"I can explain that, Robert," replied the girl. "I knew that Mr. Melville was in danger when I saw Prentice there. After going to the conservatory with his son, it suddenly occurred to me that I might find some evidence in Prentice’s car—particularly as I now knew about the gas and the mask through your adventure. Knowing that my next dances were with you, I felt that no special attention would be drawn to my absence, so I excused myself and slipped out. In the compartment under the front seat of Prentice’s car I found one of the gas cylinders with the mask attached. Hoping to avert another murder I tried to stuff my handkerchief into the tube. The handkerchief was too large, so I tore it in half and then succeeded." "And," completed Marten, "the gas being forced into the cylinder under pressure, it naturally blew out the handkerchief when released; a mechanical condition which did not occur to Miss Sturtevant. Your curious suspicions of Miss Sturtevant, by the way, made it very hard for her at times. We dared not let you into the secret, because a hot-headed young man like you might have upset our carefully laid plans. It should be unnecessary now for us to explain in detail the various little incidents which aroused your suspicions of her. With your present knowledge of the case you can easily understand the underlying cause in each instance. "Incidentally, Mr. Forrester," added Marten, soothingly, "I want to compliment you upon some of your amateur detective work. You frequently showed fine detective instincts, and it was entirely due to you that we discovered how the money was obtained from the tree. Without your visit to the cottage, that point at least might have remained a mystery." "But you have not told me how the case finally worked out, Mr. Marten," reminded Forrester. "I was coming to that," returned Marten. "We had definitely decided that the money was obtained from the tree in some way through the agency of Lucy. It was arranged with Melville to make a pretense of putting money in the tree. Miss Sturtevant managed to convey the information to Prentice that Melville would do this on a certain night—the night of your visit, to be exact. "Then, instead of watching the tree, as all had done before, _we watched the woman’s cottage_. At the time you entered, the cottage was surrounded by my men and Miss Sturtevant and I were close at hand. You may be sure you gave us a shock, but we planned to let all who wished to do so, enter the cottage, but none would be allowed to leave. You were the first. Later, Lucy appeared, and it is assumed that she discovered you in the cave. When Prentice arrived, Lucy warned him, and between them they hastily prepared the hood with which he disguised himself. Yes," added Marten, noting Forrester’s astounded expression, "that was Prentice with whom you fought. We entered the passage just as you conquered him." "And you have captured them all!" exclaimed Forrester.