HYDRODYNAMICS HYDROGEN 111 proposed, but has not been proved to be of much value as an antidote. After death and before decomposition has taken place, the pres- ence of hydrocyanic acid is rendered apparent in the blood vessels and also in the brain by its peculiar odor. To obtain the acid, the con- tents of the stomach should be washed with distilled water and filtered, and the filtrate dis- tilled in a water bath. The product may then be subjected to the various tests given in the chemical works. The therapeutic value of hydrocyanic acid is limited chiefly to a few nervous affections of the stomach, to the vom- iting of pregnancy, and to whooping cough and spasmodic derangements of the respiratory organs. Only the dilute form is used medi- cinally, of which the dose varies from two to five or six drops. llMil'.ni) VMilU s. See HYDROMECHANICS. HYDKOFLIOKIC ACID. See FLUORINE. HYDROGEN (Gr. Map, water, and yewdetv, to produce), an elementary gaseous body, named from its property of forming water by com- bining with oxygen. Its symbol is H ; chemi- cal equivalent 1 ; weight compared with air 0'06926 ; 100 cubic inches weigh under ordinary pressure and temperature 2'14 grains, being 16 times less than an equal volume of oxygen, and 14-4 times less than air. One litre of hydro- gen gas at C. and 760 mm. pressure weighs 0'08936 gramme. It was known near the close of the 17th century, and was termed in- flammable air from its burning with a flame ; it was also called phlogiston, from the suppo- sition of its being the matter of heat. Its real nature was first described by Cavendish in 1766. The gas is not found uncombined, but is readily obtained by decomposing water, of which it constitutes about one ninth by weight, the remainder being oxygen. This process is effected very much as metallic oxides are de- composed, some substance being presented to the compound which has a strong affinity for the oxygen, and combining with it liberates the hydrogen or other element. The vapor of water passed through an iron tube filled with iron shavings and kept at a red heat is thus decomposed, the oxygen uniting with the iron, and the hydrogen escaping. The common method of preparing the gas is to place some bits of zinc in oil of vitriol or sulphuric acid di- luted with five or six times its bulk of water. Chemical action immediately takes place, and the zinc is dissolved with effervescence, owing to the bubbles of hydrogen separating from the liquid. The reaction is represented by the formula Zn + HjSOj = ZnSO 4 + H. With an ounce of zinc there may be obtained 615 cubic inches of hydrogen. A common flask answers very well for the apparatus, by in- serting a bent tube through the cork for the exit of the gas, and ,1 straight tube, termi- nating above in a small funnel, and reaching below the cork nearly to the bottom of the flask, at least so as to be covered by the liquid. Through this tube the acid is poured 421 VOL. ix. 8 in as required, the zinc and water being first introduced. The sulphur and carbon which are present in almost all zinc appear in the hy- drogen as traces of sulphuretted hydrogen and carbonic acid. They may be separated by agi- tating the gas with lime water. When pure, hy- drogen has neither taste, smell, nor color. It is destructive to animal life when inhaled for a short time, and extinguishes a burning taper plunged into it. Yet it is itself highly com- bustible, burning with a faint bluish yellow flame at its contact with atmospheric air or oxygen ; and when mixed with proper propor- tions of ether and ignited by flame, an electric spark, or a glass rod heated hardly to redness, its combustion is instantaneous and explosive. A piece of spongy platinum introduced into the mixture also causes combustion to take place. The most violent effects are produced by a mix- ture of two volumes of hydrogen and one of oxygen. The only product of the combustion of hydrogen is water. The gas is made to en- ter into combination with the oxygen of the air, producing heat sufficient to cause its igni- tion, by directing a jet of it upon a piece of spongy platinum, or even upon a perfectly clean surface of sheet platinum. The metal becomes red hot, the gas ignites, and thus a light may be instantaneously obtained. A little apparatus was devised for this purpose by Prof. Dobe- reiner, which would be an excellent means of obtaining a flame in the absence of the cheap matches in common use. Though the flame of hydrogen is very slightly luminous, a bright light is emitted from the heated platinum ; and an apparatus based on this principle has been applied to purposes of illumination in the place of ordinary gas lights. Such lights were at one time in practical use in France and England. The hydrogen was produced by the decompo- sition of water, effected by passing its vapor over incandescent charcoal contained in a tube ; some carbonic oxide and carburetted hydrogen were generated, which burned with the hy- drogen, the jet of mixed gases being direct- ed against a basket constructed of fine gauze of platinum, which became intensely hot and highly luminous. Hydrogen produces intense heat by its combustion, taking up more oxygen than is required by the same weight of any other combustible. It is this property that has led to its application in the oxyhydrogen blow- pipe for melting the most refractory substances. (See BLOWPIPE.) The levity of hydrogen early suggested its use for filling balloons. The quan- tity required to fill one of the capacity of 2,000 cubic feet would weigh only 10'57 Ibs., while the same volume of air would weigh 153-26 Ibs., giving an ascensional power of 142'69 Ibs. Illu- minating gas is heavier, but is commonly used instead of hydrogen only on account of its greater cheapness. Hydrogen is so subtle and penetrating a gas that it passes with facility through paper and also through gold and silver leaves. A stream of the gas directed against one side of the leaf may be ignited on the
