitted to fall upon the reflector, B, the remainder being intercepted by a screen, having a small hole in it. By suitably arranging the thermometer, and other parts of the apparatus, it is shown, whatever may be the value of the angle of incidence, that the planes, ABC and CBD, coincide with each other, and that the angles, ABC and CBD, are equal to each other.

(197.) What is the first law of reflection? The second law? Explain the apparatus for verifying these laws. Explain the mode of verification.

Reflection of Heat from Concave Mirrors.

198. A CONCAVE MIRROR is a reflecting surface, curved towards the source of heat. For experimental purposes they are generally parabolical in shape, the axis being a normal to the surface at its middle 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 radiate from the focus they will be reflected in lines parallel to the axis.

A and B, Fig. 138, represent two parabolic reflectors, having their axes coincident, and their surface 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

(198.) What is a Concave Mirror? are rays parallel to the axis reflected?

What is their shape for experiment? How What is the focus?

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.

The property of parabolic mirrors, above explained, enables us to concentrate the heat of the sun's rays. In this case the reflector is called a burning mirror. Fig. 139 shows the manner of using a burning mirror. It is placed so that its axis is parallel to the rays of the sun, which, on falling 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