There is clear evidence that what we may venture to call the native mode of cometary circulation is absolutely exempt from the rules which impress the movements of the planets with an unequivocal stamp of congruity. The few comets showing some degree of compliance with the general plan are those which have been subjected to manifold perturbations, and can hence no longer be called as unbiassed witnesses; while their untrained associates, left relatively free to follow the impulsion of their start, betray no geometrical preferences in their manner of travelling. They revolve indifferently with or against the course of the signs; their paths are inclined at every possible angle to the ecliptic; they approach the sun in sensibly equal numbers from all quarters of the sky; they agree only in pursuing ellipses so elongated as to verge towards the parabolic limit. But just in this way, and no otherwise, we should expect to find bodies circulating which, having been aggregated at random (as Kant supposed) in the beginning, had departed to the least possible extent from the initial conditions of their systemic union. A good _primâ facie_ case can, then, be made out for regarding comets as samples of the used-up nebula, as superannuated constituents of an inconceivable chaos, which, evading the operation of laws of change, have floated down the stream of ages virtually intact and undisturbed. Yet the question has other aspects besides this purely mechanical one. They should all be harmonized by truth, which cannot be more securely guaranteed than by consilient testimony; nevertheless, there are difficulties in effecting the accommodation. Comets are not, in a chemical sense, closely related to nebulæ. They are fundamentally of carbonaceous composition--free hydrogen makes no spectroscopic show in them--while they include metallic ingredients occasionally rendered glowing by the powerful excitement of a perihelion rush-past. But gaseous nebulæ shine mainly with the light of certain unknown substances, reinforced by rays of hydrogen and helium. Carbon flutings and metallic lines are alike alien to their spectra. Nor is there any community that we yet know of between the chemistry of white nebulæ and that of comets. The nebular hypothesis of cometary origin is thus discountenanced by the results of light-analysis. Still, there are possibilities of reconcilement. Spectral conditions must be subject to change. The quality of light emitted by a body of mixed composition cannot fail to alter with the inevitable alteration of physical state brought about by external influences or internal change.