OTHER HABITABLE WORLDS.

NOT many years have elapsed since Whewell, in The Plurality of Worlds,' and Brewster, in 'More Worlds than One,' respectively oppugned and defended the belief that there exist other inhabited worlds besides our own earth; yet so many and such important discoveries have been made in astronomy and physics during the interval, that the question which was at issue between Brewster and Whewell may be said to have assumed in the present day a totally different aspect. The invention of a mode of physical analysis, the powers of which seem absolutely incredible to anyone who is unfamiliar with the laws on which they rest, has enabled the modern physicist to answer some of the very questions respecting which Brewster and Whewell were at issue. It is not a little remarkable to find that, in a controversy in which both disputants were so able, one has been shown to have been wholly in the wrong in nearly every speculative argument adduced in support of his views. It would almost seem as if there were some truth in the view which was put forward during the progress of the controversy, that Dr. Whewell was but half in earnest. We propose to discuss, very briefly, the more important of the

discoveries referred to above, and then to consider the evidence we have respecting the habitability of certain members of the solar system.

One of the arguments on which Whewell laid most stress was founded on our want of knowledge respecting the constitution of the celestial bodies. We know nothing, he reasoned, even respecting the substances of which our own moon is constituted, and this body is but a quarter of a million of miles from us. What, then, can we ever learn respecting the constitution of bodies which are many millions--in some cases, hun

dreds of millions of miles removed from us? For aught we know, not one of the elements which exist on our own earth is present in these distant globes. Nay, he even ventured to express positive opinions respecting the immense difference which he assumed to exist between several of the celestial bodies and our own earth. He held that Jupiter, Saturn, Uranus, and Neptune are but immense clouds,' or water and vapour packed into rotating masses.' The asteroids he held to be mere shreds and specks of planetary matter-watery globes, with perhaps a lump, or a few similar lumps, of planetary matter at their centre.'

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In expressing the opinion that astronomers could never obtain any certain knowledge of the constitution of the celestial bodies, Whewell can hardly be said to have been unduly confident. Even his opponent concurred with him here. Brewster held, indeed, that the physical constitution of some, at least, of the other planets may resemble that of our own earth; but he

was compelled to acknowledge that his views could never be established by positive arguments. He held that they were more probable than Whewell's, and that was all he ventured to say for them.

And indeed, if we consider the subject a little attentively, we cannot but feel that no scientific man could have hoped, with any show of reason, for positive information respecting the constitution of the celestial bodies. One might almost as well have hoped that it would one day become possible to communicate with the inhabitants of these outer worlds. A certain philosopher once said, jestingly, that if there be inhabitants on the moon, we might interchange ideas with them respecting mathematical problems, by means of trees planted in geometrical figures. For instance,' he said, 'we might construct in this way a figure illustrating the famous forty-seventh proposition of Euclid, and wait until the lunarian geometers showed by some corresponding labours their appreciation of our mathematical acquirements.' Ridiculous as this notion may appear, it certainly does not seem more absurd, at a first view, than the expectation that, by any processes man might invent, he could learn the physical constitution of bodies even more distant than the moonthat he should be able, for instance, to assert with the fullest certainty of conviction that enormous quantities of iron and copper exist in the sun's mass.

Yet it is precisely such knowledge as this which has been deduced from the application of the wonderful method of research termed spectroscopic analysis,'

the principles of which are explained in the preceding paper.

The evidence supplied by this powerful analysis affords a very complete and satisfactory reply to Dr. Whewell's argument. The rainbow-coloured streak of light which forms the solar spectrum is crossed by hundreds of fine lines-here separated by well-marked intervals, there clustered together with almost inconceivable closeness. It is evident, therefore, that the solar light reaches us through a very complex atmosphere. And when these lines are compared with the lines of the various terrestrial elements, it is found that many of the most important of these certainly exist in the solar atmosphere. Other terrestrial elements are probably present, but some of their fainter lines are not seen; and the lines of some elements are wanting altogether. We are not, of course, to assume that those elements are wanting whose lines are not seen; because, if any element were present in small quantity its lines would be proportionately faint. We find, indeed, a certain correspondence in this respect between the solar constituents and those of our own earth. Iron is present in large quantities as an element in the earth's composition; and we find the iron lines in the solar spectrum so strongly marked that no doubt whatever can exist respecting the presence of enormous quantities of iron in the solar atmosphere. The same remark applies to sodium, magnesium, calcium, and other elements. But gold and silver, mercury, antimony, arsenic, &c., which are so much less common on

our own earth, have not yet been detected in the solar atmosphere. Copper and zinc, which are moderately common terrestrial elements, are found to exist in the solar spectrum, but probably in less quantities than iron, sodium, magnesium, and calcium, since the fainter lines of the two former elements are not noticeable in the solar spectrum.

We need hardly point out how largely the discovery that terrestrial elements exist in the sun modifies the views we are to form respecting the constitution of the planets. As the planets are opaque, we cannot tell what elements exist in their substance; but when we know that the great centre of our system is formed of the elements which constitute our own earth, we are justified in accepting as highly probable the opinion that all the other planets are similarly constituted.

But this is far from being all. The range of the spectroscope extends beyond the centre of our own system. Unlike the telescope, which can do simply nothing with the fixed stars-revealing them, indeed, with heightened splendour, but affording no indication whatever of their true nature-the spectroscope tells us more about them than we could have hoped to learn even of our nearest neighbour, the moon. We obtain, in fact, precisely the same sort of evidence respecting the stars as we have already had respecting the sun, with this important difference in the evidence itself, that whereas the sun exhibits a close affinity to our own earth as respects the proportions which exist between its elementary constituents, the stars (centres,