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of adhesion, until it has reached the greatest contraction. When the temperature rises, the alcohol expands, and passing between the sides of the tube and the index does not displace B. The position of the index gives therefore the lowest temperature which has been reached in the figure this was 9 degrees below zero.

195. Pyrometers.—The name pyrometers is given to instruments for measuring temperatures so high that mercurial thermometers could not be used. The older contrivances for this purpose, Wedgwood's, Daniell's (which in principle resembled the apparatus in fig. 156), Brongniart's, etc., are gone entirely out of use. None of them gives an exact measure of temperature.

CHAPTER II.

RADIATION OF HEAT.

196. Radiant heat.—If we stand in front of a fire, or exposed to the sun's heat, we experience a sensation of warmth which is not due to the temperature of the air, for if a screen be interposed the sensation immediately disappears, which would not be the case if the surrounding air had a high temperature. Hence bodies can send out rays which excite heat, and which penetrate through the air without heating it, as rays of light through transparent bodies. Heat thus propagated is said to be radiated; and we shall use the terms ray of heat, or thermal, or calorific ray, in a similar sense to that in which we use the term ray of light, or luminous ray.

We shall find that the property of radiating heat is not confined to incandescent substances, such as a fire, or a lamp, or a red-hot ball, but that bodies of all temperatures radiate heat. Thus a bottle full of hot water and a bottle full of cold water both emit heat ; the first emits more as compared with the second, the greater the difference of temperature between the two.

197. Laws of radiation.—The radiation of heat is governed by three laws.

I. Radiation takes place in all directions round a body. If a thermometer be placed in different positions round a heated body, it indicates everywhere a rise in temperature; at equal distances from the source of heat it indicates the same rise of tempe

rature.

II. Heat is propagated in a right line. For, if a screen be placed

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Radiant Heat.

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in the right line which joins the source of heat and the thermometer, so as to stop the rays, the latter is not affected.

But in passing obliquely from one medium into another, as from air into a glass, calorific like luminous rays become deviated, an effect known as refraction. The laws of this phenomenon are the same for heat as for light, and they will be more fully discussed under the latter subject.

III. Radiant heat is propagated in vacuo as well as in air. This is demonstrated by the following experiment.

t

Fig. 169.

This having On immersing

In the bottom of a glass flask a thermometer is fixed in such a manner that its bulb occupies the centre of the flask (fig. 169). The neck of the flask is then carefully narrowed by means of the blowpipe, and then the apparatus having been suitably attached to an air-pump a vacuum is produced in the interior. been done, the tube is sealed at the narrow part. this apparatus in hot water, or on bringing near it some hot charcoal, the thermometer is at once seen to rise. This could only be due to radiation through the vacuum in the interior, for glass is so bad a conductor, that the heat could not travel with this rapidity through the sides of the flask, and the stem of the thermometer.

198. Causes which modify the intensity of radiant heat.The intensity of radiant heat transmitted to us by heated bodies depends on the temperature of the source of heat, and on its distance. The corresponding laws may be thus stated :

I. The intensity of radiant heat is proportional to the temperature of the source.

II. The intensity of radiant heat is inversely as the square of the distance.

The first law is demonstrated by placing a metallic box containing water at 10°, 20°, or 30°, successively, at equal distances from the bulb of a differential thermometer. The temperatures indicated by the latter are then found to be in the same ratio as those of the box for instance, if the temperature of that corresponding to the box at 10° be 2°, those of the others will be 4° and 6° respectively.

The second law is demonstrated experimentally by placing the differential thermometer at a certain distance from the source of heat, a yard for instance, and then removing it to double the distance. In the latter case, the amount of heat received is not

one-half but one-quarter. If the distance be three yards the quantity of heat is one-ninth, and so forth.

199. Interchange of heat among all bodies.-Owing to the radiation which is continually taking place in all directions round a body, there is a continual interchange of heat. If the bodies are all at the same temperature, each one sends to the surrounding ones a quantity equal to that which it receives, and their temperatures remain stationary. But if their temperatures are unequal, as the hot bodies emit more heat than they receive, they therefore sink in temperature; while, as the bodies of lower temperatures receive more heat than they emit, their temperature rises; thus the temperatures are all ultimately equal. The radiation does not stop; it goes on, but without loss or gain from each body, and this condition is accordingly known as the mobile equilibrium of tempe

rature.

From what has been said it will be understood, that bodies, placed in our rooms, all tend to assume a uniform temperature; generally speaking this is not the case, for many causes concur in cooling one set, and in heating the others. Thus bodies, placed near a wall, cooled by the exterior atmosphere, find a cause for cooling. Those, on the contrary, which are at the top, tend to acquire a higher temperature; for, as the air is always tending to rise as being less dense, the layers nearest the ceiling are always hotter than the lower ones.

From this continual interchange of heat, there is necessarily a limit to the cooling of bodies, for they always tend to resume, on the one hand, what they lose on the other. To have an indefinite cooling, a body should be suspended in space, not receiving heat from any body. As it then loses heat without acquiring any, there is no telling to what extent its temperature would sink.

CHAPTER III.

REFLECTION OF HEAT. REFLECTING, ABSORBING, AND
EMISSIVE POWERS.

200. Law of the reflection of heat.--When the heat rays emitted by a source of heat fall upon the surface of a body, they are divided generally into two parts; one, which passes into the mass of a body and raises the temperature; the other, which darts

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Reflection of Heat.

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off from the surface like an elastic ball striking against a hard body; this is expressed by saying that these rays are reflected. Thus let A be a source of heat, a cubical box filled with hot water (fig. 170), and near it a screen which does not allow heat to pass, but near the bottom of which is an aperture. If behind this screen a polished surface be placed on which the rays emitted by the cube impinge, and beyond this again a differential thermometer, the latter indicates an increase of temperature when one of its bulbs is so placed that it receives the rays reflected by the polished body. In this experiment, rays like AB which fall on the reflecting surface are called

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ncident rays, from a Latin word which signifies to fall; and the angle of incidence is not the angle which they make with the reflecting surface, but the angle, ABC, which they make with a right line, BC, perpendicular to this surface. In like manner the angle, CBD, which the reflected rays make with the same straight line, is called the angle of reflection.

The reflection of heat is always subject to the law, that the angle of reflection is equal to the angle of incidence. We shall subsequently see that the reflection of light is governed by the same law.

201. Reflection of heat from concave mirrors.-The effects of the reflection of heat may be very powerful when it takes place from the surface of concave mirrors, which are spherical surfaces of glass or of metal. These mirrors may be regarded as being made up of an infinite number of extremely small planes inclined towards each other in such a manner as to determine the curvature. From the symmetrical grouping of these small facets, it follows that when a group of rays fall upon a concave mirror, these rays, in obedience

to the laws of reflection coincide in a single point, to which the name focus is applied, to express the great quantity of heat which is concentrated there.

In treating of light we shall discuss in detail the properties of the focus in concave mirrors; for the present it will be sufficient to describe experiments which demonstrate the great intensity which radiant heat may acquire when concentrated in these points. Fig. 171 represents an experiment which is frequently made in physical lectures. Two reflectors, A and B (fig. 171), are arranged at a distance of 4 to 5 yards, and so that their axes coincide. In the focus of one of thein, A, is placed a small basket, n, containing a red-hot

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iron ball. In the focus of the other, B, is placed an imflammable body, such as gun-cotton or phosphorus. The rays emitted from the focus, n, are first reflected from the mirror, A, in a direction parallel to the axis; and impinging on the other mirror, B, are reflected so that they coincide in the focus m. That this is so, is proved by the fact that the inflammable substance placed in this point takes fire, which is not the case if it is above or below it.

The same effect may be produced by the sun's rays. For this purpose a concave reflector is so placed that the sun's rays strike directly against it (fig. 172), and if then a combustible substance, such as paper, wood, cork, etc., be held by means of a pincette in the focus, these bodies are seen to take fire. The effect produced dépends on

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