PRECIPITATION, in chemistry, is an operation in which decomposition Oc5nrs in fluid, either throumh the action of the air, or of a gas, or of a chemical agent in solution; and is accompanied by the deposition of a solid substance that was previously. held in solution. The substance employed to produce the precipitation is called the precipitant, and the substance which is separated by its action the precipitate. For example, if a solution of carbonate of protoside of iron be exposed to the air, a precipitate of hydrated sesqnioxide or peroxide of iron speedily falls; if a current of sulphureted hydrogen gas be passed through a solution of acetate of lead, a black precipitate of sill phide of lead is thrown down from the clear and colorless solution; and if a solution of corrosive sublimate (bichloride of mercury) be added to a solution of iodide of potassium, a yellow precipitate of biniqdide of mercury is thrown down. • The precipitant must be added with caution, as, in many cases, an excess of it re-dissolves the precipitate. In
qualitative analysis—that is to say, in determining the presence of substances without reference to their quantity—the color, solubility, etc., of the precipitate thrown down by numerous tests, as sulphureted hydrogen, solutions of nitrate of silver, iodide of potas sium, ferro-cyanide of potassium, etc., afford the most useful information; and in quanti tative analysis, the amount of precipitate thrown down from a given quantity of a solu tion is often employed to determine the strength of the latter. For example, if a.solu tion of nitrate of silver is added to an ounce of a solution of hydroevanic acid of unknown strength, till no further precipitation ensues, we may readily calculate, from the weight of the white precipitate of cyanide of silver, how much anhydrous hydro cyanic acid was present.