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 way in which our own system came into being, the white nebulæ assert their present constitution to be that from which we know our system sprang. Another suggestive fact about the present members of our solar system which has something to say about a past collision is the densities of the different planets. The average density of the four inner planets, Mars, the Earth, Venus, and Mercury is nearly four times that of the four outer ones Neptune, Uranus, Saturn, and Jupiter [see NOTE 2]. The discrepancy is striking and cannot be explained by size, as the smallest are the most massive, and if all were primally of like constitution, should be the least compressed. Nor can it be explained simply by greater heat tending to expand them, for Neptune and Uranus show no signs of being very hot. The minor differences between members of each group are probably explicable in part by these two factors, mass and heat, but the great gulf between the two groups cannot so be spanned. We are then driven to the supposition that the materials composing the outer ones were originally lighter. Now this is precisely what should happen had all eight been formed by disruption of a previous body. For its cuticle would be its least dense portion, and on disruption would travel farthest away, not because of being lighter, but because of being on the outside. Parts coming from deeper down would remain near, and be denser intrinsically. What the present densities of the planets enable us to infer of the cataclysm from which they came, a remarkable set of spectrograms taken not long ago by Dr. V. M. Slipher, at Flagstaff, seems to confirm.