5. Friction.-Friction is the resistance which one body offers to another when they are rubbed together. This resistance is accompanied with a great development of heat. In many cases, the friction is so great that the rubbing bodies are set on fire. In this way many savage tribes procure fire. Pieces of ice when rubbed together, generate heat enough to melt them. In machinery, the friction on axles often sets them on fire, especially when lubrication has been neglected.

Sources of Cold.

256. The principal sources of cold are: fusion, vaporiz ation, expansion of gases, and radiation of heat.

1. Fusion.-When a body melts, it absorbs heat from the surrounding bodies, which becomes latent in the melted body.

2. Vaporization.-When a liquid passes to a state of vapor, it absorbs heat, which becomes latent in the vapor. Both of these causes of cold have been considered already.

3. Expansion of gases.-When a gas is compressed, it gives out heat, and conversely, when it expands it absorbs heat. This heat, it is, that acts to keep the particles asunder, and the further apart the particles are kept, the greater the amount of heat required.

Heat is the repulsive force that keeps a body in a gaseous state at all, or even in a liquid state.

If air be compressed in a condenser and then allowed to escape into the atmosphere, a slight cloud will be formed; this is due to the cold generated by the expanding air, which condenses the vapor in the air. This experiment illustrates the manner in which clouds are formed in the upper regions of the atmosphere.

Explain friction as a source of heat. (256.) What are the principal sources of cold? Explain fusion as a source of cold? Vaporization. Expansion of gases. Explain the formation of a cloud when compressed air expands.

4. Radiation.-Radiation produces cold, because radiation is nothing else than giving off heat.

The earth, and all bodies on its surface, are continually radiating heat. This is compensated during the day by the heat received from the sun; in fact, the amount received is greater than that given off. But at night the reverse holds true, and a greater amount is radiated than is received. This cooling of the earth's surface is, as has been stated, the cause of dew and frost.

It is often said that it freezes harder when the moon shines than when it is concealed by clouds. This is the case, but the moon has nothing to do with the freezing. The true explanation of the phenomenon is this: When the moon shines, it is generally cloudless, and the radiation goes on more rapidly, and of course a greater degree of cold is produced. On the contrary, when the moon is obscured, it is generally cloudy; now the clouds are good radiators of heat, and the heat that they send back to the earth is nearly or quite enough to compensate for that radiated from the earth; hence the process of freezing is either retarded or entirely prevented.

Plants are good radiators, hence they are more likely to be affected by frost than other objects. To protect them from frost, we cover them with mats, which prevent radiation, or rather radiate back the heat that the plants throw off.

Explain radiation as a cause of cold. Illustrate. What effect has the moon on freezing? Why is it colder when the moon shines than when cloudy? Why are plants likely to be affected by frost? How are they protected?

257. OPTICS is that branch of Physics which treats of the phenomena of light.

Definition of Light.

258. LIGHT is that physical agent which, acting upon the eye, produces the sensation of sight.

Two Theories of Light.

259. Two theories have been advanced to account for the phenomena of light: the Emission Theory, and the Wave Theory.

According to the emission theory, light consists of infinitely small particles of matter, shot forth from luminous bodies with immense velocity, which, falling upon the retina of the eye, produce the sensation of sight. This is the theory of NEWTON and LAPLACE.

According to the wave theory, light consists of waves, or vibrations, transmitted through an impalpable medium

(257.) What is Optics? (258) What is Light? (259.) What two theories of light have been advanced? Explain the emission theory. Explain the wave theory.

of almost perfect elasticity, called ether. This medium pervades all space, and penetrating all bodies, exists in the interspaces between their molecules. Luminous bodies impart a motion of vibration to this ether, which is transmitted through it in the same way that sound is transmitted through the atmosphere, and reaching the eye, it produces the sensation of sight, just as sound falling upon the ear produces the sensation of hearing. This is the theory of HUYGHENS, FRESNEL, YOUNG, MALUS, and many others, and is the one now adopted by almost all physicists.

The vibrations, or waves, which constitute light, are far more rapid and much shorter than those which constitute sound, but the analogy between the two is extremely close. In both the intensity depends upon the excursions of the molecules. In sound, the pitch depends upon the frequency of the waves, whilst in light, colors depend upon the same condition, red corresponding to grave, and violet to acute sounds. This will be further considered hereafter.

Sources of Light.

260. Bodies which emit or give out light, are called luminous bodies. The different sources of light are the sun, the stars, heat, chemical combinations, phosphorescence, and electricity, if indeed the latter are not modifications of a common principle.

We know nothing of the cause of the light emitted by the sun and the stars. We do know, however, that bodies when heated become luminous, and their luminosity becomes greater as their temperature is raised. Thus, coal when heated becomes luminous emiting a glow; the same is the case with the metals.

Artificial light, like that of candles, lamps, gas lights, &c., is due to the combustion of substances containing carbon and hydrogen,

How do light vibrations compare with sound vibrations? What analogies exist between light and sound? (260.) What are luminous bodies? What are some of the sources of light? What do we know of the causes of natural light? Of artificial light?

which, combining with the oxygen of the air, produce a degree of heat so great that the burning bodies become luminous.

PHOSPHORESCENCE is a pale light, given out by certain substances in darkness, without any manifestation of heat. Phosphorescence is observed in animal and vegetable substances, and in some minerals. Certain insects also have the power of emitting a phosphorescent light; thus the surface of the ocean in many parts of the world is covered with an infinity of small animals, which become phosphorescent during the night, especially when disturbed. Certain minerals become phosphorescent when exposed for a long time to the solar rays, such as the diamond, white marble, fluor spar, &c.

The cause of phosphorescence is not known, but in many cases it appears to be the result of electrical excitement.

ELECTRICITY, as we shall see hereafter, is a source of light so intense, that it may be compared with the brilliancy of the sun.

Media.-Opaque and Transparent Bodies.

261. A MEDIUM is anything that transmits light; thus, free space, air, water, and glass, are media.

Media owe their property of transmitting light to the ether which pervades them. This ether exists in the spaces between the particles of all bodies, but not always in such a state as to permit the transmission of light.

A TRANSPARENT BODY is one which permits light to pass through it freely, as glass, diamonds, rock-crystal, and

water.

When bodies permit light to pass through them, but not in such quantity as to allow objects to be seen through them,

What is Phosphorescence? Illustrate. What is its cause? (261.) What is a Medium? Examples. To what is the transmission of light through a medium What is a Transparent Body?

due?