II YD 



384 



II YG 



metra, and hydrocele are some of the 

 pecies of the dloeaM. 



HT'DROSCOPE, from v^tuf, water, and 

 ffxotrut, to view. A kind of water-clock 

 anciently in use. Time was marked by 

 the rise of water, which was allowed to 

 flow slowly into a graduated tube. 



HTDROSELE'NIC ACID, seleniuretted hy- 

 drogen. A highly deleterious gas, readily 

 prepared by treating seleniurct of potas- 

 sium with liquid hydrochloric acid. 



HYDROSTAT'ICA. In natural history, an 

 order of Acalepha, characterised by having 

 buoyant air-vessels, by which they sus- 

 pend themselves in their liquid element. 



HYDROSTAT'IC BALANCE. A delicate ba- 

 lance employed in finding specific gra- 

 Tities. 



HYDROSTAT'IC BELLOWS. An apparatus 

 for illustrating the upward pressure of 

 fluids, and the hydrostatic paradox. It 

 consists of two circular boards connected 

 by leather, to rise and fall like the com- 

 mon bellows, but without valves, and 

 having a pipe three or four feet long, 

 communicating with the interior through 

 the lower board. "Water being poured 

 into this tube, the upper board of the bel- 

 lows is raised with a force equal to the 

 difference between its area and the area 

 of the tube. This is the property applied 

 by Mr. Bramah in the construction of his 

 hydraulic press. 



HYDROSTAT'IC INDEX. An apparatus for 

 demonstrating the truth of Pascal's hy- 

 drostatic paradox. 



HYDROSTAT'IC PARADOX. The principle, 

 in hydrostatics, that a portion of fluid, 

 however small, may be made to counter- 



Sse a portion of fluid, however great, 

 us, if to a wide vessel A, a small tube 

 B, be attached, com- 

 municating with the 

 vessel, and if wafr . 

 be poured into either ** 

 of them, it will stand 

 at the same height 

 in both ; consequent- 

 ly there is an equili- 

 brium between ; and 

 this holds, whatever 

 the shape and size of the vessels may be. 

 This illustrates the principle, that the 

 pressure of fluids is as their height, with- 

 out any regard to their quantity. 



HYDROSTAT'IC PRESS. See BRAMAH'S 

 PRESS. 



HYDROSTAT'ICS, from ufiiu?, water, and 

 0-70.71x0?, static. That branch of hydro- 

 dynamics which treats of the weight, 

 pressure, and equilibrium of non-elastic 

 fluids at rest. It comprehends the doctrine 

 of flotation, methods of finding specific 

 gravities, &c. 



HTDROSI-LPHXTES. Compounds of hy- 

 drosulphuric acid. 



HYDBosui'raoMn. Chemical com- 

 pounds of bases with sulphuretted hydro- 

 gen. 



HYDROSCLPHC'RIC ACID. The name 

 given by Gay Lussac to a solution of 

 sulphuretted hydrogen in water. It has 

 also been called ht/drotheionic acid, from 

 Q-.IOV, sulphur. 



HYDRO-SUL'PHTROUS ACID. When three 

 vols. of sulphuretted hydrogen gas, and 

 two of sulphurous acid gas are mixed to- 

 gether, over mercury, they are condensed 

 into a solid orange-yellow body, to which 

 Dr. Thomson has applied the name of 

 hydro-sulphurous, acid. 



HYDROTHO'RAX, from v%vf , water, and 

 0fe, the chest, dropsy of the chest. 



HYDROXAN'THIC ACID. An acid disco- 

 vered by Zeise, of Copenhagen, and since 

 considered as an oxy-acid, and described 

 under the name of xanthic acid (q.v.). It 

 is the carbo- sulphuric acid of some che- 

 mists, being a compound of two volumes 

 of bisulphuret of carbon, and one volume 

 of sulphuric ether. 



HY'DROXCRE, a hydrate. 



HYD'RURET, a compound of hydrogen 

 with a metal. See HYDROGURET. 



HYGE'INISM, the science of health, 

 ii'/mac., health. The right application of 

 medical science to the preservation or re 

 storation of health. The term has been 

 extensively abused. 



HYGROM'ETER, from v^tf, moisture, 

 and [AtT^av, measure. An instrument for 

 determining the quantity of vapour in the 

 atmosphere. Various instruments have 

 been contrived for this purpose, some of 

 them founded upon the property which 

 certain substances have of imbibing va- 

 pours, and having their dimensions there- 

 by altered ; others on the principle that 

 the moisture of the atmosphere will depo- 

 sit itself in dew-drops upon the surface of 

 a colder body in contact with it. It is 

 upon this principle that Daniel's hygro- 

 meter is constructed. Th is consists of two 

 small glass bulbs, connected together by a 

 glass tube, bent at right angles, as shown 

 in the figure. A very delicate thermo- 

 meter is inclosed in one bulb, which also 

 contains some ether, and the other bulb is 

 covered with a piece of fine muslin. "When 

 an observation is to be made, the covered 

 bulb is moistened with ether, which by 

 evaporation produces intense cold, and 

 thereby condenses the etherous vapour 

 within. Evaporation from the further 

 bulb is consequently promoted, and the 

 temperature of its exterior surface falls : 

 a slight ring of dew, coinciding with the 

 surface of the liquid within, forms on the 

 glass. The thermometer within is to be 

 observed during the oponuion. and the 

 temperature indicated by it at the instant 

 when the dew appears on (or disnppcari 



