PRECIPITATION, that process by which bodies dissolved, mixed, or sus pended in a fluid, are separated from the fluid, and made to gravitate to the bot tom of the vessel: this is one of the great operations in chemistry, and is op. posed to that of solution. In truth the chief operations in the laboratory may be resolved into solution and precipitation. When a base is employed to precipitate a soluble acid, the substance thrown down is always a compound, consisting of the acid united to the base employed. In this case the acid is sometimes completely separated, and sometimes not, according to the energy of the base employed, and the degree of insolubility of the salt form ed. The same explanation aunties as in the first case. When a neutral salt is employed as a precipitant, the substance which falls is always a compound.. It is composed of one of the ingredients of the precipitating sal( united to one ingredient of salt in solution. Such salts alone can be employed as are known to form in soluble compounds with the acid or base which we wish to throw down. In these cases the separation is complete, when the new salt formed is completely insolu ble. Neutral salts perform the office of
precipitants in general much more readily and completely than pure bases or acids. Thus the alkaline carbonates throw clown the earths much more effectually than the pure alkalies, and sulphate of soda sepa rates barytes much more rapidly than pure sulphuric acid. This superiority is owing partly to the combined action of the acid and base, and partly to the com paratively weak action of a neutral salt upon the precipitate, when compared to that of an acid or alkali- For the preci pitation takes place, not because the salts are insoluble in water, but because they are insoluble in the particular solu tion w which the precipitate appears. Now if this solution happens to be capa ble of dissolving any particular salt, that salt will not precipitate, even though it be insoluble in water. Hence the reason why precipitates so often disappear; when there is present in the solution an excess of acid, of alkali, &c.