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131. Will the Sun, then, keep up for ever a supply of this force? It cannot, if it be not replenished, any more than a fire can be kept in unless we put on fuel; any more than a man can work without food. At present, philosophers are ignorant of any means by which it is replenished. As, probably, there was a time when the Sun existed as matter diffused through infinite space, the coming together of which matter has stored up its heat, so, probably, there will come a time when the Sun, with all its planets welded into its mass, will roll, a cold, black ball, through infinite space.*

132. Such, then, is our Sun the nearest star. Although some of the stars do not contain those elements which on the earth are most abundant-a Orionis and ẞ Pegasi, for instance, are worlds without hydrogen-still we see that, on the whole, the stars differ from each other, and from our Sun, only by the lower order of differences of special modification, and not by the more important differences of distinct plans of structure. There is, therefore, a probability that they fulfil an analogous purpose; and are, like our Sun, surrounded with planets, which by their attraction they uphold, and by their radiation illuminate and energize. As has been previously pointed out, the elements most widely diffused through the host of stars are some of those most closely connected with the constitution of the living organisms of our globe, including hydrogen, sodium, magnesium, and iron.

The probable past and future of the Sun are, therefore, the probable past and future of every star in the firmament of heaven.

* Sir W. Thomson.

CHAPTER III.

THE SOLAR SYSTEM.

LESSON X.-GENERAL DESCRIPTION.

DISTANCES OF

THE PLANETS FROM THE SUN. SIZES OF THE
PLANETS. THE SATELLITES. VOLUME, MASS, AND
DENSITY OF THE PLANETS.

133. From the Sun we now pass to the system of bodies which revolve round it; and here, as elsewhere in the heavens, we come upon the greatest variety. We find planets—of which the Earth is one-differing greatly in size, and situated at various distances from the Sun. We find again a ring of little planets clustering in one part of the system; these are called asteroids, or minor planets : and we already know of at least two masses or rings of smaller planets still, some of them so small that they weigh but a few grains: these give rise to the appearances called meteors, bolides, or shooting-stars. We find also comets, some of which break in, as it were, upon us from all parts of space; and then, passing round our Sun, rush back again : we find others so little erratic that they may be looked upon as members of the solar household. Besides these there is another ring which is rendered visible to us by the appearance called the Zodiacal Light.

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