THE MATERIALS OF A COMET.
A comet is made of very unsubstantial material. This we can show in a very interesting manner, when we see it moving over the sky between the earth and the stars. Sometimes a comet will pass over a cluster of very small stars, so faint that the very lightest cloud that is ever in the sky would be quite sufficient to hide them. Yet the stars are distinctly visible right through the comet, notwithstanding that it may be hundreds of thousands of miles thick. This shows us how excessively flimsy is the substance of a comet, for while a few feet of haze or mist suffice to extinguish the brightest of stars, this immense curtain of comet stuff, whatever it may be made of, is practically transparent.
I have often told you that we are able to weigh the heavenly bodies, but a comet gives us a great deal of trouble. You see that the weighing machine must be of a very delicate kind if you are going to weigh a very light object. Take, for example, a little lock of golden hair, which no doubt has generally a value quite independent of the number of grains that it contains. Suppose, however, that we are so curious as to desire to know its weight, then one of those beautiful balances in our laboratories will tell us. In fact, if you snipped a little fragment from a single hair, the balance would be sensitive enough to weigh it. If, however, you were only provided with a common pair of scales like those which are suited for the parcel post, then you could never weigh anything so light as a lock of hair. You have not small enough weights to begin with, and even if you had they would be of no use, for the scale is too coarse to estimate such a trifle. This is precisely the sort of difficulty we experience when we try to weigh a comet. The body, though so big, is very light, and our scales are so cumbersome that we are in a position of one who would try to weigh a lock of hair with a parcel-post balance. We cannot always find suitable scales for weighing celestial bodies. We have to use for the purpose whatever methods of discovering the weights happen to be available. So far, the methods I have mentioned are of the rudest description; they serve well enough for weighing heavy masses like planets, but they will not do for such unsubstantial bodies as comets.
But, though we fail in this endeavor, i.e. to weigh comets, yet skilful astronomers have succeeded in something which at first you might think to be almost impossible. They have actually been able to discover some of the ingredients of which a comet is made. This is so important a subject that I must explain it fully.
The most instructive comet which we have seen in modern days is that which appeared in the year 1882. It was an object so great that its tail alone was double as long as from the earth to the sun. It was discovered at the observatories in the southern hemisphere early in September of that year. A little later it was observed in the northern hemisphere in extraordinary circumstances. It must be remembered that a comet is generally a faint object, and that even those comets which are large enough and bright enough to form glorious spectacles in the sky at night are usually invisible during the brightness of day. For a comet to be seen in daylight was indeed an unusual occurrence; but on the forenoon of Sunday, September 17, Mr. Common at Ealing saw a great comet close to the sun. Unfortunately clouds intervened, and he was prevented from observing the critical occurrence just approaching. An astronomer at the Cape of Good Hope—Mr. Finlay—who had also been one of the earliest discoverers of the comet, was watching the body on the same day. He followed it as it advanced close up to the sun; bright indeed must that comet have been which permitted such a wonderful observation. At the sun’s edge the comet disappeared instantly; in fact, the observers thought that it must have gone behind the sun. They could not otherwise account for the suddenness with which it vanished. This was not what really happened. It was afterwards ascertained that the comet had not passed behind the sun; it had, indeed, come between us and our luminary. In its further progress this body showed in a striking degree the incoherent nature of the materials of which a comet is composed. It seemed to throw off portions of its mass along its track, each of which continued an independent journey. Even the central part in the head of the comet—the nucleus, as it is called—showed itself to be of a widely different nature from a substantial planetary body. The nucleus divided into two, three, four, or even five distinct parts, which seemed, in the words of one observer, to be connected together like pearls on a string.
The comet of 1882 was also very instructive with regard to the actual materials from which such bodies are made. Astronomers have a beautiful method by which they find out the substances present in a heavenly body, even though they never can get a specimen of the body into their hands. We know at least three materials which were present in this comet. The first of them is an ingredient which is very commonly found in comets—a chemist calls it carbon. It is an extremely familiar material on the earth; for instance, coal is chiefly composed of carbon. Charcoal when burned leaves only a few ashes. All the substance that has vanished during combustion is carbon; in fact, it is not too much to say that carbon is found abundantly not only in wood, but in almost every form of vegetable matter. The food we eat contains abundant carbon, and it is an important constituent in the building up of our own bodies. Generally speaking, carbon is not found in a pure state—it is associated with other substances. Soot and lampblack are largely composed of it; but the purest form of this element carbon that we know is the diamond.
It is interesting to note that carbon is certainly found as a frequent constituent of comets. The great comet of 1882 undoubtedly contained it, as well as certain other substances. Of these we know two: the first is the element sodium, an extremely abundant material on earth, inasmuch as the salt in the sea is mainly composed of it. It was also discovered that the same great comet contained another substance very common here and extremely useful to mankind. Dr. Copeland and Dr. Lohse at Dunecht showed that iron was present in this body which had come in to visit us from the depths of space.
These discoveries are especially interesting because they seem to show the uniformity of material composing our system. We already knew that sodium and iron abounded in the sun, and now we have learned that these bodies and carbon as well are present in the comets. In the next chapter we shall learn that the very same materials—sodium and iron—are met with in bodies far more remote from us than any bodies of our own system.
Comets have such a capricious habit of dashing into the solar system at any time and from any direction, that it is worth while asking whether a comet might not sometimes happen to come into collision with the earth. There is nothing impossible in such an occurrence. There is, however, no reason to apprehend that any disastrous consequences would ensue to the earth. Man has lived on this globe for many, many thousands of years, and the rocks are full of the remains of fossil animals which have flourished during past ages; indeed, we cannot possibly estimate the number of millions of years that have elapsed since living things first crawled about this globe. There has never been any complete break in the succession of life, consequently during all those millions of years we are certain that no such dire calamity has happened to the earth as a frightful collision would have produced, and we need not apprehend any such catastrophe in the future.
I do not mean, however, that harmless collisions with comets may not have occasionally happened; in fact, there is good reason for knowing that they have actually taken place. In the year 1861 a fine comet appeared; and it is not so well remembered as its merits deserve, because it happened, unfortunately for its own renown, to appear just three years after the comet of 1858, which was one of the most gorgeous objects of this kind in modern times. But in 1861 we had a novel experience. On a Sunday evening in midsummer of that year, we dashed into the comet, or it dashed into us. We were not, it is true, in collision with its densest part; it was rather the end of the tail which we encountered. There were, fortunately, no very serious results. Indeed, most of us never knew that anything had happened at all, and the rest only learned of the accident long after it was all over. For a couple of hours that night it would seem that we were actually in the tail of the comet, but so far as I know no one was injured or experienced any alarming inconvenience. Indeed, I have only heard of one calamity arising from the collision. A clergyman tells us that at midsummer he was always able in ordinary years to read his sermon at evening service without artificial light. On this particular occasion, however, the sky was overcast with a peculiar glow, while the ordinary light was so much interfered with that the sexton had to provide a pair of candles to enable him to get through the sermon. The expense of those candles was, I believe, the only loss to the earth in consequence of its collision with the comet of 1861.
Fig. 76.—How the Tail of a Comet arises.
The tail of a comet appears to develop under the influence of the sun. As the wandering body approaches the source of central heat it grows warm, and as it gets closer and closer to the sun, the fervor becomes greater and greater, until sometimes the comet experiences a heat more violent than any we could produce in our furnaces. The most infusible substances, such as stones or earth, would be heated white-hot and melted, and even driven off into vapor, under the intense heat to which a comet is sometimes exposed. Comets, indeed, have been known to sweep round the sun so closely as to pass within a seventh part of its radius from the surface. It seems that certain materials present in the comet, when heated to this extraordinary temperature, are driven away from the head, and thus form the tail ([Fig. 76]). Hence we see that the tail consists of a stream of vaporous particles, dashing away from the sun as if the heat which had called them into being was a torment from which they were endeavoring to escape.
The tail of a comet is, therefore, not a permanent part of the body. It is more like the smoke from a great chimney. The smoke is being incessantly renewed at one end as the column gets dispersed into the air at the other. As the comet retreats, the sun’s heat loses its power. In the chills of space there is, therefore, no tail-making in progress, while the small mass of the comet renders it unable to gather back again by its attraction the materials which have been expelled. Should it happen that the comet moves in an elliptic orbit, and thus comes back time after time to be invigorated by a good roasting from the sun, it will, of course, endeavor to manufacture a tail each time that it approaches the source of heat. The quantity of material available for the formation of tails is limited to the amount with which the comet originally started; no fresh supply can be added. If, therefore, the comet expends a portion of this every time it comes round, an inevitable consequence seems to follow. Suppose a boy receives a sovereign when he goes back to school, and that every time he passes the pastry-cook’s shop some of his money disappears in a manner that I dare say you can conjecture, I need not tell you that before long the sovereign will have totally vanished. In a similar way comets cannot escape the natural consequences of their extravagance; their store of tail-making substance must, therefore, gradually diminish. At each successive return the tails produced must generally decline in size and magnificence, until at last the necessary materials have been all squandered, and we have the pitiful spectacle of a comet without any tail at all.
The gigantic size of comets must excite our astonishment. A pebble tossed into a river would not be more completely engulfed than is our whole earth when it enters the tail of one of these bodies. But we now pass by a sudden transition to speak of the very smallest bodies, of little objects so minute that you could carry them in your waistcoat pocket. You will perhaps be surprised that such things can play an important part in our system and have a momentous connection with mighty comets.