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LEFT HYPOPHOSPHOROUS ACID 103 HYBCANIA gestive organs, and have also been found of benefit in consumption. HYPOPHOSPHOBOUS ACID, H3: PO2, hydric hypophosphite. It can be obtained as a barium salt. By boiling phosphorus with barium hydrate, phos- phuretted hydrogen escapes as a gas. HYPOPHYSIS (-pof'i-sis), a disease of the eyelids, when hairs grow so much as to irritate and offend the pupil. HYPOSTASIS, in alchemy, the prin- ciple or element of anything; specifically, mercury, sulphur, and salt, which the alchemists deemed the principles or ele- ments of all material bodies. In botany, the name given by Dutrochet to what is more commonly called the suspensor or suspensory cord of the embryo of an ovule. HYPOSULPHITES, salts of hypo- sulphurous acid. Among the most im- portant are the hyposulphites of sodium and calcium, the former of which is used in medicine as an external remedy in parasitic skin disorders and an internal one in checking fermentation in zymotic diseases. It is variously used in bleach- ing, photography, and other arts as an antichlore, a dissolvent of bromide and iodide of silver, etc. HYPOTHECATION, in civil law, an engagement by which the debtor assigns hia goods in pledge to a creditor as a security for his debt, without parting with the immediate possession; differing, in the last particular, from the simple pledge. In commerce, the pawning of a ship for necessaries, or to raise money in some critical emergency. HYPOTHENUSE, or HYPOTENUSE, the name given to that side of a right- angled triangle which subtends, or is opposite to, the right angle. Its prop- erty — that the square described on it is equal to the sum of the squares described on the other two sides — is demonstrated and generalized, so as to appl^ ^0 any figure in Euclid. The discovery of this property is attributed to Pythagoras. HYPOTHESIS, etymologically a sup- position, is popularly used to denote something not proved, but assumed for the sake of argument. In scientific and philosophical usage it denotes either a probable theory of phenomena not yet fully explained, or a strictly scientific theory which accounts for all the known facts of the case, and which only needs the verification of subsequent observa- tions and deductions to become a cer- tainty. HYPSOMETBY, that department of geodesy which treats of the measure- ments of the altitudes or relative heights of various points on the earth's surface. In all cases in which great accuracy is essential, trigonometrical methods must be employed, but in other cases suffi- ciently accurate results may be obtained by levelling, by the use of the barometer, or by the boiling point of water as given by the thermometer. The trigonomet- rical method is often the only one avail- able, as the height to be measured may be quite inaccessible. The barometric method is based on the fact that as the mercurial column is supported by the atmospheric pressure, it must fall when conveyed from a lower to a higher level, as in the latter case the pressure is diminished. Were the atmosphere uni- form in density throughout, nothing could be simpler than the measurement of heights by the barometer, but gases being very compressible, the lower strata of the atmosphere are denser than the upper strata, being exposed to greater pressure. Moreover, increase of tem- perature affects the density of the mer- cury in the barometer, and also that of the air, and further complicates the problem. Hence for the greatest accu- racy in determining the difference of levels two mercurial barometers and four thermometers are required. Two of the thermometers are used for determin- ing the temperature of the air at the stations, and two are attached to the barometers for determining the tempera- ture of the mercury. The observations are made simultaneously. The aneroid barometer is in some respects more suit- able than the mercurial, being much more portable, and requiring two ther- mometers only. After the necessary ob- servations are made the required height may be calculated by the use of certain logarithmic formulae. HYBAX, a genus of pachydermatous mammalia, intermediate in their char- acter between the rhinoceros and the tapir. It is the only genus of the or ler Hyracoidea, characterized by having no canine teeth, but long curved incisors. The front feet have four toes, and the hind feet three. The Cape hyrax is by the colonists of South Africa called rock badger and rock rabbit, from the fact that they inhabit rocky places. They are also called damans. HYBCANIA, an ancient district of Asia, of indefinite extent, bordered on the Caspian Sea (sometimes called Hyr- canum Mare) and the river Oxus. It corresponded with the modern Mazan- deran and Asterabad.