2. The progress of scientific discovery, so far from tending to verify, has served rather to invalidate the fundamental assumption on which the whole theory depends. That assumption was the existence of a Nebulous Fluid or Fire-Mist, capable of being distinguished, by certain characteristic marks, from unresolved nebulae or clusters of stars. The existence of any such fluid has become more and more doubtful, in proportion as astronomers have been enabled, with the aid of larger and better constructed telescopes, to resolve several nebulae which had previously defied the power of less perfect instruments. We do not affirm that every cluster has been already resolved, nor is it necessary for the purposes of our argument to suppose that, at any future time, this stupendous achievement is likely to be effected; for it is a very obvious consideration, that just in proportion as our telescopic powers are enlarged so as to enable us to resolve many of the nearer nebulae, they must also bring within the range of our extended vision _others_ more remote and hitherto unperceived, which may continue to exhibit the same cloud-like appearance as the former, until, by a new improvement of the telescope, we may succeed in separating them into distinct stars; and even then the march of discovery is not ended,--we may reasonably expect that with every fresh increase of telescopic vision, new clusters will be brought into view, and new clouds appear in the utmost verge of the horizon. But, unquestionably, the progress which has already been made in this direction affords a strong presumption in favor of the idea, that the apparent nebulosity of those masses which still appear, even to our best telescopes, as cloud-like vapors, is to be ascribed rather to the imperfection of our instruments than to any difference between them and such as have been already resolved. Sir John Herschell, a high authority in such a case, tells us that "we have every reason to believe, at least in the generality of cases, that a nebula is nothing more than a cluster of stars."[33] Sir David Brewster is equally explicit: "It was certainly a rash generalization to maintain that nebulae differed essentially from clusters of stars, because existing telescopes could not resolve them. The very first application of Lord Rosse's telescopes to the heavens overturned the hypothesis; and with such unequivocal facts as that instrument has brought to light, we regard it as a most unwarrantable assumption to suppose that there are in the heavenly spaces any masses of matter different from solid bodies, composing planetary systems."[34] And Professor Nichol, while he gracefully acknowledges that he has "somewhat altered the views which he formerly gave to the public, as the highest then known and generally entertained, regarding the structure of the heavens," states, as the result of more mature reflection, that "the supposed distribution of a