doubtless, of other systems) exhibit no such affinity. It may seem rashly speculative to found a theory on this evidence alone; but we cannot but regard it as a legitimate deduction that, in all probability, all the members of a planetary system circulating around any star are similarly constituted, and that the nature of their common constitution is exhibited by the spectroscopic analysis of their central sun.

But there is evidence of yet another kind to show that the elements we have been in the habit of speaking of as 'terrestrial' exist in other parts of the solar system. Although meteoric stones, or aërolites, have fallen on the earth at intervals during many hundreds of years, it is but recently that the scientific world has accepted as indubitable the fact that these stones are really visitants from the interplanetary spaces. Now that this fact is recognised, the chemical analysis of aërolites becomes the chemical analysis of portions of the solar system. • There is an interest attached to aërolites,' says Humboldt, wholly different from that connected with any other objects of astronomical or physical research, inasmuch as by means of them we are brought into contact, so to speak, with external space, and are permitted to weigh, to handle, and to analyse masses not belonging to our terrestrial formations.' The analysis of aërolites exhibits to us the same fact which has been revealed by the spectroscopic analysis of the sun. We find that the very elements which are most common on our own earth occur most commonly also as components of meteoric stones. But,

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remembering that the stones which reach the earth are few in number compared with those which are wholly dissipated in the upper regions of air, the inquiry is suggested whether we cannot learn anything respecting the structure of these objects also. They are luminous through intensity of heat, and therefore they are suitable objects for spectroscopic analysis. But the difficulty is to view them with a spectroscope during their hasty swoop across the sky. Patient observers have, however, overcome this difficulty; and although it is impossible to obtain a well-defined spectrum from the light of a shooting-star, yet it has been found that certain elements which happen to have well-marked lines, and notably sodium—which, it will be remembered, is one of the elements most plentifully distributed throughout the solar atmosphere-exist in the masses of these wandering and minute members of the great planetary family.

Another argument on which Dr. Whewell laid great stress was founded on the doubt whether any planet has an atmosphere resembling that of our own earth. Astronomers had been led to suspect that most of the planets-if not all of them—are surrounded with atmospheric envelopes of some sort; but there was no certainty on this point, and far less respecting the constitution of the planetary atmospheres. Here was another negative argument, which it seemed wholly impossible that men should ever be able to oppugn satisfactorily. Yet here again the spectroscope has afforded the clearest evidence. We have said that, the planets

being opaque, it is impossible to learn in what manner they are constituted. But we can learn-or, at least, there is a possibility of our learning-whether the light reflected from a planet's surface has passed through an atmospheric envelope; for, if the planet's spectrum is crossed by dark lines not existing in the solar spectrum, these lines must be caused by vapours existing either in the earth's atmosphere or in the atmosphere of the planet; nor is it so difficult as, at first sight, might be supposed to determine in which of the two atmospheres those vapours exist. In observing the planet Mars, Dr. Huggins noticed that the spectrum was crossed by a number of lines which appear in the solar spectrum when the sun is low down, that is, when his light passes through the denser strata of our atmosphere. Now, although Mars was not so low down as to suggest the probability that the lines were caused by the earth's atmosphere, yet it was not wholly impossible that they might have been, because the constitution of the atmosphere, as respects the amount of aqueous vapour present in it, &c., is not absolutely constant. Therefore it did not become certain that the vapours indicated by these lines exist in the atmosphere of Mars until the following crucial test had been applied :-The spectroscope was directed towards the moon, then lower down than Mars; so that if the vapours were due to the earth's atmosphere their lines must have been more strongly shown in the moon's spectrum than in that of Mars. But they were not seen in the moon's spectrum. Thus it was proved

that there is a Martial atmosphere, and that it is loaded with the very vapours that are found in the earth's atmosphere."

It has been shown that the same vapours exist, also, in the atmosphere of Venus, Jupiter, and Saturn; but their lines are not quite so distinctly seen as in the spectrum of Mars-for this reason, probably, that the light received from the former planets is not reflected from their true surface, but from vaporous masses floating above the denser atmospheric strata. Thus the light has traversed a smaller quantity of these characteristic vapours, and their lines are proportionately indistinct.

Sir David Brewster laid great stress on the analogy between the planet Mars and our own earth. He pointed to the continents and oceans of the ruddy planet; to its snow-crowned poles; to the clouds which float in its atmosphere; and to numerous other analogies which mark it as well fitted to be the abode of creatures resembling those which exist on our own earth. Dr. Whewell was not ready to admit that all these analogies really exist. He argued that what we call continents and oceans may not be so; and that it is assuming too much to say that the white specks of light which cap the Martial poles are certainly masses of snow and ice. On these points recent discoveries do not speak quite so positively as on the others. But this has been done: it has been shown that the socalled lands and seas are permanent features. They have even been charted and named, and a globe of

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Mars has been constructed. It has been shown that the red colour of the continents' is not due to the Martial atmosphere. The waxing and waning of the polar snow-caps have been more carefully watched than before, and found to correspond closely with the progress of the Martial seasons. Then, as we have seen, the existence of aqueous vapour in the Martial atmosphere has been established, so that we cannot doubt that water exists on Mars in large quantities. And lastly, clouds, covering extensive regions, have been observed to melt away with the progress of the Martial day, exactly as the morning mists are dissipated by the heat of one of our summer days. The words applied by Brewster to long past ages of the earth's history will at once suggest themselves as applicable to the planet Mars. If this orb be indeed uninhabited, then it exhibits to us physical relations fulfilling no purpose that human reason can conceive; lamps lighting nothing; waters quenching nothing; clouds screening nothing; breezes fanning nothing; and everything around, mountain and valley, hill and dale, earth and ocean, all meaning nothing.'

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But perhaps the most important of all Wheweil's mistakes was his assumption that the climate of each planet must necessarily correspond with the planet's distance from the sun. He argued that Mercury and Venus must be as unfit for habitation, through excessive heat, as Jupiter and Saturn through excessive cold. He drew, in particular, a dismal picture of the climatic relations presented by the giant planet Jupiter,