"Just so," replied Paul. "Those are the crystals of arsenious acid. It forms characteristic eight-sided crystals. So you see we have determined the presence of arsenic by three independent tests. It therefore must be arsenic, as nothing else will give these reactions. In the case of alkaloids the tests are much more difficult, because one may poison a person with a very small quantity indeed.
"For example, here are the remains of the contents of the stomach of a child. In this particular instance we found it extremely difficult to detect the poison. We tested for all the ordinary poisons in vain. Here our menagerie came to our aid; for on injecting a small quantity of the fluid under a guineapig's skin with this Pravaz syringe the little animal rapidly died with convulsions and syncope. Hence we knew at once that we had to do with a very poisonous alkaloid. By using nearly the whole contents of the stomach, and extracting the alkaloid,[7] we recovered about the 1/30th part of a grain of a white powder which we proved to be Aconitine—one of the most deadly poisons known.
"So you see if anyone tries to poison a person even with these alkaloids he is sure to be found out."
"But are there no poisons which are beyond your powers to detect?"
"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."