As I write, I have Lord Rosse's elaborate drawing of this nebula before me, and on the margin of this stupendous object the nebula fades away so tenderly that it is almost impossible to say where the luminosity terminates. Probably this nebula will in some remote age condense down into more solid substances. It contains, no doubt, enough material to make many globes as big as our earth. Before, however, it settles down into dark bodies like the earth, it will have to pass through stages in which its condensing materials will form bright sun-like bodies. It seems as if this process of condensation might almost be witnessed at the present time in some parts of the great object. There are also some very striking nebulæ which are often spoken of as planetary. They are literally balls of bluish-colored gas or vapor, apparently more dense than that which forms the nebula now under consideration. Such globes are doubtless undergoing condensation, and may be regarded as incipient worlds." Of these spiral nebulæ it is said, in Appleton's Cyclopædia, "Many of them had been long known as nebulæ, but their characteristic spiral form had never been suspected. They have the appearance of a maelstrom of stellar matter, and are among the most interesting objects in the heavens." Of their spectra it is said, "The bright-line spectrum is given by all the irregular nebulæ hitherto examined and by the planetary nebulæ." That is to say, these nebulæ are gaseous in constitution, and have not yet reached the stage of solar condensation which marks the existence of individual suns. CHAPTER XII. THE NEBULAR HYPOTHESIS: ITS BASIS AND ITS DIFFICULTIES. "There sinks the nebulous star we call the Sun, If that hypothesis of theirs be sound."--Tennyson.