HYDROMETER. 380 HYDROPHOBIA. HYUBOMETER. represents the weight of the water displaced hy the substiinee, and if divided into the sum of the weiglits removed when the body was placed in the upper pan, will (jive the .-.pciihc gravity. The hydrometer of Fahrenheit is based on a similar principle, but is made of glass instead of metal, and has a bulb filled with mercury at its lower end instead of a weighted cup. It can only I>o used for liquids. These instruments arc not as reliable as the orilinary hy- drometers, and are not as widely usi'd. In the most accurate deter- minations of s|X'cific gravity a chem- ical balance is employed, and equal volumes of the liquid actually weighed, or if the substance is a solid it is weighed both in the air and in distilled water, suitable cor- rections being a])plicd for tempera- ture, and other disturbing inlluences. The hydrometer is without doubt one of the earliest pieces of physical apparatus, its invention being gen- erally ascribed to Archimedes, to whom is due the ])rinciple on which it is based. The instrument is men- tioned by Prisoian. who died about A.M. .500, and it is also described by Syncsius of I'tolemais in a letter to llypatia of Alexandria, under the name of hydroscopiuni, as follows: "It is a cylindrical tube the size of a reed or pipe, a line drawn along it lengthwise which is intersected by others, and these point out the weight of water. At the end of the tube is .a cone the base of which is joined to that of the tube so that both have b>it one base. This part of the instrument is called huriillion. If it be placed in water it remains in perpendicular di- rection so that one can readily discover the weight of the duid." The date of this letter can he approximately fixed by the fact that llypatia was murdered in a.r. 41.5. The use of the hydrometer was known to the Saracens of the tenlli and elevenlh centuries, and one of tlieir writers, Al- Khftzinl, attributes its invention to a Greek phi- losopher named Poppius, a contemporary of Theodosius the (Jreat. In this connection this same writer refers to the fundamental discovert' of Archimedes. HY'DRONI'TRITES. See Hydronitkois Acid. HY DEONI'TROUS ACID, IIX,. An acid gaseous cniiriound of liydrogen and nitrogen first obtained liy (iirtius in 1800. An easy method of prepaiing this acid was subsequently worked out by Angeli, and consists in adding hydrazine hy- drate (see IIvmtAZiNE) to a concentrated solu- tion of silver nitrate, the result being a precipi- tate of silver, hydronitrite, AgXj, from which free hydronitrous acid may be obtained V)V decom- posing with sulphuric acid. The salts of hydro- nitrous arid, or htjdronitrUes, are violently ex- plosive compounds, and their preparation should not be undertaken by inexperienced persons. HYDROP'ATHY (from Gk. f.U./,, hydor, water -+- ■miflnr, pathos, disease). A synonym for hydrotlierajiy. the method of treating dis- ca.ses by external and internal use of water. See HVDBOTIIER.PY. HY'DROPHANE (from Gk. C-Iup, luidfir. water -|- <priv6r, jihanos, clear, from tpmvciv. phaincin, to appear). A translucent variety of opul that is either whiiish or light-colored, and becomes more transparent or translucent in water. HYDROPH'ID.^;. See SeaSnake. HYDROPHIL'IDiE (Neo-Lat. nom. pi., from Cik. tiiu/i, hydOr, water + 0//«f, philos, loving). A family of scavenger beetles, most of wliieh swim in the water or crawl on the sub- merged parts of plants in quiet pools. They are elliptical black beetles with club-shaped anten- nx'. Because of the fact that they carry a film of air on the ventral surface of the body, this part has a silvery appearance. The larvic are carnivorous, but the ailult beetles live eliielly on decaying vegetation. There are one hundre<l and fifty species of this family in the United States. One of the most common species is Ili/drophihix triangularis. A few forms live in moist earth or dung, and subsist upon maggots which in- habit such places. HYDROPH'ILOtrS. A term which has been applii-d piirticuhuly to plants which arc pol- linated tlirougli the agency of water. It has also been apjilied l)y some authors to liydropbylic plants, but should Ix' entirely discarded there, both because of its previous use in another s*'nse and because it is an undesirable word in all senses. The term water-pollinated is preferable in the first sense. See Poi.unation. HY'DROPHO'BIA ( Lat., from Gk. i.V'"*"/", hydropliobiii. from ii^p'Kpipnr , hiidrophohns, dread- ing water, from Mui>, hijdSr, water + (pi'i't'K, phobos, fear), or Kahies, also called Ly.ssa. An acute infectious disease of warmblooded animals produced in man by the implantation of a spe- cific virus through the bite of an animal sick with the disease. This disease has Ix-en known since the earliest historical times. Dcmocritus of .bdcra gave an account of it in the fifth cen- turj- n.c. Aristotle in the fourth century n.c. described it. Celsus, R.c. 21, detailed .some of its symptoms. Xcnophon, Ovid, Vergil, Horace, Plu- tarch, all referred to rabies in their works. Boer- haave (q.v.) and Van Swieten described the di.s- ease intelligently. .John Hunter (q.v.), ^Magen- die (q.v.), Marocbetti, Morgatmi. Trousseau, ami Dupuy all wrote of it. Youatt (1850), the cele- brated veterinarian, considered rabies at length in his books, and relied on early cauterization with nitrate of silver to obviate infection. Yet after being bitten seven times with impunity, he is reported to have died of rabies at the last. Virchow and Von Ziemssen propagated the dis- ease by the hypodermic injection of saliva of a rabid animal, .fter many others had added to our knowledge of hydrophobia, Pasteur (q.v.), in 1882, began to treat the disease with scientific accuracy. The geographic distribution of the disease is wide. It has been found in (Jretmland. where it was epidemic in 18C0; in Constantinople, where it prevailed in 1839; in Eg>-pt, in London, in Hamburg, in Saxony, in Bavaria, and in many parts of the I'nited States, the prevalence of the disease in Washington. D. C. in 1809-1000 rais- ing grave apprehension. In Russia and Siberia very many instances of r.ibid wolves have been recorded. The disease is found most frequently in the dog, tho wolf, the cat, and the cow. The skunk, also, seems to be peculiarly liable to hydrophobia, and the disease in this animal has
