same ray only, all nature would display the monotony of a single color, and our senses would never have known the charms of that variety which we now behold.

All bodies appear of the color of that ray, or of a tint depending on the several rays which it reflects, while all the other rays are absorbed, or, in other terms, are not reflected. Black and white, therefore, in a philosophical sense, cannot be considered as colors, since the first arise from the absorption of all the rays, and the reflection of none, and the last is produced by the reflection of all the rays, and the absorption of none. But in all colors, or shades of color, the rays only are reflected, of which the color is composed. Thus the color of grass, and the leaves of plants is green, because the surfaces of these substances reflect only the green rays, and ab. sorb all the others. For the same reason the rose is red, the violet blue, and so of all colored substances, every one throw. ing out the ray of its own color, and absorbing all the others.

To account for such a variety of colors as we see in differ. ent bodies, it is supposed, that all substances, when made suf. ficiently thin, are transparent, and consequently, that they transmit through their surfaces, or absorb, certain rays of light, while other rays are thrown back, or reflected, as above described. Gold, for example, may be beat so thin as to transmit some of the rays of light, and the same is true of several of the other metals, which are capable of being ham. mered into thin leaves. It is therefore, most probable, that all the metals, could they be made sufficiently thin, would permit the rays of light to pass through them. Most, if not quite, all the mineral substances, though in the mass they may seem quite opaque, admit the light through their edges, when broken, and every kind of wood, when made no thinner than writing paper, becomes translucent. Thus we may safe ly conclude, that every substance with which we are acquainted, will admit the rays of light, when made sufficiently thin.

Transparent, colorless substances, whether solid, or fluid, such as glass, water, or mica, reflect, and transmit light of the same color; that is, the light seen through these bodies, and reflected from their surfaces, is white. This is true of all

Why are not black and white, considered as colors? Why is the color of grass green? How is the variety of colors accounted for, by considering all bodies transparent? What is said of the reflection of calored light by transparent substances?

transparent substances under ordinary circumstances; but if their thickness be diminished to a certain extent, these substances will both reflect, and transmit colored light of various hues, according to their thickness. Thus the thin plates of mica, which are left on the fingers, after handling that substance, will reflect prismatic rays of various colors.

There is a degree of tenuity, at which transparent substan. ces cease to reflect any of the colored rays, but absorb, or transmit them all, in which case, they become black. This may be proved by various experiments. If a soap bubble be closely observed, it will be seen, that at first, the thickness is sufficient to reflect the prismatic rays from all its parts, but as it grows thinner, and just before it bursts, there may be seen a spot on its top, which turns black, thus transmitting all the rays at that part, and reflecting none. The same phenome. non is exhibited, when a film of air, or water, is pressed between two plates of glass. At the point of contact, or where the two plates press each other with the greatest force, there will be a black spot, while around this, there may be seen a system of colored rings.

From such experiments, Sir Isaac Newton concluded, that air, when below the thickness of half a millionth of an inch, ceases to reflect light; and also that water, when below the thickness of three eighths of a millionth of an inch, ceases to reflect light. But that both air and water, when their thick. ness is in a certain degree above these limits, reflect all the colored rays of the spectrum.

Now all solid bodies are more or less porous, having among their particles either void spaces, or spaces filled with some foreign matter, differing in density from the body itself, such as air, or water. Even gold is not perfectly compact, since water can be forced through its pores. It is most probable, then, that the parts of the same body, differing in density, either reflect, or transmit the rays of light according to the size, or arrangement of their particles; and in proof of this, it is found that some bodies transmit the rays of one color, and reflect those of another. Thus the color, which passes through a leaf of gold is green, while that which it reflects is yellow.

From a great variety of experiments on this subject, Sir

What substance is mentioned, as illustrating this fact? When is it said that transparent substances become black? How is it proved that fluids of extreme tenuity, absorb all the rays and reflect none ?

Isaac Newton concludes that the transparent parts of bodies, according to the sizes of their transparent pores, reflect rays of one color, and transmit those of another, for the same rea. son that thin plates, or minute particles of air, water, and some other substances, reflect certain rays, and absorb, or transmit others, and that this is the cause of all their colors.

In confirmation of the truth of this theory, it may be observ. ed, that many substances, otherwise opaque, become transparent, by filling their pores with some transparent fluid.

Thus the stone called Hydrophane, is perfectly opaque, when dry, but becomes transparent when dipped in water; and common writing paper becomes translucent, after it has absorbed a quantity of oil. The transparency, in these cases, may be accounted for, by the different refractive powers which the water and oil possess, from the stone, or paper, and in consequence of which the light is enabled to pass among their particles by refraction.

ASTRONOMY.

Astronomy is that science which treats of the motions and appearances of the heavenly bodies; accounts for the phenomena which these bodies exhibit to us, and explains the laws by which their motions, or apparent motions, are regulated. Astronomy is divided into Descriptive, Physical, and Prac

tical.

Descriptive astronomy demonstrates the magnitudes, distances, and densities of the heavenly bodies, and explains the phenomena dependent on their motions, such as the change of seasons, and the vicissitudes of day and night.

Physical astronomy explains the theory of planetary motion, and the laws by which this motion is regulated and sus tained.

Practical astronomy details the description and use of astronomical instruments, and developes the nature and application of astronomical calculations.

The heavenly bodies are divided into three distinct classes,

What is the conclusion of Sir Isaac Newton, concerning the tenuity at which water and air cease to reflect light? What is said of the porous nature of solid bodies? What is astronomy? How is astronomy divided? What does descriptive astronomy teach? What is the object of physical astronomy? What is practical astronomy?

or systems, namely, the solar system, consisting of the sun, moon, and planets, the system of the fixed stars, and the system of the comets.

The Solar System.

The Solar system consists of the sun, and twenty-nine other bodies, which revolve around him at various distances, and in various periods of time.

The bodies which revolve around the sun as a centre, are called primary planets. Thus the Earth, Venus and Mars, are primary planets. Those which revolve around the primary planets, are called secondary planets, moons, or satellites. Our moon is a secondary planet or satellite.

The primary planets revolve around the sun in the following order, and complete their revolutions in the following times, computed in our days and years. Beginning with that nearest the sun, Mercury performs his revolution in 87 days and 23 hours; Venus, in 224 days, 17 hours; the Earth, attended by the moon, in 365 days, 6 hours; Mars, in 1 year, 322 days; Ceres, in 4 years, 7 months, and 10 days; Pallas, in 4 years, 7 months, and 10 days; Juno, in 4 years and 128 days; Vesta, in 3 years, 66 days, and 4 hours; Jupiter, in 11 years, 315 days, and 15 hours; Saturn, in 29 years, 161 days, and 19 hours; Herschel, in 83 years, 342 days, and 4 hours.

is

A year consists of the time which it takes a planet to perform one complete revolution through its orbit, or to pass once around the sun. Our earth performs this revolution in 365 days, and therefore this is the period of our year. Mercury completes her revolution in 88 days, and therefore her year no longer than 88 of our days. But the planet Herschel is situated at such a distance from the sun, that his revolution is not completed in less than about 84 of our years. The other planets complete their revolutions in various periods of time, between these; so that the time of these periods, is generally in proportion to the distance of each planet from the sun.

Ceres, Pallas, Juno, and Vesta, are the smallest of all the planets, and are called Asteroids.

How are the heavenly bodies divided? Of what does the solar system consist? What are the bodies called, which revolve around the sun as a centre? What are those called, which revolve around these primaries as a centre? In what order are the several planets situated, in respect to the sun? How long does it take each planet to make its revolution around the sun? What is a year? What planets are called asteroids?

Besides the above enumerated primary planets, our system contains eighteen secondary planets, or moons. Of these, our Earth has one moon, Jupiter four, Saturn seven, and Herschel six. None of these moons, except our own, and one or two of Saturns, can be seen without a telescope. The seven other planets, so far as has been discovered, are entirely without moons.

All the planets move around the sun, from east to west, and in the same direction do the moons revolve around their primaries, with the exception of those of Herschel, which appear to revolve in a contrary direction.

The paths in which the planets move round the sun, and in which the moons move round their primaries, are called their orbits. These orbits are not exactly circular, as they are commonly represented on paper, but are elliptical, or oval, so that all the planets are nearer the sun, when in one part of their orbits, than when in another.

In addition to their annual revolutions, some of the planets are known to have diurnal, or daily revolutions, like our earth. The periods of these daily revolutions have been ascertained in several of the planets, by spots on their surfaces. But where no such mark is discernible, it cannot be ascertained whether the planet has a daily revolution or not, though this has been found to be the case, in every instance where spots are seen, and therefore there is little doubt but all have a daily, as well as a yearly motion.

The axis of a planet is an imaginary line passing through its centre, and about which its diurnal revolution is performed. The poles of the planets, are the extremities of this axis.

The orbits of Mercury and Venus are within that of the earth, and consequently they are called inferior planets. The orbits of all the other planets are without, or exterior to that of the earth, and these are called superior planets.

That the orbits of Mercury and Venus, are within that of the earth, is evident from the circumstance, that they are never seen in opposition to the sun, that is, they never appear

How many moons does our system contain? Which of the planets are attended by moons, and how many has each? In what direction do the planets move around the sun? What is the orbit of a planet? What revolutions have the planets, besides their yearly revolutions? Have all the planets diurnal revolutions? How is it known that the planets have daily revolutions? What is the axis of a planet? What is the pole of a planet? Which are the superior, and which the inferior planets?