perimeters torrents of sparks are flying, the Orion Nebula rather recalls the aspect of a cloud of smoke and fragments produced by the explosion of a shell. This idea is enforced by the look of the outer portion farthest from the bright half of the nebula, where sharply edged clouds with dark spaces behind seem to be billowing away as if driven by a wind blowing from the center. [Illustration: The Orion nebula] Next let us consider what scientific speculation has done in the effort to explain these mysteries. Laplace’s hypothesis can certainly find no standing ground either in the Orion Nebula or in those of a spiral configuration, whatever may be its situation with respect to the grand Nebula of Andromeda, or the “ring” and “planetary” nebulæ. Some other hypothesis more consonant with the appearances must be found. Among the many that have been proposed the most elaborate is the “Planetesimal Hypothesis” of Professors Chamberlin and Moulton. It is to be remarked that it applies to the spiral nebulæ distinctively, and not to an apparently chaotic mass of gas like the vast luminous cloud in Orion. The gist of the theory is that these curious objects are probably the result of close approaches to each other of two independent suns, reminding us of what was said on this subject when we were dealing with temporary stars. Of the previous history of these appulsing suns the theory gives us no account; they are simply supposed to arrive within what may be called an effective tide-producing distance, and then the drama begins. Some of the probable consequences of such an approach have been noticed in Chapter 5; let us now consider them a little more in detail.