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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.