_Fatal Period._--Four hours; more commonly two to four days. _Detection of Phosphorus in Organic Mixtures._--Mitscherlich's method is the best. Introduce the suspected material into a retort. Acidulate with sulphuric acid to fix any ammonia present. Distil in the dark, through a glass tube kept cool by a stream of water. As the vapour passes over and condenses, a flash of light is perceived, which is the test. XXI.--ARSENIC AND ITS PREPARATIONS =Arsenic= is the most important of all the metallic poisons. It is much used in medicine and the arts. It occurs as metallic arsenic, which is of a steel-grey colour, brittle, and gives off a garlic-like odour when heated; as arsenious acid; in the form of two sulphides--the red sulphide, or realgar, and the yellow sulphide, or orpiment; and as arsenite of copper, or Scheele's green. It also exists as an impurity in the ores of several metals--iron, copper, silver, tin, zinc, nickel, and cobalt. Sulphuric acid is frequently impregnated with arsenic from the iron pyrites used in preparing the acid. It is a constituent of many rat pastes, vermin or weed killers, complexion powders, sheep dips, etc. =Arsenious Acid= (White Arsenic, Trioxide of Arsenic).--Colourless, odourless, and almost tasteless. It occurs in commerce as a white powder or in a solid cake, which is at first translucent, but afterwards becomes opaque. Slightly soluble in cold water; 1 ounce of water dissolves about 1/2 grain of arsenic. Fowler's solution is the best-known medicinal preparation of arsenic, and contains 1 grain of arsenious anhydride in 110 minims. _Symptoms._--Commence in from half to one hour. Faintness, nausea, incessant vomiting, epigastric pain, headache, diarrhoea, tightness and heat of throat and fauces, thirst, catching in the breath, restlessness, debility, cramp in the legs, and convulsive twitchings. The skin becomes cold and clammy. In some cases the symptoms are those of collapse, with but little pain, vomiting, or diarrhoea. In others the patient falls into a deep sleep, while in the fourth class the symptoms resemble closely those of English cholera. The vomited matters are often blue from indigo, or black from soot, or greenish from bile, mixed with the poison. Should the patient survive some days, no trace of arsenic may be found in the body, as the poison is rapidly eliminated by the kidneys. In all suspected cases the urine should be examined. The symptoms of _chronic_ poisoning by arsenic are loss of appetite, silvery tongue, thirst, nausea, colicky pains, diarrhoea, headache, languor, sleeplessness, cutaneous eruptions, soreness of the edges of the eyelids, emaciation, falling out of the hair, cough, haemoptysis, anaemia, great tenderness on pressure over muscles of legs and arms, due to peripheral neuritis, and convulsions.