tiple, and sometimes even of abnormal tails, must all be taken fully into account. The yet more perplexing phenomenon of the breaking up of a comet into two distinct comets, each with its own nucleus, coma, and tail, and even-if ancient records can be trusted -the formation of a multiple system of comets out of a single comet, must also be interpreted. And many other matters, which it would be tedious to enter upon here, must be explained satisfactorily before any theory of comets can take its place in the rank of physical truths.

In conclusion, I must remark that it would be unfair to form an estimate of Professor Tyndall's views—or at any rate to decide finally on their value-until he has had time to arrange and co-ordinate them with reference to all the facts which lie at his disposal. It is not to be expected, and he doubtless would be the last to suppose, that a discovery so recently made as the one on which the theory is founded, should in a moment remove all the difficulties and reconcile all the incongruities presented by cometary phenomena. If we were to estimate the theory as at present exhibited, we could hardly look upon it (based though it be on observed facts) as other than a highly ingenious speculation. It is because I look upon the views which Professor Tyndall has brought before the scientific world, as affording promise of further researches on the same subject, and that such researches made by such a physicist as Professor Tyndall cannot fail to bear useful fruit, that I have dealt at length with

views which, however ingenious, must be looked upon at present as speculative. It must be remembered, also, that astronomers have not been so successful in theorising respecting comets, that they can claim (or afford) to reject the assistance which one of the most eminent of living physicists is offering them in the treatment of a question which they have been too much in the habit of considering as peculiarly their

own.

Fraser's Magazine, October, 1869.

219

COMETS AND COMETS' TAILS.

AMONG the many startling suggestions recently thrown out by men of science, not one, perhaps, has seemed more amazing to the general public than the idea put forward by Sir W. Thomson in the able address with which he inaugurated the late meeting of the British Association that life on the earth may have had its origin from seeds borne to our planet by meteors, the remnants of former worlds. Coupling this startling theory with the partly-admitted view that the tails of comets and comets themselves consist of meteoric flights, he presented the 'hairy stars' which men so long viewed with terror in a somewhat novel light. Regarded not so many years ago as probably the vehicles of the Almighty's wrath, comets are made by this new hypothesis to appear as the parents of universal life. How would Whiston, and those who thought with him that a comet in old times effected the destruction of all living things (save å chosen few) with water, and that a comet at perhaps no very distant future would destroy the whole earth with fire, have contemplated a theory according to which the seed-bearing fragments of a comet's tail peopled the earth with all the living things which at present exist upon its surface? The fear of change' with which

in old times comets perplexed the nations must be replaced, it would seem, by another sort of fear. We need not dread the approaching dissolution of the world through cometic agency, though the thought of a vast catastrophe may be suggested by the consideration that we see in the comet but the fragments of another world. But if this new theory should be accepted, we have reason to regard with apprehension the too close approach of one of these visitants; because, if one comet supplied the seeds of the living things now existing on the world, another may supply myriads of seeds of undesirable living things; and perhaps the sequent struggle for life may not result in the survival of the fittest.

It is hardly necessary for me to say, perhaps, that I am not troubled by such misgivings. I can scarcely bring myself to believe, indeed, that the eminent professor was serious in urging his hypothesis of seed-bearing meteors. Englishmen speak sometimes of the slowness with which a Scotsman apprehends a jest; but the Scotsman may return the compliment-so far, at least, as the southern estimate of Scottish humour is concerned. For a true Scot makes his jests with a gravity and aplomb unequalled among Sassenach humorists. It is far from improbable that the seriousness with which the seed-bearing meteorites have been discussed proved infinitely amusing to the gathering of the clans in Edinburgh. Thomson and Tait, Andrews and Geikie, Stewart and Lockyer, in fine, all the Scottish men of science who were present at the gathering, may

be ready to retort Sydney Smith's gibe, maintaining henceforth that nothing short of a surgical operation will enable an Englishman to appreciate Scottish humour.

For it will be noticed that the explanation of the origin of life upon our globe leaves the real question of the origin of life where it was. The theory, in this respect, resembles that undoubtedly humorous account which the Hindoo sages gave of the manner in which our earth is supported; and precisely as the Hindoo student of science might ask how the tortoise who supports the earth is himself supported, so may we ask how the worlds which, by bursting, supplied space with seed-bearing meteors, were themselves peopled with living things. This circumstance of itself throws an air of doubt over the new hypothesis, as a seriouslyintended account of the origin of life on our earth. It may seem superfluous to add that in a collision by which a world was shivered into fragments the seeds of life would have what may be described as a warm time, since the collision could hardly fail to vaporise the destroyed world. The fiery heat generated by the collision, followed by a voyage during myriads of millions of ages through the inconceivable cold of space, and, lastly, by the fierce heat which accompanies the fall of meteoric masses upon our earth, would seem so unfavourable to the germs of life, that Pouchet himself might accept with confidence the belief that all such germs had been completely destroyed before reaching this planet.