34. Nebulæ which assume a regular, for instance, a circular or oval shape, with whatever variation of luminous density from the inner to the outer parts, may have a form of equilibrium, if their parts have a proper gyratory motion. Still, we see no reason for supposing that these differ so much from irregular nebulæ, as to be denser bodies, kept in their forms by rapid motions. We are rather led to believe that, though perhaps denser than the spiral nebulæ, they are still of extremely thin and vaporous character. It would seem very unlikely that these vast clouds of luminous vapor should be as dense as the tail of a comet; since a portion of luminous matter so small as such a tail is, must have cooled down from its most luminous condition; and must require to be more dense than nebular matter in order to be visible at all by its own light. 35. Thus we appear to have good reason to believe that nebulæ are vast masses of incoherent or gaseous matter, of immense tenuity, diffused in forms more or less irregular, but all of them destitute of any regular system of solid moving bodies. We seem, therefore, to have made it certain that _these_ celestial objects at least are not inhabited. No speculators have been bold enough to place inhabitants in a comet; except, indeed, some persons who have imagined that such a habitation, carrying its inmates alternately into the close vicinity of the sun's surface, and far beyond the orbit of Uranus, and thus exposing them to the fierce extremes of heat and cold, might be the seat of penal inflictions on those who had deserved punishment by acts done in their life on one of the planets. But even to give coherence to this wild imagination, we must further suppose that the tenants of such prison-houses, though still sensible to human suffering from extreme heat and cold, have bodies of the same vaporous and unsubstantial character as the vehicle in which they are thus carried about the system; for no frame of solid structure could be sustained by the incoherent and varying volume of a comet. And probably, to people the nebulæ with such thin and fiery forms, is a mode of providing them with population, that the most ardent advocates of the plurality of worlds are not prepared to adopt. 36. So far then as the Nebulæ are concerned, the improbability of their being inhabited, appears to mount to the highest point that can be conceived. We may, by the indulgence of fancy, people the summer-clouds, or the beams of the aurora borealis, with living beings, of the same kind of substance as those bright appearances themselves; and in doing so, we are not making any bolder assumption than we are, when we stock the Nebulæ with inhabitants, and call them in that sense, "distant worlds." FOOTNOTES: [1] Herschel, _Outl. of Astr._ Art. 893. [2] Herschel, _Outl. of Astr._ Art. 874, and Plate 11, Fig. 3. [3] Ibid. Art. 897. [4] Hersch. 874. [5] Ibid. 881-8.