POISONS, those substances which, when applied externally, or taken into the human body, uniformly cause such a derangement of the animal economy as to produce disease. As it is extremely dif ficult, however, to give a definition of a poison, the above is subject to great in accuracy. Poisons are divided, with re spect to the kingdom to which they be long, into animal, vegetable, mineral, and vaporous poisons. Poisons are only de leterious in certain doses ; for the most active, in small doses, form very valuable medicines. There are, nevertheless, certain poisons, which are really such in the small est quantity, and which are never adminis tered medicinally, as many of the animal poisons, the poison of hydrophobia, &c. There are likewise substances which are innocent when taken into the stomach, but which prove deleterious when taken into the lungs, or when applied to an abraded surface. Thus carbonic acid gas is con tinually swallowed with fermented liquors, and thus the poison of the viper may be swallowed with impunity; whilst inspir lug carbonic acid instantly destroys, and the poison of the viper inserted into the flesh produces formidable effects. Many substancei also act as poisons when ap plied either externally or internally, as arsenic, lead, &c. When a deleterious substance produces its effects, not only in mankind, but in all other animals, it is distinguished by the term common poison, as arsenic, caustic, alkali, &c. whilst that which is poisonous to man only, or to brute animals, and often to one genus only, is said to be a relative poison ; thus, aloes is said to be poison ous to dogs and wolves ; the phellandri um aquaticum kills horses, whilst oxen devour it greedily and with impunity. It appears, then, that substances act as poi sons only in regard to their dose, the part of the body they are applied to, and the subject on which their powe'rs are exert ed.
It is often of great importance to be able to discover, by certain chemical tests, copper and lead, particles of which frequently find their way into the stom ach, either through inadvertencies, as by the employment of certain kitchen uten.
sils made of these materials, or by fraud, as when acetate of lead (sugar of lead) is made use of to revive wines that have grown sour by long keeping. If copper be suspected in any liquor, its presence may be ascertained by adding to it a so lution of pure ammonia, which will strike a beautiful blue colour. If the so lution be very dilute, it may be concen trated by evaporation ; and if it contain a great excess of acid, as in the liquor of pickles, so much alkali must be added as will be sufficient to saturate the acid.
Lead is affirmed by Pr. Lamb to exist in water that passes through leaden pipes, in such quantities as to be injurious to the human frame ; this has, however, been much doubted ; but it is well known that petty dealers in wine have occasion ally recourse to the acetate of lead to re vive bad wines. Lead may be discovered in water, by adding to a portion of it about half its bulk of water, impregnated with sulphuretted hydrogen gas. if lead be present, it will be manifested by a dark brown, or blackish tinge. For discover. ing the presence of lead .in wine, a test is employed, called, from the name of the inventor, Ifahnemann's wine test. This is prepared by putting together into a small phial, sixteen grains of sulphuret of lime prepared in the dry way, and twen ty grains of acidulous tartrite of potash (cream of tartar). The phial to be filled with water and well corked, and occa sionally shaken for a few minutes. When the powder has subsided, decant the clear liquor, and preserve it in a well stopped bottle for use. The test, when newly prepared, discovers lead by a dark coloured precipitate. Lead may be like wise discovered by adding to the wine a solution of the sulphate of soda, which will throw down a precipitate. If a large quantity of the acetate of lead has been taken, as by a child, inadvertently, on ac count of its saccharine taste, an active emetic must first be given, and then the hydro sulphuret of potash, or of ammonia, be taken ; a solution of the common sul phuret will answer.