"Undoubtedly there are," replied Paul, warming up with his subject. "The ptomaines for example. These are soluble ferments which are formed when any animal tissue putrifies. But although we cannot so readily test them by chemical means, we can easily prove their presence by observing their effect on some one or other of the animals in our invaluable menagerie. "I could give you many more examples if you wanted them. Muscarine, for instance, the alkaloids of certain fungi, many snake poisons, and countless different microbes." "But can't you tell me of something which will defy detection even by means of your animals?" Paul puffed away at his cigarette in deep thought, and then, slowly removing it from his lips, looked up at Pierre and gave a characteristic nod. "Yes, now I think of it, I can give you one. There is a peculiar fluid sent to me from Japan recently," and he pointed to a bottle on the top shelf. "This has hitherto defied all detection by chemical means or otherwise. I alone have discovered how to detect its presence, but I have not had time to publish my discovery, and the poison is quite unknown in Europe. I am told it has the property of sending the person off into a gentle sleep from which he never wakes, if only a teaspoonful be injected under the skin. A friend of mine who is a professor of toxicology at Tokio wrote to me about it, and told me of several murders that had been committed through some mysterious drug which he ultimately managed to get hold of. Being unable to analyse it he sent me a sample to see what I could do with it. It arrived only about two weeks ago." "Well," said Duval, rising to go, "thanks very much for the charming hour I have spent with you." "Don't mention it. I see it is nearly dinner time; will you have dinner with me? I know of a select restaurant where the viands and wines are admirable." Pierre cordially thanked him, and taking up his hat and stick proceeded to follow him out of the room. Before doing so, however, he allowed his cigarette case to fall noiselessly on a duster which lay partly hidden by the table. On leaving the room, Paul turned round and locked the door, and the two left the house together. "Allow me to offer you one of my cigarettes," said Pierre, as they stood in the portico waiting for a fiacre. "With pleasure, mon ami." "Diable!" exclaimed Pierre, fumbling in vain for his cigarette case. "What have I done with it? Oh, I remember, I left it in your laboratory. Pray don't trouble to go back," he added, as Paul turned round to enter the house. "Give me the keys, I can find it much quicker than you can, as I know exactly where I left it in the laboratory. I will be back in a moment."