Page:The New International Encyclopædia 1st ed. v. 10.djvu/431

* HYDROGEN. 375 HYDROGEN SULPHIDE. 0° C. and under normal atmospheric pressure (760 millimeters of mercury), weighs .08!)5228'.) gram. In accordance with the atomic and molecular theory, the molecule of ordinary gaseous hydrogen is assumed to consist of two atoms, and is therefore usually represented by the formula H^. The gas burns in the air with a non-luminous flame of very liigh t<?mperaturc, combining with the oxygen to form water. When pure, though it cannot support animal life, hydrogen is not poisonous, and when mixed with a sufficient quantity of oxygen or atmospheric air, may be inhaled for some time without incon- venience; but it weakens the voice, and renders it higl»-pitched. Hydrogen gas is capable of be- ing absorbed by certain metals, such as iron, phitinum, and especially palladium. Graham, who studied this phenomenon, maintained that when tlius 'occluded' hydrogen assumed a true metallic form, and named that form of the ele- ment hydrogenium. Troost and Hautefeuille, too. believed that hydrogen is capable of forming alltti/K with metals, and some of the alloys ob- tained by them were found to have the composi- tion of true chemical compounds ; for example, the hydride of sodium, Na,H, and the hydride cf potassium, KjH. If this is true, then hydro- gen cannot be assumed to be invariably univ- alent in its compounds. ( See Valency. ) At ordinary temperatures, hydrogen is chemically inert toward most of the other elements; with chlorine, however, it combines gradually in dif- fused, and with explosive violence in direct, sun- light. Its mixture with double its volume of oxygen explodes violently if ignited ; the explo- sion is somewhat less violent if instead of two volumes of oxygen, about tw-o and a half volumes of air are mixed with one volume of hydrogen. At elevated temperatures, or when in the nascent state (i.e. while being formed, say by the action of dilute acid on zinc), hydrogen is a powerful reducing agent, i.e. it readily abstracts oxygen from compounds or adds itself to them. Nascent hydrogen is thus often employed in the prepara- tion of organic compounds from given materials. With oxygen, hydrogen forms two different com- pounds: water, H,0, and hydrogen peroxide, HjO,. With sulphur it combines directly, at the boiling temperature of the latter, forming sul- phureted hydrogen, H2S. The ordinary com- pound of hydrogen with nitrogen is ammonia, NH3; another compound of hydrogen and nitro- gen, termed hydrazine, has the formula N;H,. Hydrochloric acid, HCl, is the compound of hydrogen with chlorine, the two elements com- bining, as stated above, under the influence of sunlight at ordinary temperatures. At some- what elevated temperatures, hydrogen similarly comfcines with the vapors of bromine and iodine, forming, respectively, hydrobromic acid, HBr, and hydriodie acid, HI. For the compounds of hydrogen with phosphorus, see Phosphorus. For its compounds with carbon, see Hydrocab- BONS. Among the ordinary uses of hydrogen may be mentioned its use for the production of "high temperatures, as in the ordinary oxy-hydrogen flame. In ordinary illuminating gas, hydrogen acts as a diluent for the light-giving constituents, and its combustion yields much of the heat with- out which- the flame could not be luminous. On account of its great lightness, hydrogen is used to give buoyancy to balloons. HYDROGEN, Sulphubeted. See Sulphu- RETED Hydrogen. HYDROGEN DIOXIDE, Peroxide of Hy- drogen, or Oxygenated W ateb, H,Oi. A colorless compound of hydrogen and oxygen containing, for the same amount of hydrogen, twice as much o.ygen as water. It was discovered in 1818 by the French chemist Thenard. Minute quantities of it occur in the air, in rain-water, and in snow, and, according to some, in the juices ot certain plants. It may be prepared by the action of cold diluted hydrochloric or sulphuric acid on hydraled barium peroxide. It can also be pre- pared by the action of hydrofluoric, hydrottuo- silicic, phosphoric, or compressed carbonic acid on barium peroxide; and it has been prepared on a somewhat large scale by the action of acids on the peroxide of sodium. By any of these methods the peroxide is obtained in the form of a dilute aqueous solution. This may be con- centrated by allowing the water to freeze out, or by passing a current .of dry air through the solu- tion kept at ordinary temperatures, or by allow- ing it to evaporate in a vacuum over sulphuric acid. The anhydrous peroxide thus obtained is a colorless, odorless, oily liquid with a harsh, bitter, metallic taste. It remains unfrozen at — 30° C.,and when heated to the boiling-point of water it decomposes with violence into oxygen and water. When brought in contact with the skin it causes a white blister, which after a time produces an irritating, itching sensation. It is a powerful bleaching agent and is largely used to remove color; it is particularly valuable for bleaching ostrich-feathers, bones, ivory, silver, wood, silk, cotton, etc. Dilute solutions of it under special trade names are employed to pro- duce a light color in hair. It has also been used to restore paintings that have become darkened by age. The peroxide is very useful in medicine and surgery as an antiseptic, rapidly destroying pus and similar discharges, in consequence of which it has been employed in diphtheria. Hydrogen dioxide has the power of freeing water from microorganisms, which has led to its use in brewing. It also destroys the acid and mold ferments in the wort. If taken internally, it has the effect of improving digestion, and has there- fore been used in certain forms of dyspepsia. Ordinarily the peroxide acts as a strong oxidiz- ing agent, owing to the ease with which it parts with half of its oxygen. In certain cases, how- ever, the same property has the efTcct of causing reduction. This happens, namely, whenever the peroxide is brought into contact with substances like oxide of mercury, or ozone, which readily undergo decomposition. The reduction of such substances is accompanied by the evolution of much heat, great volumes of oxygen being given off by the peroxide itself as well as by the sub- stances undergoing reduction. The peroxide itself is readily decomposed into water and free oxygen by mere contact with certain substances, e.g. gold, without these undergoing any change. Among the substances that have this effect on the peroxide are most alkalies, and hence the peroxide can only be preserved in slightly acid .solution. Before using such a solution, a few- drops of ammonia may be employed to neutralize the acid. HYDROGEN SULPHIDE. BETED Hydrogen. See SuLPHU-