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while it is very easy to convert mechanical energy wholly into heat, it is impossible to convert heat wholly back into mechanical energy.

The connexion between heat and the various other forms of molecular energy will be considered when these energies are described.

CHAPTER VI.

RADIANT ENERGY.

LESSON XXVII. PRELIMINARY.

250. WHEN a substance is heated, it gives out part of its heat to a medium which surrounds it (Art. 106). This heatenergy is propagated as undulations in the medium, and proceeds outwards with the enormous velocity of 186,000 miles in a second. If the temperature of the hot substance be not very great these undulations do not affect the eye, but are invisible, forming rays of dark heat, such, for instance, as are given out by boiling water; but as the temperature rises we begin to see a few red rays, and we say that the body is red-hot. As the temperature still continues to rise the body passes to a yellow and then to a white heat, until it ultimately glows with a splendour like the sun.

It thus appears that we have two kinds of rays, those which do not affect the eye, or rays of dark heat, and those which the eye perceives, or rays of light.

Let us first proceed to those rays which affect the eye. 251. The term optics is given to that branch of physics which treats of luminous rays.

The old idea regarding light was that it consisted of very minute particles given out by a luminous body, but of late men of science have come unanimously to the conclusion that it consists of waves which traverse a medium pervading space. There is thus no addition to the weight of a body which receives light, and no diminution in the weight of one

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which gives it out, nor is any blow given to a delicatelysuspended substance upon which the light of the sun is made to fall.

But while there are many properties of light which can hardly be explained except on the supposition that it consists of waves, there are others which can be studied indifferently under either hypothesis.

252. Suppose now that we have a very small luminous body, in fact a mere shining point, and that light is radiated on all sides by this small body, which forms, as it were, a centre from which waves of light proceed in all directions. If the eye look towards this point, a cone of rays strikes the eye. If the bulb of a thermometer be held near it, a cone or bundle or pencil of rays strikes the bulb.

Now just as in a sphere we can geometrically conceive of a single radius, so in the case of the radiant point we can conceive of a single ray of light, and we may imagine a vast number of such rays to form a luminous pencil.

If the luminous point be near the eye, the pencil of rays which strikes the eye is divergent. On the other hand, the light which strikes the eye from a star or body at a very great distance may be regarded as a pencil of parallel rays. We may likewise have a pencil whose section lessens as it proceeds, in which case it is called a convergent pencil.

253. Substances are divided into two distinct classes with reference to light there are those which are opaque and those which are transparent. The former stop a ray of light, while the latter allow it to pass ; nevertheless no substance is perfectly opaque or perfectly transparent, but a very thin slice of the most opaque substance will allow a little light to pass through it, while a very thick stratum of the most transparent substance will stop some light.

As long as a ray of light moves through the same medium it moves in straight lines, but on passing from one medium to another, one part of the light is reflected, or thrown back, and another part enters the medium, but in a different direction from that which it previously pursued ; this bending of the ray is termed refraction.

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254. Velocity of Light.-Römer, a Danish astronomer, in 1675 was the first to determine the velocity of light from the eclipses of Jupiter's satellites. It so happens that at equal intervals of 42h. 28m. 36s. the first of Jupiter's satellites passes within his shadow, and is thus obscured. Now if light travelled instantaneously from Jupiter to the Earth, we should see this phenomenon precisely at the moment when it took place. But Römer found that when the Earth was farthest away from Jupiter there was a retardation in the time of the occurrence equal to 16m. 36s.

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Now it will at once be seen (Fig. 68) that the Earth is nearest Jupiter when both are in one line with the sun and on the same side of him, and that the Earth is farthest from Jupiter when both planets are in one line with the sun but on different sides of him, and that the difference of distance in these two cases is the diameter of the Earth's orbit. Hence Römer argued that a ray of light takes 16m. 36s. to cross the diameter of the Earth's orbit. From this it may be calcuis about 186,000 miles per

lated that the velocity of light
second.

ment.

The velocity of light has also been determined by experiFizeau's apparatus for this purpose will be most easily understood. Suppose we have a toothed wheel, and that a ray of light is made to pass through the interval between two teeth. It is then allowed to proceed to a mirror some distance off, from which it is reflected back precisely in the direction in which it came, so as to return through the same interval between the two teeth of the wheel from which

it originally proceeded to the mirror. If, however, the toothed wheel is made to revolve with great velocity, the ray of light, when it comes back from the mirror, may find itself stopped by the next tooth, and may not be able to pass.

Whether this will happen or not will depend upon the time the ray of light takes to go from the toothed wheel to the mirror and back again, and upon the velocity with which the wheel is made to revolve.

M. Fizeau so performed the experiment as to stop the ray of light on its return, and knowing at the same time the rate of revolution of the toothed wheel he was able to estimate the time the ray took to go from the wheel to the mirror and back again, and he thus determined the velocity of light.

255. Intensity of Light.-If the light from a luminous body fall upon a surface, the quantity of light which the surface receives will vary inversely as the square of its distance from the source

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To prove this, let s (Fig. 69) denote the luminous source, and let P denote a circular plate upon which the light falls, the distance Sc being unity. Also let P' denote a plate similarly placed, but twice as far from the source, the distance s c' being equal to 2. Now it is evident from the figure that the same amount of light which would fall on P will, if allowed to pass, spread itself out so as to fall on P', so that both circles receive the same amount of light from the source. But the figures being similar, P' is four times as large as P, and hence the light which falls on a portion of P' equal in

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