762 POTASSIUM and in the composition of fireworks. The fol- lowing composition is applied to the interior of percussion caps : chlorate of potash 26, nitre 30, fulminate of mercury 12, sulphur 17, ground glass 14, gum 1 = 100 parts. Chlorate of potash is extensively used as an oxidizing agent in heightening the intensity of steam colors on printed goods, and in the chemical laboratory as a source of oxygen. (See OXYGEN.) It is not adapted to the manufacture of gunpowder, as it is too violently explosive, endangering the bursting of the gun, and is also liable to become ignited by friction. (See EXPLOSIVES.) It is one of the most valuable agents of the materia medica, and is employed in the treat- ment of scurvy, rheumatism, pseudo-membra- nous diseases, as croup, diphtheria, and scarlet fever, and as a prophylactic in mercurial sali- vation. Internally it may be given in doses of from 5 to 30 grains three or four times a day, but is generally used in much smaller quanti- ties ; as a gargle in diphtheria, from half an ounce to an ounce may be dissolved in a pint of water. 12. Several salts (phosphates of po- tassium) result from the union of potassium with the different forms of phosphoric acid (see PHOSPHORUS), which are not of sufficient general interest to warrant a description. Potassium forms important salts in combina- tion with cyanogen (ON), and ateo with this radical and other elements. 13. Cyanide of potassium, KON or KCy, is formed by burn- ing potassium in cyanogen gas, and also when the vapor of hydrocyanic acid is passed into a cold alcoholic solution of potash ; but it is usually prepared from ferrocyanide of potas- sium. Eight parts of this salt are mixed with three of carbonate of potash and heated to redness in a covered iron crucible. The iron constituent separates in the form of a metallic powder and settles to the bottom of the cru- cible, and the fused cyanide being poured off solidifies to a milk-white mass. Potassium cy- anide is also produced in considerable quan- tities in blast furnaces in which ores are smelted with coal or coke. It crystallizes in anhydrous cubes or octahedrons, which deliquesce in the air and are very soluble in water. It is a pow- erful reducing agent ; the oxides of many of the rnetals, when thrown into the melted salt, are readily reduced to the metallic state, cya- nate of potassium being formed. On account of this property it is useful in removing stains of metallic oxides, as indelible ink and the juices of fruits. With many of the metallic salts it forms precipitates of cyanides, which are generally soluble in excess of potassic cya- nide, forming crystallizable double salts. Thus, when it is added to a solution of nitrate of silver, cyanide of silver is precipitated, which being washed and treated with another portion of the potassic cyanide, there is formed a solu- tion of a double cyanide of silver and potas- sium, AgKCya. A similar solution of gold and of other metals may be formed in the same way, and these are in common use in electro- plating. (See GALVANISM, vol. vii., p. 600.) 14. Ferrocyanide of potassium, potassic fer- rocyanide, or yellow prussiate of potash, is an important salt, met with in commerce nearly pure, arid is the source from wljich the cyano- gen compounds are usually obtained. It may be formed by digesting ferrous hydrate in a solution of potassic cyanide ; but it is made on a large scale by heating azotized matter to red- ness with potassic carbonate and iron. Dried refuse animal matters, as blood, horns, and parings of hides, are mixed with an equal weight of crude commercial potash, which contains sulphate of potash and about one third as much iron filings, and heated to redness in a large iron retort from which the air is ex- cluded. The melted mass is then treated with hot water, which completes the chemical com- bination. The reactions by which the salt is produced are complicated. Cyanogen is first formed by the reaction of the potash and iron on the azotized matter, and the cyanogen then combines with the potassium, forming potassic cyanide. A quantity of iron is at the same time converted into sulphide from the action of potassic sulphide, which is produced by the action of the sulphate of potash contained in the crude potash. When the mass is treated with water the potassic cyanide attacks both the metallic iron and the sulphide of iron, forming potassic ferrocyanide, which is ob- tained from the filtered liquor by evaporation in large transparent lemon-yellow crystals, derived from an octahedron with a square base, with easy cleavage parallel to the base, hav- ing the formula K 4 FeCy 6 + 3H 2 O. They dis- solve in four parts of cold and in two parts of boiling water, but are insoluble in alcohol. Exposed to a gentle heat, the three equivalents of water are expelled. Raised in a closed cru- cible to a high heat, potassic cyanide, iron car- bide, nitrogen, and other gaseous products are formed. Potassic ferrocyanide is one of the most valuable of chemical reagents. With neutral or slightly acid salts of the heavy met- als it forms precipitates having characteristic colors, the potassium being generally displaced by the new metal. It is not poisonous, and is sometimes used in medicine as a sedative tonic. When it is distilled with sulphuric acid, an abundance of hydrocyanic (prussic) acid is pro- duced. It is largely used in dyeing and calico printing in connection with certain mordants upon which the color produced depends. It is used in the preparation of Prussian blue, which is the dye formed when cloth mordanted with an iron salt is treated with a solution of po- tassic ferrocyanide. With ferrous salts (proto- salts of iron) potassic ferrocyanide produces a white precipitate of potassic-ferrous ferrocy- anide (KaFeaOye), which on exposure to the air is converted into Prussian blue or ferrocy- anide of iron. Prussian blue is more directly produced by adding potassic ferrocyanide to a ferric salt (per-salt or iron) ; a deep blue pre- cipitate is immediately formed, consisting of
