_Fatal Dose (Smallest)._--Two grains. Exceptionally, recovery from very large doses if rejected by vomiting.
_Fatal Period (Shortest)._--Twenty minutes. Exceptionally, death as late as the sixteenth day. The effects of arsenic are modified by tolerance, some persons being able to take considerable quantities. The peasants of Styria are in the habit of eating it.
_Method of Extraction from the Stomach._--The coats of the stomach should be examined with a lens for any white particles. These, if present, may be collected, mixed with a little charcoal in a test-tube, and heated. If arsenic is present, a metallic ring will be formed in the cooler parts of the tube. If this ring be also heated, octahedral crystals of arsenic will be deposited farther up the tube, and are easily recognized by the microscope. The contents of the stomach, or the solid organs minced up, should be boiled with pure hydrochloric acid and water, then filtered. The filtrate can then be subjected to Marsh's or Reinsch's process.
_Tests._--In _solution_, arsenic may be detected by the liquid tests. (1) Ammonio-nitrate of silver gives a yellow precipitate (arsenite of silver). (2) Ammonio-sulphate of copper gives a green precipitate (Scheele's green). (3) Sulphuretted hydrogen water gives a yellow precipitate.
_Marsh's Process._--Put pure distilled water into a Marsh's apparatus with metallic zinc and sulphuric acid. Hydrogen is set free, and should be tested by lighting the issuing gas and depressing over it a piece of white porcelain. If no mark appears, the reagents are pure, and the suspected liquid may now be added. The hydrogen decomposes arsenious acid, and forms arseniuretted hydrogen. The gas carried off by a fine tube is again ignited. A piece of glass or porcelain held to the flame will have, if arsenic be present, a deposit on it having the following characters: In the centre a deposit of metallic arsenic, round this a mixture of metallic arsenic and arsenious acid, and outside this another ring of arsenious acid in octahedral crystals. The deposit is dissolved by a solution of chloride of lime, turned yellow by sulphide of ammonium after evaporation; on the addition of strong nitric acid, evaporated and neutralized with ammonia and nitrate of silver added, a brick-red colour is produced--arseniate of silver.