Poison, any substance which, introduced in small quantities into the animal economy, seriously disturbs or destroys the vital functions. Under this head are obviously included a vast number of bodies belonging to the mineral, vegetable, and animal kingdoms, some solid, others fluid, and others gaseous, and deleterious vapors and miasmata imperceptible to the senses. The science which treats of these is designated toxicology. Until of late years the true nature and effects of poisons were little understood, although poisonous mixtures were much used among the ancient Greeks and Romans. What these preparations were we cannot in all cases ascertain, but some of them can be identified with a considerable degree of probability. (See Hemlock.) Among the multitude of substances that rank as poisons are many, some possessing the most active qualities, which are also useful drugs, and which, administered in suitable quantities, are recognized among medicines in universal employment and of the most beneficial character. The difference between a medicine and a poison is frequently a mere question of dose, and the line which divides them is sometimes narrow.

Certain poisons manifest their activity chiefly by their effect upon the alimentary canal, and are styled irritants, although they sometimes have also an action upon the constitution of the blood and secondarily upon the solid tissues. Such are the caustic alkalies, mineral acids, oxalic acid, arsenic, corrosive sublimate, to a certain extent tartar emetic, phosphorus, and many vegetables acting as drastic cathartics, such as scammony, croton oil, gamboge, and various other plants of the order euphorbiacece. Some vegetables when handled, or even by their emanations, give rise to cutaneous eruptions. The manchineel of the West Indies (hippomane maneinella) and our native rhas toxicodendron and rhus venenata, or poison ivy, and poison dogwood, are of this class. The latter are supposed to owe their poisonous properties to the volatile toxi-codendric acid. The number of vegetables acting as poisons upon the nervous system, and secondarily upon the heart and other organs, is enormous. They are often called narcotic or narcotico-Acrid, although their action is by no means always narcotic or stupefying. Such are opium, belladonna, henbane, savin, tansy, conium, tobacco, lobelia, digitalis, aconite, ve-ratrum, Calabar bean, woorara, nux vomica (containing strychnia), and many others less known.

To these may be added alcohol, chloroform, and chloral hydrate. Some volatile poisons appear to act chiefly upon the blood; such are carbonic oxide, illuminating and sewer gas, nitrous oxide, nitro-benzole, and hydrocyanic acid. But the line between this class and the preceding cannot be drawn with accuracy. The action of the poisons of venomous reptiles and insects is less easily understood, from the small quantities usually secreted at a time and obvious difficulties in obtaining it. These poisons are rapidly absorbed from the skin or subcutaneous tissues, but are much less noxious when taken by the mouth, provided no abrasion exists. The same is true of the woorara, curare, or ticunas, the South American arrow poison. The virus or contagions of many diseases, in some instances almost demonstrable, as in smallpox or syphilis, at others hypothetical, as in typhoid, yellow fever, diphtheria, etc, may be with some propriety classed among poisons, and their actions are seen in the symptoms of the special disease developed. - Chronic poisoning may take place from the gradual introduction of small quantities of various deleterious substances.

Notable examples are lead poisoning, occurring in lead workers, type founders, painters, and persons using water standing in lead pipes, or liquor contaminated therewith; mercurial poisoning in dry gilders and manufacturers of looking glasses, and from the medicinal use of the drug; arsenical, from green paper hangings, clothing, etc. Chronic poisoning by tartrate of antimony has probably often taken place for criminal purposes. A considerable degree of tolerance may however be observed with certain drugs. Opium may be taken, by persons accustomed to it, in doses enormously larger than would be fatal to a person not habituated to its use. . Certain persons in Styria and elsewhere are able to use poisonous doses of arsenic not only with impunity, but, as they allege, with benefit. Stille's " Medical Jurisprudence " speaks of a case, on the authority of Dr. Hartshorne, where a man was in the habit of taking five grains of corrosive sublimate every day. The number of substances from which accidental or criminal poisoning frequently takes place is smaller than might be supposed from the above statements.

In France, during 12 years from 1851 to 1862, 26 substances only were employed, and of these the most common were arsenic, phosphorus, sulphate of copper, verdigris, sulphuric acid, and cantharides. In New York in 1872 deaths by accident and suicide took place from opium and its preparations in 20 cases; Paris green (arsenite of copper) in 24 (all suicides); arsenic, 2; carbolic acid, 2; hydrate of chloral, 2; sulphuric acid, oxalic acid, phosphorus, rat poison (probably phosphorus), colchicum, prussic acid, chloroform, ether, yellow wash, wine, each 1; chronic from lead, 5. In medico-legal cases evidence of poisoning depends on the symptoms and on the discovery of a poison. Symptoms alone can seldom if ever prove conclusively the action of a poison, but organic chemistry has made such progress in recent years that criminal poisoning is much less easily concealed than formerly, and the alkaloid or other active principle which caused death can often be actually exhibited in court. Many alkaloids, however, are fatal in such exceedingly small doses that their separation is a work of very great delicacy, and may even be impossible.

In hardly any circumstances is it more important that the so-called expert should be really a competent chemist than in cases of suspected poisoning, and nowhere has an ignorant or careless man a better opportunity to mislead a jury and defeat the ends of justice. - The numerous popular antidotes for snake bites are all alike useless, and thus far science has failed to replace them with any agent deserving the name of antidote. In case of a bite by a venomous serpent, the proper plan is to isolate the part bitten by a ligature, and then if it be a small member, as a finger end, to have it promptly removed, but in any case to take enough of stimulus to carry the weakened heart over the period of depression which belongs to the early stage of venom poisoning. In very grave cases artificial respiration may help to prolong life. - See Antidotes, Medical Jueispeudence, and the articles on the various substances mentioned above.