jaYDROCARBONS. 370 HYDROCHLORIC ACID. of the compounds with increase of their molec- ular weight. The ntetylonca are unsaturated; in fact, 'doubly unsaturated' compounds (see Acetv- lene), as is shown by their tapaeity for taking on broniim- and other elements to form additive products. The acetylene hydrocarbons burn with a very smoky tiamc, unless by some method enough o.ygon is supplied to burn up completely the large amount of carbon they contain. The general formula of the scries is C„H;„_.. Xasccnt hydrogen adds itself readily to tlio acetylene hydrocarbons, yielding still unsaturated hydro- carbons of the ethylene series. Aromatic HYr)ROCABBONS. The principal groups into which these hydrocarbons are sub- divided include the Benzene series, the yaphlha- lene series, and the Anihraeenc scries. The typical formulas representing these three series are. respectively, C„ ir.„_o. C„II:„_,:. and C„H. „_„. Other hydrocarbons, not included in these series, are represented bv the formulas C„H,„.., C„n,„_,„ C„U.„_«, etc.; but these are not important enough to be mentioned here. The most important members of the benzene series are benzene (C«H,), toluene (C,H,), and xylene (CjHio)- The most impurtant substance of the naphthalene series is naphthalene itself {C,;>H,); that of the anthracene series, anthra- cene (C„li,o). All these are described in special articles. Sulhce it to mention here that the chief source of aromatic hydrocarbons is the coal-tar (q.v. ) obtained as a byproduct in the manufac- ture of coal-gas. and that it is prinei|>ally from these hydrocarbons that innumerable artificial (lyestufTs. drugs, etc., are now made on a very large industrial scale. Sec also C.vrbon Com- poiNns. The hydrocarbons should not be confounded with the 'carbohydrates,' like sugar, starch, or cellulose, which contain besides carbon and hy- drogen also oxygen. HY'DROCELE (Lat.. from Gk. vdpuKi/?.l, hy- drokclv, from Ci'cjp, hi/dor, water + Kr/Ai/, ki'lC, tumor). The medical terra for dropsy of the tunica vaginalis, a serous membrane or sac in- vesting the testes. Hyilrocele occurs as a smooth, pear-shaped swelling, fluctuating when pressed, devoid of pain or tenderness, but sometimes caus- ing a slight uneasiness from its weight. The quantity of serous fluid in the sac is usually from (i to 20 ounces, but it occasionally exceeds 100 ounces. Hydrocele may occur as a result of acute inflammation, but it most commonly comes on without any apparent local cause. It is most frequently met with about or beyond the middle period of life, and gcner.TjIy in jx-rsons of feeble power, or with a tendency to gout ; sometimes, however, it occurs in young children, either in the same form as in adults, or as what is termed congenital hi/drocelc, when the communication between the tunica vaginalis and the abdominal peritoneum is not obliterated, as it normally should be. Palliative treatment consists in the use of suspensory bandages, .strapping, and tap- ping with a fine trochar. Tapping seldom gives more than temporary relief, the swelling usually regaining its former bulk in three or four months. A cure is sometimes eTected after tapping by the injecting of irritating substances, such as tincture of iodine and carbolic acid. The only certain method of cure, however, consists in excision of the tunica vaginalis. HYDROCEPH'ALUS (Nco-Lat., from Gk. uSpoKntiaKof, lii/dicil.t plialon, water in the head, from tSSup, htjdur, water -+- K«paX-lt, kepliair, head). A collection of serous fluid in the cra- nium. If it is within the ventricles of the brain, the condition is known as internal hi/drocei>liulus. If beneath the arachnoid membrane, external hydroeephalns. The disease may be primary, being then usually tuberculous in origin, or it may follow sonic other disease, in which case it is termed secondary. Three types, the uculc, the chronic, and the eungenilal, are descrilH'd. Hydro- cephalus may occur in children or in adults as a sccondarj' lesion resulting from meningitis. In such cases, the bones of the skull having imited, there is much less opportunity for distention. Acute llvniiocEruAi.is. This is u?ually a sudden disease, marked by an acute collection of fluid within the skull, internal or external to the brain ventricles. H may be primary or secondary to such diseases as whooping cough, heart disease, rachitis, tumors within the skull- cavity, or acute fevers, such as pneumonia or typhoid. The symptoms are various, much de- pending on its cause. The most characteristic symptoms are those due to the pressure on thi» brain. In addition to the enlargement of the head, there may be much restlessness, inability to sleep, nausea and vomiting, paralysis or con- vulsions. The pupils are apt to be unequal and sluggish. In the .severer cases pressure upon the brain causes death. CiiBOMC Hydrocephalus. This may follow an acute attack, or the disease may begin very insidiously. The symptoms again are those due to pressure. The fontancUes in the skull are apt to bulge, the eyeballs protrude somewhat, the blood-vessels on the outside of the head are apt to be swollen and prominent. Headache is com- mon. A peculiar sidewise shifting movement of the eye, tcriiied nystagmus, is very constant, and crossed eyes are not uncommon, 'ihere is usually much restlessness, and if the patients live they are apt to be dull mentally or idiotic. Some of the.se patients live to middle age. but most of them die young. The treatment requires the greatest medical and surgical skill. Congenital Hvorocephalus. The distention may be so slight at birth as to be scarcely evi- dent, or the distention may be considerable, or so advanced as to preclude the birth of a living child. After birth the progress of the disease may be gradual or rapid. The lateral ventricles usually show the greatest distention, although all the ventricles may be involved as well as the central canal of the cord. The condition of the brain-tissue depends entirely upon the amount of pressure. It may be little difTerent from normal, or the convolutions may be flattened, or the hemispheres may he reduced to a thin biycr under the dura mater. The brain-tissue itself is apt to be soft and flabby, and the base of the brain is usually less atTected than the hemi- spheres. The openings between the bones of the .skull are large in size, the bones thin, and tend- ing to bulge forward, making the forehead promi- nent. HYDROCHARIDE^, hi'drft-ka-rld'ft-e, or HY'DROCHARTDA'CEiE. An order of aquatic plants. Sec allisnkbia. HYDROCHLOrRIC ACID (from hydrogen ■{■ chlorine), or Miriatio .Vcid. HCI. .. gas- eous compound of hydrogen and chlorine, the
