"Watch me closely. I first stir the contents well, and then filter some of it through this filter paper into this little beaker. Now I add a few drops of acid, and then allow some of the sulphuretted hydrogen gas to bubble through. Observe a bright canary yellow precipitate is forming. This shows me that arsenic is probably present. But to make quite sure I apply some further tests." Paul then poured another small quantity of the suspected fluid into a tiny porcelain dish, to which he added a few drops of pure hydrochloric acid and gently warmed it. "Now," said Paul, "I take this slip of pure polished copper-foil and just dip it into the liquid--so, and see, it is slowly becoming covered with an iron-grey metallic film. In order to be quite sure that the coating is not due to accidental impurity, I repeat the experiment with the contents of another stomach which I know is free from any poison, and observe when I dip the foil in there is no deposit. This shows me that both the acid and the copper-foil are pure, and that in the former case the grey deposit was due to arsenic. In order to make doubly sure, I take the coated slip of copper, wash it well in water, then in ether alcohol, and gently heat it in this reduction tube. Now, let us put it under the microscope and tell me what you see." "I see a number of shiny square crystals like little diamonds." "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?"