HYDROGRAPHY HYDROPHOBIA. 



be their relative quantity in the mixture. Hem c the 

 composition of water, by weight and measure, is, 



By weight. By volume. 



Orypen, 8 1 



Hydrogen, 1 2 



(For a further account of the properties of water, see 

 that article.) The processes for procuring a supply 

 ot hydrogen, described at the commencement of the 

 present article, will now be intelligible. The first is 

 founded on the fact that iron, at a red hent, decom- 

 poses water, the oxygen of which unites with the 

 metal, while the hydrogen gas is set free. That the 

 hydrogen which is evolved when zinc or iron is put 

 into dilute sulphuric acid, is derived from the water, 

 is obvious from the consideration, that of the three 

 substances, iron or zinc, sulphuric acid, and water, 

 the last is the only one which contains hydrogen. 

 The product of the operation, besides hydrogen, is 

 the sulphate of the protoxide of iron, if iron is used, 

 or of the oxide of zinc, when zinc is employed. Hy- 

 drogen, therefore, is one of the most abundant sub- 

 stances in nature. It forms, as has been stated, eight- 

 ninths of water; besides, with carbon and oxygen, it 

 enters into the composition of all vegetable sub- 

 star ces ; and, with oxygen, carbon, ana nitrogen, it 

 forms a part of all animal substances. Large quan- 

 tities of it, often united with more or less carbon, are 

 continually evolved into the atmosphere from the 

 decomposition of vegetable and animal matters. 



HYDROGRAPHY; that part of geography which 

 treats of waters. Hydrographic maps; such as make 

 the rivers and other collections of water their chief 

 subject. 



HYDROMETER, (Greek), measurer of density 

 (for fluids), is an instrument, wliich, being immersed 

 in fluids, as in water, brine, beer, brandy, determines 

 the proportion of their densities or their specific 

 gravities, and thence their qualities. The use of the 

 hydrometer depends on the following propositions 

 1 . The hydrometer will sink in different fluids in an 

 inverse proportion to the density of the fluids ; 2. the 

 weight required to sink a hydrometer equally far in 

 different fluids, will be directly as the densities of the 

 fluids. Each of these two propositions gives rise to 

 a particular kind of hydrometer ; the first with the 

 graduated scale, the second with weights. The latter 

 deserves the preference. (See Traite d ' Areometrie 

 de M. Francoeur, and Le Cours de Physique de M. 

 Biot ) There are various instruments used as hydro- 

 meters ; one is a glass or copper ball, with a stem, 

 on which is marked a scale of equal parts or degrees. 

 The point to which the stem sinks in any liquid being 

 ascertained and marked on this scale, we can tell how 

 many degrees any other liquid is heavier or lighter, 

 oy observing the point to which the stem sinks in it. 

 Another kind is formed by preparing a number of 

 hollow glass beads, of different weights, and finding 

 which bead will remain stationary in any liquid, 

 wherever it is placed. An instrument of great deli- 

 cacy, which will even detect any impurity in water 

 too slight to be detected by any ordinary test, or by 

 the taste, consists of a ball of glass three inches in 

 diameter, with another joining it, and opening into 

 it one inch in diameter. A wire, about ten inches 

 long and one-fortieth of an inch in diameter, divided 

 into inches and tenths, is screwed into the larger 

 ball. A tenth of a grain, placed on the top of 

 the wire will sink it a tenth of an inch. Now 

 it will stand in one kind of water a tenth of an inch 

 lower Uian in another, which shows that a bulk of one 

 kind of water, equal to the bulk of the instrument 

 (which weighs 4000 grs ), weighs one tenth of a 

 grain leas than an equal bulk of the other kind of 

 water; so that a difference in specific gravity of one 



part in 40,000 is detected. The areometer is more 

 simple and accurate. A glass phial, about two 

 Indus in diameter, and seven or eight long, is 

 corked tight ; into the cork is fixed a straight wire, 

 one twelfth of an inch in dinmeter, and thirty inches 

 long. The phial is loaded with shot, so as to sink in 

 tlir heaviest liquid, leaving the wire just below the 

 surface. The liquor is then placed in a t;!a^i 

 cylinder, three or four feet long, with a scale of 

 equal parts on the side, by which the point to which 

 the top of the wire sinks is marked. This instru- 

 ment is so delicate, that the sun's rays, falling upon 

 it, will cause the wire to sink several inches ; and it 

 will rise again when carried into the shade. 



Nicholson made an improvement by which the 

 hydrometer is adapted to the general purpose of 

 finding the specific gravity both ot solids and fluids. A 

 is a hollow ball of copper, B a dish affixed 

 to the ball by a short slender stem D; 

 C is another affixed to the opposite side 

 of the ball by a kind of stirrup. In the 

 instrument actually made, the stem D is 

 of hardened steel 1-40 of an inch in diame- 

 ter, and the dish C is so heavy as in all [ 

 cases to keep the stem vertical when the 

 instrument is made to float in any liquid. 

 The parts are so adjusted, that the addi- 

 tion of 1000 grains in the upper dish B, 

 will just sink it in distilled water, at the 

 temperature of 60 of Fahrenheit's ther- 

 mometer, so far that the surface shall in- 

 tersect the middle of the stem D. Let it now be 

 required to find the specific gravity of any fluid. 

 Immerse the instrument in it, and by placing weights 

 in the dish B cause it to float, so that the middle of its 

 stem D shall be cut by the surface of the fluid. Then, 

 as the known weight of the instrument, added to 1000 

 grains, is to the same known weight added to the 

 weight used in producing the last equilibrium, so is 

 the weight of a quantity of distilled water displaced 

 by the floating instrument, to the weight of an equal 

 bulk of the fluid under examination. And these 

 weights are in the direct ratio of the specific 

 gravities. Again, let it be required to find the 

 specific gravity of a solid body, whose weight is 

 less than 1000 grains. Place the instrument in dis- 

 tilled water, and put the body in the dish B. Make 

 the adjustment of sinking the instrument to the 

 middle of the stem, by adding weight in the same 

 dish. Subtract those weights from 1000 grains, and 

 the remainder will be the weight of the body. Place 

 now the body in the lower dish C, and add more 

 weight in the upper dish B, till the adjustment is 

 again obtained. The weight last added will be the 

 loss the solids sustain by immersion, and is the 

 weight of an equal bulk of water. Consequently 

 the specific gravity of the solid is to that of water, as 

 the weight of the body to the loss occasioned by the 

 immersion. This instrument was found to be suffi- 

 ciently accurate to give weights true to less than one- 

 twentieth of a grain. 



HYDROPHANE. See Opal. 



HYDROPHOBIA (from vS e , water, and <p/3, 

 fear) ; a specific disease arising from the bite of a 

 rabid animal. The animals most liable to be afflicted 

 with madness are dogs ; but cats, wolves, foxes, &c., 

 are also subject to it. The following description of 

 the way in which rabies affects dogs, is from a com- 

 munication in the Sporting Magazine, September, 

 1825 : The symptoms of rabies in the dog are the 

 following, and are given nearly in the order in which 

 they usually appear : An earnest licking, or 

 scratching, or rubbing, of some particular part ; 

 snllenness, and a disposition to hide from observa- 

 tion ; considerable costiveness, and occasional vomit- 



