HYDKO-FERROCYANIC ACID. 374 HYDROGEN. solution ul' potussiuni cyanide is boiled with fer- rous hydroxide,, the reaction lakiiij^ place accord- ing to the following chemical equation: OKCX -f Fe(OH), = K<Fe{CX). + 2K0H C.vniildpof I'lrrniiH Potiuwlum Caustic iJotosHluui liydroxlile ferroc.vuulde potash On a large scale, potassium ferrocvanide is made from animal refuse, such as dried blood, hair, horn-shavings, etc. These products are charred and then heated to a high teiiiperature with scrap iron and crude potash, liy this process the potassium carbonate contained in crude potashes combines with the charcoal and nitrogen of the refuse, and yields potassium cyanide; on the other hand, the scrap iron combines with the sulpluir likewise contained in crude potash, forning sulpliide of iron; and when potassium cyanide combines with iron sulphide, potassium ferrocyanide is produced. The chemical trans- formation taking place during this manufactur- ing process may, therefore, be represented by the following equations: 1. KjCO, + 4C + 2N = 2KCN + 3C0 Potassium CarboD Nitrogen Cyanide of Carbon carbonate potassium mon- oxide 2. GKCN -f FeS = K.Fe(CX), -f K,S C.vani<io of Sulphide Potassium PotasHlum potassium of iron ferrocyanide sulphide l^arge quantities of potassium ferrocyanide are at present made also from one of the byproducts of the manufacture of coal-gas. When coal is heated out of contact with air, part of its nitro- gen is obtained in the form of ammonium cyanide. Jn the manufacture of illuminating gas the cyanogen thus produced is for the most part retained by the iron oxide used in purifying the gas. and when tlie iron oxide has become use- less for the latter purpose, it is employed in mak- ing potassium ferrocyanide after another valuable product, ammonium sulpho-cyanate, has been ex- tracted from it with warm water. The mass re- maining behind is mixed with lime and heated with steam; the ealciiim ferrocyanide thus pro- duced is first transformed into potassium-calcium ferrocyanide K;CaFe(('X)o, by boiling with po- tassium chloride; finally, potassium ferrocyanide is obtained by heating the potassium-calcium salt with caustic potash. Potassium ferrocyanide is extensively used for the manufacture of cyanide of potassium and of Prussian blue, as well as for a variety of other purposes. Prussian hJiie, or Berlin blue, ferric ferro- cyanide, Fe,[Fe(CX),l, , is obtained by mixing potassium ferroeyanide with ferric salts, in aqueous solution. HYDROFLUORIC ACID (from /ii/cfro-gen + fluorine). IIF. An acid compound of hydro- gen and fluorine, analogous to hydrochloric, hydrobromic, and hydriodic acids. It may be prepared by the action of sulphuric acid upon cryolite (q.v.) in an appropriate apparatus made of lead or of platinum. It may thus be readily obtained in liquid form, or rather in the form of an aqueous solution. The anhydrous acid may be obtained by heating acid potassium fluoride. As thus prepared, hvdrnfluoric acid is a colorless liquid, boiling at l6.5»C. (67.1°F.). Its vapors are exceedingly poisonous, and the liquid itself, even when mixed with more or less water, causes swellings on the skin that heal with extreme difficulty. Great care must therefore be taken in working with the acid. Solutions of hydro- lluorie acid are used in analytical chemistry for breaking up minerals that are insoluble in strong hydrochloric and sulnhuric acid; another way of breaking up such minerals consists in fusing them with the carbonates of sodium and potas- sium, but then, of cour.se, the umounts of these metals in the mineral cannot be determined, and hence it is usually necessary to treat a separate portion of the mineral with hydrofluoric acid. Jlydrolluoric acid dissolves glass, foniiing hydro- Ituosilicic acid (HjSiF,) with its silica, hence its use for making etchings on glass, the acid being, for this purpose, often replaced by its ammonimu and certain other salts. (See Etching.) The acid is now often kept in bottles made of a wax- like substance called 'ceresinc.' HY'DROFLU OSFLIC'IC ACID. See IIy- DHOFi.rdUic Aciii. HY'DROGEN (from Gk. Hup, hyddr, water "t" -yc'K, •f/(H'".s', producing, from yiyvcadai, ijigncslhai, to become). A gaseous element, dis- covered in 17<i(i by Cavendish, who called it 'inflammable air.' Paracelsus had already ob- tained hydrogen by treating iron with dilute mineral acids. But the existence of gases es.sen- tially difl"erent from air was not generally recog- nized until about the middle of the eighteenth century, and the properties of hydrogen gas were not dclinitely known before the researches of Cavendish. Lavoisier recognized its elementary nature, and gave it its present name. In the free state, hydrogen is found in the exhalations from volcanoes, in the gases issuing from the salt beds at Stassfurt and Wieliczka, in gases given oil' by oil-wells, in the intestinal gases of animals, etc. The spectroscope reveals its exist- ence in the atmosphere of the sun and of many stars, and it has been found 'occluded' in 7neteor- ites. In combination it occurs as water, of which it forms very near one-ninth by weight, and as a constituent of almost all organic mat- ter. It may be obtained by the electrolysis of acidulated wator: by the action of sodium, potas- sium, and certain other metals on water; by passing steam over red-hot iron wire, etc. But the most convenient method, and the one gen- erally emploved. consists in allowing dilute hydrochloric acid, or preferably dilute sulphuric acid, to act on metallic zinc. Owing, however, to the impurities ordinarily present in zinc, as well as in mineral acids, the hydrogen thus ob- tained is impure and has a disagreeable odor. A large percentage of hydrogen is contained in ordinary coal-gas, in which it is formed as a • product of the destructive distillation of coal (organic matter). JIuch hydrogen is also con- tained in ordinary water-gas, in which it is formed by the action of red-hot coal on water- vapor. Hydrogen (svmbol H; atomic weight— either 1, the standard of atomic weights, or 1.01, when the figure 16 for oxygen is adopted as the stand- ard; see Atomic Weights) is a colorless, taste- less, and odorless gas. whose critical temperature is — 234. 5°C.. the critical pressure being 20 at- mospheres (."JOO pounds to the square inch). It was first solidified in 180!), by causing it rapidly to evaporate when in the liquid state. In the gaseous state, it is the lightest substance known, l>eing fourteen and one-half times lighter than atmospheric air. and about 2.'>fl.000 times lighter than the mineral platiniridium. One liter, at
