There could be no doubt that the fire which had destroyed the laboratory had been far fiercer than that which had burnt down the cottage. The charred beams of the latter still existed, while nothing whatever remained of the wooden fittings of the former. And yet, in case of a concrete-built laboratory, one would have expected a fire merely to have burnt out the interior without doing much damage to the structure. The place must certainly have contained a large quantity of substances whose combustion produced great heat, such as thermite. And Dr. Priestley knew enough of the methods of research chemists, who as a rule deal in very small quantities, to wonder why Dr. Morlandson required such large quantities, and why, even supposing that his isolated situation made it necessary for him to obtain his supplies in bulk, he should store them in the laboratory itself. The suspicion that Dr. Morlandson’s activities had not been solely concerned with researches into the properties of medicinal drugs seemed to the Professor to be thoroughly well grounded. To the Professor’s mind, there was even something suspicious about the origin of the fire itself. Dr. Morlandson had been aware of the danger of such an outbreak; he had warned the police superintendent against smoking in the laboratory. This being so, was it likely that he would lock himself into what had proved to be a regular death-trap, and leave himself with no means of escape? The doubt raised by these queries set the Professor’s mind to work upon an entirely fresh train of thought. Had Dr. Morlandson’s death been as accidental as it had appeared? Or was it possible that he too had had an enemy, who had somehow contrived the whole affair? And, if so, what became of the theory which had brought the Professor to the Isle of Purbeck? The Professor awoke with a start from his meditations, suddenly conscious that the light was failing rapidly. It was already past sunset, the ruined Castle which was to be his landmark was now indistinguishable against the universal grey of its background. A faint, silvery mist was rising above the heath, wreathing every object within sight with a curious opalescent halo. The Professor realized that he had already spent long enough contemplating this deserted heap of ashes. Their secret, did they hold one, was not to be wrested from them by further study that night. He turned his back upon the gaunt skeleton of the laboratory, and walked round to the further side of the mound. Here the stumps of a rotten post or two showed where a fence had probably surrounded the front garden, now long since overgrown. Beyond the posts a white thread caught the Professor’s eye, and he made his way up to it. It was a narrow track, apparently disused for years, and had no doubt once been the path by which the cottage was approached.