H-SDROCYANIC ACID. 3"-2 HYDRODYNAMICS. applied; the solution is now acidilied with liydro- cliliiiie luid, and some ferriu ehlnride is lidded. If h.vdroejunic acid was present in the original solution, a precipitate of Prussian blue, or at least a blue coloration, is produced. (2) A few drops of aMiiiioniuni sulphide are lidded to the susjiected liquid, and the nii.xture is evaporated un the water-bath; the dry residue is moistened with ferric cliloridc. wlien, if liydro- cyanic acid was present in the original solution, an intense red coloration is produced. The quantity of hydrocyanic acid in a solution may be determined by adding to the latter a solution of silver nitrate, separating the precipi- tated cyanide of silver by filtration, washing, drying, and weighing. The amount of hydro- cyanic acid may be readily calculated from the weight of silver cyanide found. We are indebted to the Italians for the introduction of hydro- cyanic acid in the materia medica. There are nc cases in which it is so serviceable as in those alfections of the stomach in which pain is a leading symptom, as in gastrodynia, water-brash, and in cases of intense vomiting. It is also use- ful in allaying spasmodic cough, and has been emploj'cd with advantage in chronic skin- diseases, to allay pain and irritation, no matter what the cause. A mixture of one part of the dilute acid (of 2 per cent, strength) with about 48 parts of water forms a good lotion. Hydrocyanic acid is one of the most energetic poisons, and is frequently employed both in mur- der and suicide. A single drop of the pure acid causes instant death if placed inside the eye. When a small poisonous dose (about half a dram of the 2 per cent, acid) has been taken, the first symptoms are weight and pain in the head, with confusion of (liought. giddiness, nause.t (ami .sometimes vomiting), a quick pulse, and loss of muscular power. If death result, this is preceded by tetanic spasms and involuntary evacuations. When a large dose has been taken (as from half an ounce to an ounce of the 2 per cent, acid), the symptoms may commence instantaneously, rnd it is seldom that their appearance is delayed beyond one or two minutes. The patient is in- sensible; his eyes are fixed, the pupils are dilated and unaffected by light; the limbs are flaccid, and the skin is cold and clammy; the respiration is slow and convulsive, and the pulse is imperceptible. Hydrocyanic acid causes paralysis of the heart, of the respiratory M-ntre, and of tlie vasomotor centre in the medulla. The spinal cord. too. is paralyzed shortly before death. The immediate cause of death is. in most cases, obstruction of the respiration ; but in some cases it lies in stop- page of the heart's action. Where the fatal action is so rapid antidotes are of comparatively little value. Chlorine, am- monia, peroxide of hydrogen, cold affusion, and artificial respiration are the most important agents in the treatment. Cold affusion on the head, neck, and down the spine is a valuable remedy, and it is asserted that its efficacy is almost certain when it is employed before the conviIsive stage of poisoning is over, and that it is often successful even in the stages of insen- sibility and paralysis. One-fiftieth grain of atro- pine should be injected subcutaneously. Artifi- cial respiration should never hp oniittrd. Among the compounds of hydrocyanic acid may be mentioned the cyanides of potassium and mer- cury. The more important ferrocyanides and fcrricyanides are described in the articles on liydro-ferrocyanic and hydro-ferricyanic acids (qq.v.). I'otassium cyanide is an extremely poisonous white crystalline substance soluble in water, but insoluble in absolute alcohol. It may be melted without undergoing chemical decomposition; its aqueous solutions, however, readily decompose on boiling. It is made on a large scale by heat- ing yellow ]>russiate of potash (potassium ferro- cyanide), the decomposition of the latter taking place according to the following equation: K.Fe(CN'), = 4KCX + FeC, -f N, Potassium Potassium Iron ferrocyanlde cyautde carbide The cyanide is separated from the carbide of iron formed along with it, by filtering through hot porous crucibles. Potassium cyanide i.s largely employed in extracting gold and in electroplating. In the molten .itate it acts as a powerful reducing agent, readily setting free metals from their oxides; for example, lead oxide is reduced by it according to the following equa- tion: PbO -f KCN = Pb -f KCNO Mercuric Cyanide, Hg(CX)j. may be obtained by heating potassium ferrocyanide with mercuric sulphate in water, or by dissolving mercuric oxide in aqueous hydrocyanic acid. It is a color- less substance of a bitter metallic taste, and is moderately soluble in water and in alcohol. When heated it decomposes into metallic mer- cury and cyanogen gas. It is very poisonous, but is. in highly dilute solution, sometimes use- ful in syphilis and diphtheria. HYDRODYNAMICS (from Gk. Mup. hy- dOr, water -j- I'liia/xi^, dynamis, power, from iii'aaSiu, dynasthiii, to be able). Strictly speak- ing, that branch of mechanics which is the ap- plication of dynamics to liquids; it should there- fore iiiclud.T both the statics and the kinetics of liquids; that is, a study of their properties both when they are in equilibrium and wlien rot. In general, however, the former phenomena are treated separately under hydrostatics (q.v.), leaving for hydrodynamics simply the phenomena of kinetics. Further, although gases arc easily compressed, while liquids are not. yet, if a gas is flowing slowly and without great fivictuations, it will have properties closely resembling those of a flowing liquid. Consequently hydrody- namics also includes most of the phenomena of the kinetics of gases. If a liquid is flowing regularly through a tube of an irregular cross- section, or if a gas is flowing slowly and regu- larly through such a tube, the mass of the fluid which passes each point in the tube in a given time must be the same; otherwise there would be a state of compression or rarefaction some- where in the tube; it follows, then, that where the tube is narrow the velocity must be large, and conversely, like a river flowing first through a lake and then through a narrow channel. If the velocity at any point in the tube is greater than at another, it shows that there must be a force acting in the direction from the second point toward the first, so as to increase the velocity of the moving fluid; hut the force must always be produced by a fall in pressure, and so the pressure at the second point is greater than at the first. It follows, then, that in a fluid
