which are marked after a fashion which can not be found anywhere else on earth; produced in a laboratory which has not yet been discovered on the planet. Fourthly, we are to find something that would produce cyclonic convulsions upon a scale for which the ordinary operations of nature furnish us no parallel. Fifthly, we are to find some external force so mighty that it would crack the crust of the globe like an eggshell, lining its surface with great rents and seams, through which the molten interior boiled up to the light. Would a comet meet all these prerequisites? I think it would. Let us proceed in regular order. {p. 65} CHAPTER II. WHAT IS A COMET? IN the first place, are comets composed of solid, liquid, or gaseous substances? Are they something, or the next thing to nothing? It has been supposed by some that they are made of the most attenuated gases, so imponderable that if the earth were to pass through one of them we would be unconscious of the contact. Others have imagined them to be mere smoke-wreaths, faint mists, so rarefied that the substance of one a hundred million miles long could, like the genie in the Arabian story, be inclosed in one of Solomon's brass bottles. But the results of recent researches contradict these views: Padre Secchi, of Rome, observed, in Donati's comet, of 1858, from the 15th to the 22d of October, that the nucleus threw out intermittingly from itself appendages having the form of brilliant, coma-shaped masses of incandescent substance twisted violently backward. He accounts for these very remarkable changes of configuration by the influence first of the sun's heat upon the comet's substance as it approached toward perihelion, and afterward by the production in the luminous emanations thus generated of enormous tides and perturbation derangements. Some of the most conspicuous of these luminous developments occurred on October 11th, when the comet was at its nearest approach to the earth, and on {p. 66} October 17th, when it was nearest to the planet Venus. He has no doubt that the close neighborhood of the earth and Venus at those times was the effective cause of the sudden changes of aspect, and that those changes of aspect may be accepted _as proof that the comet's substance consists of "really ponderable material."_ Mr. Lockyer used the spectroscope to analyze the light of Coggia's comet, and he established beyond question that-- "Some of the rays of the comet were sent either from _solid particles_, or from vapor in a state of _very high condensation_, and also that beyond doubt other portions of the comet's light issue from the vapor _shining by its own inherent light_. The light coming from the more dense constituents, and therefore giving a continuous colored spectrum, was, however, deficient in blue rays, and was most probably emitted _by material substance at the low red and yellow stages of incandescence_."