H YDROC EPH ALUS 97 HYDROIDA bon, with atoms of hydrogen. Carbon is a tetrad element, capable of uniting with four atoms of H, as CHi methane. But carbon atoms can unite with each other, by one or more pair of bonds. Hydrocarbons having the C atoms united by only one pair of bonds are called par- affins, Cn:H2n-f 2:, as ethane. Hydro- carbons having two C atoms united by two pairs of bonds are called olefines Cn H::n:, as ethene. By three pairs of bonds, are called acetylenes CnH^n-^ as acetylene. Besides these there are hy- drocarbons belonging to the aromatic series, having the carbon atoms ar- ranged to form a closed ring, as Ben- zene, CoHo (g. v.), and the naphthalene series containing two closed rings. The constitution of the other groups of hy- drocarbons is described under their re- spective names. HYDROCEPHALUS, a form of dropsy in the ventricles of the brain, or in the arachnoid cavity, in children, and usu- ally rapidly fatal, though cases have been known to go on to adult life. HYDROCHLORIC ACLD, (HCI), a gaseous compound of equal volumes of hydrogen and chlorine. It is evolved during volcanic eruptions, and is found in the water which collects in the crev- ices of mountains, as well as in rivers which take their rise in volcanic forma- tions, especially in South America. It may be produced by decomposing com- mon salt with sulphuric acid (2Na CI + H=SO. = 2HCl + Na.S04), or by bring- ing equal volumes of chlorine and hydro- gen together and exposing the mixture to diffused daylight with condensation, as it explodes in direct sunlight. HYDROCYANIC ACID, cyanide of hydrogen, hydric cyanide, hydrogen cy- anide, prussic acid. Discovered by Scheele in 1782. HYDRODYNAMICS, a department of science which treats of the nature of liquids in motion, as opposed to hydro- statics, which investigates the condition of their equilibrium when at rest. HYDRO-ELECTRIC MACHINE, a mechanism in which electricity is gen- erated by the friction of steam against the sides of orifices through which it is allowed to escape under high pressure. HYDROFLUORIC ACID, hydrogen fluoride, fluoride of hydrogen, hydric fluoride, HF. The anhydrous acid is ob- tained by neutralizing in a platinum dish the aqueous solution of hydrofluoric acid with caustic potash, and evaporating the solution. HYDROGEN, a monatomic metallic element, which exists in the state of gas. Symbol H; at. wt., 1; density, 0.06927; air being 1; weight of a liter of hydro- gen, 0.0896 grammes, called a crith. Hy- drogen can be obtained by the electroly- sis of water, H^O, the H being liberated at the platinum pole; by the action of metallic sodium and water; also by pass- ing steam over iron filings, 3Fe 4-4HOH = FenOi 4- 8H'; by boiling zinc with caustic potash, 2KH0 + Zn = Kc ZnO: + 2H'; but H is generally made by the action of dilute H2S0i on zinc. Hydro- gen can be purified by passing it through a solution of two parts of KjCrsOT in 20 parts of water and one part of H,SO„ then washing it with KHO, and drying it by passing it through concentrated H.SO,. HYDROGEN DIOXIDE, hydrogen peroxide, H^Os, or HO — OH. Obtained by the action of dilute sulphuric acid and barium peroxide, BaO;. HYDROGEN PEROXIDE (H,0.), a compound of hydrogen and oxygen, con- taining a larger proportion of oxygen than water, the other compound of these elements. HYDROGEN SULPHIDE, H.S, sul- phide of hydrogen, hydric sulphide, sul- phydric acid, hydrosulphuric acid, sul- phuretted hydrogen. A gas obtained by passing hydrogen through sulphur, or by burning sulphur vapor in hydrogen. HYDROGRAPHY (hl-drog-ra-fe) , a branch of science which deals with the measurement and description of the seas, lakes, rivers, and other waters, as used for purposes of commerce or navigation; the art of marine surveying and of the construction of charts. HYDROMECHANICS, a term some- times used to include hydrodynamics and hydrostatics, as also hydraulics, or the department of engineering which deals with the application of liquids in motion to machinery. Hydromechanics is some- times limited to the latter department alone. On the other hand, hydraulics is sometimes made to cover hydrodynamics. HYDROIDA (hi-dro'e-da), in zo- ology, a typical sub-class of Hydrozoa. They have an alimentary region or poly- pite provided with an adherent disk or hydrorhiza and prehensile tentacles. Unlike hydra, the type of the sub-class, most of the hydroida live in societies, each of which constitutes a polypidom so like a seaweed that it is often col- lected as such. But while the apertures on an ordinary seaweed are only minute pores, the Hydroida have little cup-like
