and to Kirchhoff and Bunsen is due the credit of applying this method of research to astronomical science. They arranged their apparatus so that one-half was lighted by the Sun, the other by the incandescent gas they were examining. When the vapour of sodium was treated in this way they found that the bright line in the flame of soda exactly coincided with a line in the Sun's spectrum. The conclusion was obvious; there is sodium in the Sun. It must, indeed, have been a glorious moment when the thought flashed upon them; and the discovery, with its results, is one of the greatest triumphs of human genius. One set of lines could not be accounted for by any known substance, and were therefore referred to a new element, which was named "Helium," after the Greek name for the Sun. This element has since been discovered in a Norwegian mineral, but its existence was determined in the Sun before its actual discovery on our earth-a remarkable testimony to the truth of the result derived from spectrum analysis.

The Sun has thus been proved to contain hydrogen, sodium, barium, magnesium, calcium, aluminium, chromium, iron, nickel, man

ganese, titanium, cobalt, lead, zinc, copper, cadmium, strontium, cerium, uranium, potassium, etc., in all 36 of our terrestrial elements, while as regards some others the evidence is not conclusive. We cannot as yet say that any of our elements are absent, nor, though there are various lines which cannot as yet be certainly referred to any known substance, have we clear proof that the Sun contains any element which does not exist on our earth. On the whole, then, the chemical composition of the Sun appears closely to resemble that of our earth.

THE PLANETS

The Syrian shepherds watching their flocks by night long ago noticed-and they were probably not the first-that there were five stars which did not follow the regular course of the rest, but, apparently at least, moved about irregularly. These were appropriately named Planets, or wanderers.

Further observations have shown that this irregularity of their paths is only apparent, and that, like our own Earth, they really revolve round the Sun. To the five first

observed - Mercury, Venus, Mars, Jupiter, and Saturn-two large ones, Uranus and Neptune, and a group of minor bodies, have since been added.

The following two diagrams give the relative orbits of the Planets.

MERCURY

It is possible, perhaps probable, that there may be an inner Planet, but, so far as we

Fig. 52.--Relative distances of the Planets from the Sun.

know for certain, Mercury is the one nearest to the Sun, its average distance being 36,000,000 miles. It is much smaller than the Earth, its weight being only about th

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of ours. Mercury is a shy though beautiful object, for being so near the Sun it is not easily visible; it may, however, generally be seen at some time or other during the year as a morning or evening star.

VENUS

The true morning or evening star, however, is Venus-the peerless and capricious Venus.

Venus, perhaps, "has not been noticed, not been thought of, for many months. It is a beautifully clear evening; the sun has just set. The lover of nature turns to admire the sunset, as every lover of nature will. In the golden glory of the west a beauteous gem is seen to glisten; it is the evening star, the planet Venus. A week or two later another beautiful sunset is seen, and now the planet is no longer a glistening point low down; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. Again a little longer and Venus has gained its full brilliancy and splendour. All the heavenly host-even