Their number, however, being finite, the store of energy they can supply in falling even from an infinite distance is also finite. The process of solar sustentation is then terminable; it had a beginning, and it will assuredly come to an end. Now the _terminus ad quem_ is of a calculable remoteness: it can be located (unless shifted by radio-active processes) within certain limits of time. But the _terminus a quo_ depends upon too many conditions to be satisfactorily defined. It is only certain that the sun is to-day slightly more condensed than it was a year ago. It might a few millenniums back have been measurably larger, had modern micrometrical methods been available in the Stone Age; while, looking into the geological past, we discern a continually more diffuse globe, filling the orbit of Mercury when the earth was perhaps still red-hot, then successively ampler spheres, out to, and beyond, that of Neptune. And just such a vastly diffused sun realizes the nebula of Laplace. The state of things he imagined can be reached accordingly, either by tracing forward the development of a tenuous rotating mass, or by pursuing backward the surely indicated, unceasing, and inevitable distension of the sun. Hence, no sooner was it acknowledged that energy may be transformed, but cannot be destroyed, than the nebular cosmogony assumed a new and authoritative aspect. But here a _caveat_ has been entered by the latest inquirers--a _caveat_ not to be ignored, though based upon modes of action still exceedingly obscure. Radio-activity is a fledgling science; its capabilities, though immense, are vaguely outlined. Until they more fully approve themselves, it would be unwise to admit conclusions which they may eventually enforce. Subversive ideas are in the air; the theory of atomic dissociation goes to the very root of things, and it insistently claims assent. Its verification, by disclosing the presence in the universe of a measureless store of unsuspected energy, would overthrow all the calculations of cosmic time heretofore attempted, and might protract indefinitely the radiative span of the sun. Mr. W. E. Wilson pointed out in 1903[6] that its entire thermal output could be supplied by the spontaneous liberation of energy from 3·6 grammes of radium in each cubic metre of its volume; and although we have no evidence of the actual existence of radium in the sun, the possibility that chromospheric helium represents the decay of solar radio-active elements[7] must be taken into consideration. The ground here is undermined with pitfalls. We can only see that although Helmholtz's gravitational rationale of the sun's long life-history remains true, the results derived from it may be profoundly modified by co-ordinate processes, variously efficacious according to circumstances, perhaps knowable, but as yet unknown.