Page:The story of the comets.djvu/69

III. in many of the engravings of the tail of Donati's Comet of 1858 are streamers of this type, composed, according to Bredichin, of hydrogen.

Tails of the second type are by far the most common, and in them the repulsive force is much less than in the first type, and is least of all at the inner edge of the tails of this type. It may be supposed that such tails are composed of some hydro-carbon gas.

Tails of the third type, examples of which are not numerous, owe their short stubby form to the twin facts that the repulsive force to which they are due is only a fraction of Gravitation; and that they are composed of something much heavier than is the case with the other kinds of tails, namely, the vapour of iron, with possibly an admixture of the vapours of other substances, especially sodium.

Thus far we have been considering the tails of comets looked at as regards their whole length, and the consequent outlines which they exhibit, but something must be said as to where the material of the tail comes from, and how it is evolved. Here again, whilst we can see many interesting transformations going on there is still the difficulty to be faced of what is the material and whence come the boundless supplies which mark the career of all the large and brilliant comets which we sometimes see and can always read about.

Of course the obvious and necessary answer is that this material is ejected from the nucleus, a fact which will be fully realized by the most cursory inspection, say for instance, of Plate III (ante), but no clue is afforded us as to what the material is, and speculation, it is admitted, is futile.

Speculation has been indulged in by many astronomers as to what becomes of the matter ejected from the heads of comets which after forming for a while part of the tail goes off into Space. Is it simply dispersed in Space, or what happens?

The generally accepted idea is that the fragments of a comet thus sent adrift are first of all dispersed hither and thither through Space, where if a planet falls in with them it annexes them, and they become, shall we say, "shooting stars" to that