Poisons.  A poison may be defined as “any substance or matter which, when introduced into the body in any way, can destroy life by its own inherent qualities without acting mechanically.”  Poisons are sometimes classified according to their source as mineral, vegetable and animal; or, more generally, according to their action as irritants, narcotics and narcotic irritants.  The irritants, when taken in ordinary doses, produce intense vomiting and purging and severe abdominal pain.  They act directly upon the stomach and intestines, which they inflame and frequently corrode.  The narcotics act specially on the brain and spinal cord.  Among their most common symptoms are headache, giddiness, obscurity of sight, stupor, loss of muscular power, convulsions and finally complete coma.  They have no acrid, burning taste, and, except a slight fullness of the cerebral vessels, they leave no clearly marked postmortem appearance.  The narcotico-irritants have a mixed action.  At varying periods after they have been swallowed they produce stupor, coma, paralysis and convulsions, owing to their effect on the brain and spinal marrow.  As examples of irritant poisons we may mention sulphuric, nitric and oxalic acids (in doses of half an ounce or more), carbolic acid, arsenic, corrosive sublimate, blue vitriol, lunar caustic and chloride of antimony.  Among the narcotics are opium, belladonna, alcohol, ether, India hemp, chloral and chloroform.  The narcotico-irritants include prussic acid, strychnine, nux vomica, hemlock, aconite and nicotine (the active principle of tobacco).  The evils resulting from the use of alcohol in its various forms have caused laws restricting or prohibiting the sale of intoxicating liquors to be passed in most of the states of the Union; and, as a preventive of the evils resulting both from alcohol and tobacco nearly all the states have laws providing for instruction in the public schools as to the effect of narcotics upon the brain and nervous system.