This immense globe of radium, he believed, in the first place, was the parent-body of all the infinitesimal particles of this remarkable substance that had recently been found in different parts of the world. The mysterious properties of radium, he said, were only dimly understood as yet by physicists who had experimented with it. Apparently it was a mineral; but as it revealed a constant and amazing activity, throwing out a force that so far had baffled analysis, there were those who held that it was a living, or, better yet, a life-giving substance. The existence of this immense body of radium here, in the center of the cave, explained, to the satisfaction of Leighton, much of the strange phenomena they had seen. Here, obviously, was the source of the soft, diffused light that had puzzled them ever since they passed through the Condor Gate; and it was to this center of energy that they must attribute the increase in buoyancy and physical well-being experienced the further they penetrated into this subterranean world. The peculiar growths, also, half vegetable, half mineral, that had given the appearance of groves and gardens to certain portions of the cave through which they traveled, were undoubtedly due to this marvelous force, occupying the same relative position towards subterranean life that the sun did to the outside world of nature. Moreover, Leighton firmly believed that the supremacy of radium as the life-giver in this cave, involved the existence, as they would discover, of other phenomena having still more subtle, even psychic, qualities. Narva grunted significantly at this observation, and Una confirmed the truth of it by relating how the floor of the court where they were standing had, only a short time before, reflected a series of pictures of events taking place in the outside cave, by means of which they had been able to follow Leighton's approach to the palace and watched the collision of his party with that of Raoul. It was through this peculiar photographic power of radium, indeed, that Sajipona could discover whatever was taking place in the remotest regions of her domain. This information did not surprise Leighton in the least. On the contrary, he appeared to take it as a matter of course, one of many marvels that might be expected in a land run, so to speak, by radium.