Here, then, the meteorites tell us of another, an earlier, stage of our solar system’s history, one that mounts back to before even the nebula arose to which we owe our birth. For the large body to whose dismemberment the meteorites were due can have been no other than the one whose cataclysmic shattering produced that very nebula which was for us the origin of things. The meteorites, by continuing unchanged, link the present to that far-off past. And they tell us, too, that this body must have been dark. For solid, they inform us, it was, and solidity in a heavenly body means deficiency of light. That such corroborative testimony to a cataclysmic origin is forthcoming in the sky we shall see by turning again to the spiral nebulæ. Of the two classes of nebulæ which we contemplated in the last chapter, the amorphous and the structural, there is more to be said than we touched on then. [Illustration: NEBULA ♅ V. 14 CYGNI—AFTER ROBERTS.] Not only in look are the two quite unlike, but the spectroscope shows that the difference in appearance is associated with dissimilarity of character. For the spectrum of the amorphous proves to consist of a few bright lines, due to hydrogen and nebulium chiefly, in the green, whence the name green nebulæ. That of the spirals, on the other hand, is continuous, and therefore white. The great nebula in Andromeda was one of the first in which this was recognized; and the perception was pregnant, for no nebula defies resolution more determinedly than it. We may, therefore, infer that it is not made up of stars, certainly big enough for us to see. On the other hand, from the fact that its spectrum is continuous it must be solid or liquid. Young pointed out that this did not follow, because a gas under great pressure also gives a continuous spectrum. But he forgot that here no such pressure could exist. A nebula of compressed gas could not have an irregular form and would have, in the case of the Andromeda nebula, a mass so enormous as to preclude supposition. Continuity of spectrum here means discontinuity of mass. The spectral solidity of the nebula speaks of a _status quo ante_, not of a condition of condensation now going on. [Illustration: NEBULA N. G. C. 1499 PERSEI—AFTER ROBERTS.] [Illustration: NEBULA N. G. C. 6960 IN CYGNUS—AFTER RITCHEY.]