nebulæ are composed of meteorites, the present argument differs from his. The way in which their spectra establish their constitution may be outlined as follows: the white nebulæ are from their structure evidently in process of evolution, and if they are in stable motion, as we suppose them to be, their parts are moving round their common centre of gravity. As the white nebulæ resist resolution as obstinately as the green, these parts must be not only solid but comminuted (composed of small particles). Now this would be the case were they flocks of meteorites such as we have seen composed our own system once upon a time. Though all are travelling round the centre of gravity of the flock, each is pursuing its own orbit slightly different from, and intersecting those of, its neighbors. Collisions between the meteors must therefore constantly occur, and the question is, are these shocks sufficient to cause light. Let us take our own system and consider two meteorites at our distance from the Sun, travelling in the same sense, the one in an ellipse, the other in a circle, with a major axis five per cent greater and meeting the other at aphelion. This would be no improper jostle for such heavenly bodies. If we calculate the speeds of both and deduct the elliptic from the circular, we shall have the relative speed of collision. It proves to be a half a mile a second or 30 times the speed of an express train. As such a train brought up suddenly
