No. 4.

The Origin of Nebulæ.

The possible varieties of "partial impact" seem competent to account for the origin of almost all the varieties of nebulae having definite shape. I will attempt to show how some of these may have originated. I need not refer to the fact that the coalesced part in the case of the collision of large cosmical bodies must be chiefly gas, and I have already shewn that in "partial impact" there will be a want of balance in the momentum on the two sides of the coalesced mass, also that this inequality will act in opposite directions, producing a couple, so that rotation is a necessary consequence. I have also shewn how this coalesced part will at first be drawn out into a spindle-shaped mass, which is a form very common with nebulæ. I have also shewn that the tendency to rotation is at right angles to its length. It will need little thought to see that this must certainly develop into a spiral nebula after a time. It is not difficult to shew that in many cases, if the two parts of the spindle have been drawn out to a great length, the centre of the mass will acquire a greater angular velocity than the ends. A spiral will appear at the centre before the outside has developed into this form. We have an illustration of this in the elliptical annular nebulæ of the Lion. In many cases the two pieces that did not coalesce will return, especially when the coalesced part formed a large ratio of the whole, or the original proper motion was small. In this case the nebula would be of a much more stable character. May not the dark lines, often seen in a nebula, be due to the passage cleared through the gas by the attraction of the moving body? The nebula of Andromeda, as observed by Bond, seems exactly to correspond with such an event. It is easy to show that a body passing out of such a nebula would cut a conical passage. There are two causes for this—1st, by the mass becoming larger by the attracted matter, and secondly from the fact that the body would move slower, and page 6 would hare more time for attraction to act. It appears, however, that such openings would not, generally speaking, last long. If the impact be more partial, that is, the coalesced mass of small ratio, it is easy to see that the tendency to rotation will be greater, and it is probable that the ends of the spindle will be thrown off, often coalescing with the retreating mass, The remaining mass will be acted on by the radial force of heat, and by the tangential centrifugal foroe. It can be shewn that in the case where only a small ratio is struck off, the attractive force of the mass is enormously less than the repulsive force. The heat motion may be shewn to be chiefly pressure at first, but finally it will be almost wholly direct radial molecular motion. It is certain, under these circumstances, that after a time a ring will be formed which will, in almost all cases, go on increasing in diameter, and ultimately be dissipated into space; sometimes, however, the principle of selective escape, due to chemical compositions, which I explained in my first letter, would cause a portion to be condensed at the centre; there would then result an annular nebula with cendensed centre, It is also easy to shew that, at the extremity of the axis of such a mass, nebulous matter would often be found. In many cases where coalescence was considerable, and the original temperature before impact high, it may be shown that the centrifugal force would not be sufficient to form a ring, but a hollow sphere would then result, and a planetary nebula; be formed. Let selective escape have left a nucleus, and we have a nebulous star. In cases where the impact of the two bodies was oblique, or the bodies very unequal in mass, the coalesced part would have a high proper motion in space, and a conical or cometsry nebula must result. It is easy to see that the extremely unequal escape of matter at the extremities of the spindle, together with the returning bodies, in many cases broken into many pieces, would produce the display of stars so often accompanying nebula. I need not refer to the fact that both the attraction of the nebulous matter and the resistance offered by it to the stars would generally tend to make both occupy the same field in the telescopes, I have thus glanced at the possible genesis of most of the varieties of nebula;, and it is seen how easily partial impact explains them. The more fully the matter is discussed, the more consistant the results appear; but the space I have allotted myself for this letter is exhausted. In a future letter I hope to shew that the evidences accumulating seem most unmistakeably to point to the tremendous conclusion that our own galaxy, and with it almost everything luminous in the sky, has been developed by the partial impact of two large bodies in space. The explanation of the irregular nebulæ of the milky way, and the enormous aggregation of nebulae at its poles, I shall reserve for a future letter.