against a stone wall would certainly elicit sparks, we see that a speed 30 times as great, whose energy is 900 times greater, is quite competent to a shock sufficient to make us see stars en masse. But, indeed, there must be collisions much more violent than this; both because the central mass is often much greater and because the orbits differ much more, and the effect would increase as the square of the speed. The heat thus generated would cause the meteorites to glow, and at the same time raise the temperature of the gases in and about them. Furthermore, the light would come to us through other non-affected portions of gas between us and the scene of the collision. Thus all three peculiarities of the spectra stand explained: we have a continuous background of light due to heated solid meteorites, the bright lines of glowing gases, and dark lines due to other gases not ignited, lying in our line of sight.
In addition we should perceive another result. Collisions would be both more numerous and more pronounced toward the centre of the nebula, for it must speedily grow denser toward its core owing to the falling in of meteorites, in consequence of shock. Being denser in the centre, the particles would there be thicker and be travelling at greater speed. The nebulæ, therefore, should be brightest at their centres, which is accordant with observation.
Thus from having offered themselves exemplars of the
