"I wonder rather what intruders have been doing here!" exclaimed Langton. "You forget that both doors have been forced." The inspector stood gazing round the place in silent wonder. "Well," he exclaimed at last, "I don't see the slightest evidence of burglars here, sir." "They may be hidden upstairs," suggested the young man. "Remember there are many people very anxious to obtain knowledge of the Professor's discoveries. That is why he is always so careful to keep these doors locked. His daughter, Ethelwynn, is the only person he ever allows in here. He and she even carry in the coal for the furnace, the servants being excluded." "But thieves would hardly light up the furnace!" said the officer. "Unless they wished to destroy something in the fire," responded the other. That suggestion held me aghast. Upon me, like a flash, came the astounding suspicion that that furnace might have been lit for the purpose of destroying the evidence of the mysterious crime. I remembered Kirk's curious and guarded response when I had referred to the burial of the body. Was this, then, the reason why I had found him alone in the house? I stood staggered by the suggestion. I was near the furnace--nearer than the others. Then, when I found speech again, I said: "If there are intruders in this place, they could not have escaped; they must certainly be upstairs. I agree with Mr. Langton that it is certainly very curious that these doors should have been forced." "How did you know that the Professor is in Scotland?" he inquired of me eagerly. In an instant I had a ready reply. "Antonio told me so when I called on Monday." "Did he say when his master would be back?" asked the inspector. "He said he expected him to return last night, as he had an engagement to go with his daughter to a ball." "Then he may have returned and gone to the dance," remarked the officer. "He may also have lost his keys and been compelled to break open the doors--quite a likely circumstance. Three persons dined downstairs to-night. He and his daughter and a friend probably dined and afterwards went out; while the servants, knowing they would not return before midnight, may have followed them out to spend the evening. That at least is my theory at the present." "That certainly seems to be the most logical conclusion, inspector," I remarked. "We must search the upper premises before I accept it," exclaimed Langton, who, I could see, was still very suspicious that something unusual had happened. The meeting with Antonio in the buffet at Calais had caused him to doubt, and most naturally so. My eager eyes were fixed upon the glowing furnace, the large, square, iron door of which was still red-hot, though the heat was now decreasing. At the side was a large air-shaft, in which were fitted electric fans, while on the wall were three switches by which a strong forced draught could be obtained.