When he finds a poison belonging to a certain group, he has still to ascertain which of five or six bodies it is.
For instance, after adding the first two test-liquors, if he sees a yellow coloration or precipitate, he knows that he has either arsenic or tin or cadmium. He then adds some strong ammonia, after boiling the liquid till the smell of rotten eggs has disappeared. If the powder dissolves, and the colour goes, he is quite sure he has found arsenic.
In this business-like way the murderer is convicted.
But now arises the necessity for making doubly sure, and another kind of test altogether is employed. Life and death hanging on the result, the test must be beyond all doubt. But arsenic is one of those self-assertive things about whose presence there cannot be the most infinitesimal doubt. Give a man a particle the size of a mustard-seed, and let him swallow it. When he dies bury him, and let him lie under the earth for a quarter of a century. Then gather the few remnants, give them to a chemist, and he will return you a considerable portion of the poison in the same state as that in which it was administered.
ARSENIC CRYSTALS.
Probably the most famous special test for arsenic is Marsh's, the invention of a Woolwich chemist, and equally famous is Reinsch's, which is performed as follows: The suspected liquid is put in a little glass test-tube with some hydrochloric acid. Then a small bit of bright copper is dropped in, and the test-tube is held over a flame.
Now, arsenic has the wildest love for copper, and every trace of it in the tube flies to the slip of copper and covers it with a grey coat. Another metal does the same, certainly, but they can be distinguished subsequently.
Presently the copper is removed, washed, dried, and placed in a tough glass tube, very narrow at one end. This is held over a flame and carefully heated, and then a phenomenon, not unknown, either, in the loves of mortals, occurs. The arsenic abandons the copper, and clings in crystals to the sides of the glass tube, where it can be recognised by the aid of a magnifying-glass or microscope; and if the crystals are heated with a bit of acetate of potash the odour drives the chemist from the room.
To this curious fact, that arsenic loves copper when it is wet with warm hydrochloric acid, and hates it when it is hot and dry, is due the discovery of many a crime.