flames, and even discover the chemical composition of the substances which produce them. So great is the delicacy with which these effects are produced, that the presence of particular elements may be detected, though their quantity may be so minute as to defy the manipulations of mere chemical analysis. There is, therefore, a wide field of discovery in this direction open to us, possessed of deep interest. By means of a prismatic telescope the spectrum of every star may be read off with great accuracy;* and from such a catalogue we may yet obtain a chemical classification of the stars, which may lead to important generalisations.

Another fact to which we must allude, is the variability in the brightness of many of the stars, and a periodicity which has been observed in the change. Some of them undergo all their changes in less than three days; others in a period of many years, in which the star waxes and wanes in a regular manner, which it is very difficult to account for. It is the opinion of some that the two sides of a star may not be of equal brightness, and that when it revolves on its axis there will be a corresponding periodic change in its appearance. This is a supposition not inconsistent with the gaseous theory of the sun's light. The spots on our own sun are confined to the equator, and are comparatively unimportant in size; in other suns they may be much more extensive. Were the polar regions of such a sun presented to us, they would be very much brighter than the equatorial regions; and it is not

* This might be done by making the clockwork of an equatorial telescope move slightly faster or slower than the star itself. Its progression through the whole extent of the spectrum might occupy any length of time that might be necessary to record with sufficient accuracy each line or colour as it presented itself successively to the eye.

difficult to understand how a periodic movement would present these successively to our view. Another explanation has been offered, by supposing that a large opaque body is the central orb around which a bright star revolves, and that its periodic eclipse behind this dark object would account for its variable luminosity. This, though a possible, is not a very probable explanation, because it would. scarcely account for the relative proportions of light and darkness. Such an obscuration would not only be sudden but short, unless we suppose that the diameter of its central body is nearly as large as the orbit of the star which revolves around it.

If light be produced by the vibrations of a universal ether, may there not be lacunæ, or vacuities in that ether, which would as effectually intercept the rays as a solid body interposed? This suggestion is made, not so much to account for such phenomena as variable, and appearing, and disappearing stars (although it may be connected with them), as to offer an explanation of certain black streaks and patterns which are projected with great sharpness upon some of the nebula, and which have all the appearance of dark objects in the heavens interposed between the nebula and our eye; and yet they are so much larger than, and their shapes so different from what we should expect such bodies to be, that even the imagination refuses to accept their possibility. If there be lacunæ in the heavens having such forms, their effect would be exactly such as we see inscribed on the nebulæ ; but in that case they must very numerous, because, unless they happen to interpose between our eye and a nebula, we should have no knowledge of their existence. There might be bright stars behind them invisible to us, but we could not know it, unless, by some grand movement going

on among the stars, their relative position might be changed, and then they might suddenly emerge from behind the lacuna, as if they had been newly kindled. Such phenomena do occur, but it seems more likely that these are produced by another cause. For example, in 1572 a star suddenly appeared in the heavens as bright as Sirius itself, and after increasing in brilliancy for a short time, began to fade, and in three months entirely disappeared. Such a phenomenon could not be produced in the manner we have supposed, because in that case it would continue equal in brightness so long as it was seen. It is more likely to be produced by the fall of one star into another. If such an event ever takes place, and there seems to be no reason why it should not, the effect would exactly correspond with the description given by Tycho Brahe of the star we have referred to.

CHAPTER XII.

ARE THE PLANETS INHABITED?

WE are now prepared to resume consideration of the question, Whether the earth be not, even in a scientific point of view, a very remarkable exception to other worlds, and therefore one which the infidel is not warranted in regarding as unlikely to be the scene of the incarnation?

On a review of the whole solar system, as it has passed before us, the first thing that strikes us is the extreme diversity of all its members-not one of them resembling another, even in a remote degree. If, therefore, there be a certain warmth, a certain quantity and quality of atmosphere, a certain inclination of axis, and a certain size and density of mass, which are most favourable to the development of animal and vegetable life, one thing is certain, that these have not been all attended to in the creation of any of the planets except our own. In regard to it, indeed, these concurrent advantages are so marked, and so extraordinary, that it would be difficult to say how they might be improved; while, in regard to the others, our only difficulty is to conceive how life can possibly exist in any one of them.

Those, therefore, who plead that all the stars must be inhabited, must take their choice of one of two lines of argument, but they cannot resort to both of them. They

must either hold that animal and vegetable life is independent of such circumstances-that plants and animals receive a constitution suited to the circumstances of each world, whether it be wet or dry, burning or freezing, naked or aerated. In that case, they must regard the constitution of our own world as altogether unimportant -the wisdom and goodness of the Creator being exercised, not in its preparation, but in the production of vegetables and animals adapted to this particular concurrence of circumstances, just as His wisdom and goodness are exercised in creating animals and vegetables for the sun, the moon, and the stars, in all their infinite varieties of climate and circumstances.

Or they must hold that there are certain requisites to animal and vegetable life, and that these may be more or less favourably combined in every celestial body. In this case, they will not only adore the infinite wisdom and goodness of God, that fitted the animal and vegetable life for being developed under such circumstances, but they will also admit that, in the formation of the earth as the abode of man, these requisite favourable circumstances have, by the wisdom and goodness of God, been provided by the operation of the natural laws. But they who adopt this latter position, must at the same time admit that, although these advantages have all united in the formation of the earth, they have not been so united in the formation of any other world, so far as we have an opportunity of knowing. They must not plead that, because they observe an atmosphere in one, a similarity of seasons in another, and hills and valleys in a third, therefore there must be animals, and vegetables, and moral and intelligent beings in them all. If all these requisites be not united, so as to provide the necessary