In one respect perhaps no more stupendous subject of contemplation than this has ever been offered to the mind of man. In comparison with the length of time thus required to efface the tiny individual atom, the entire cosmical career of our solar system, or even that of the whole starry galaxy, shrinks into utter nothingness. Whether we shall adopt the conclusion suggested must depend on the extent of our speculative audacity. We have seen wherein its probability consists, but in reasoning upon such a scale we may fitly be cautious and modest in accepting inferences, and our authors, we may be sure, would be the first to recommend such modesty and caution. Even at the dimensions to which our theorizing has here grown, we may for instance discern the possible alternative of a simultaneous or rhythmically successive generation and destruction of vortex-atoms which would go far to modify the conclusion just suggested. But here we must pause for a moment, reserving for a second paper the weightier thoughts as to futurity which our authors have sought to enwrap in these sublime physical speculations. PART SECOND. UP to this point, however remote from ordinary every-day thoughts may be the region of speculation which we have been called upon to traverse, we have still kept within the limits of legitimate scientific hypothesis. Though we have ventured for a goodly distance into the unknown, we have not yet been required to abandon our base of operations in the known. Of the views presented in the preceding paper, some are wellnigh certainly established, some are probable, some have a sort of plausibility, others--to which we have refrained from giving assent--may possibly be true; but none are irretrievably beyond the jurisdiction of scientific tests. No suggestion has so far been broached which a very little further increase of our scientific knowledge may not show to be either eminently probable or eminently improbable. We have kept pretty clear of mere subjective guesses, such as men may wrangle about forever without coming to any conclusion. The theory of the nebular origin of our planetary system has come to command the assent of all persons qualified to appreciate the evidence on which it is based; and the more immediate conclusions which we have drawn from that theory are only such as are commonly drawn by astronomers and physicists. The doctrine of an intermolecular and interstellar ether is wrapped up in the well-established undulatory theory of light. Such is by no means the case with Sir William Thomson's vortex-atom theory, which to-day is in somewhat the same condition as the undulatory theory of Huyghens two centuries ago. This, however, is none the less a hypothesis truly scientific in conception, and in the speculations to which it leads us we are still sure of dealing with views that admit at least of definite expression and treatment. In other words, though our study of the