HYD 



HYD 



Hydrogen is not, as several of the other 

 gases are, noxious to vegetable life ; at 

 the same time it appears to contribute 

 little to the nourishment of plants, Dr. 

 Priestley having found, that it still con- 

 tinued inflammable after a growing vege- 

 table had been confined in it for several 

 months. It can apparently supply, to a 

 certain extent, the place of light, in sup- 

 porting vegetation. Von Humboldt ob- 

 served, that some cryptogamic plants in 

 mines, and of course secluded from light, 

 were not pale, but of a green colour, such 

 as they would have had from growing 

 under exposure to the light of day; and 

 he concluded, with sufficient probability, 

 that the agency of light had, in this case, 

 been siipplied by the hydrogen gas, 

 which is evolved in greater QT less abun- 

 dance in such situations. 



Hydrogen gas is so sparingly soluble 

 in water, that, when agitated with it, it suf- 

 fers no perceptible diminution of volume. 

 When the water has been previously freed 

 from atmospheric air, Mr. Henry found, 

 that one hundred cubic inches take up 

 1.5 of the gas under a common atmosphe- 

 ric pressure ; under increased pressure, 

 a larger quantity, equal to one-third of 

 the volume of the water, is absorbed. 



The affinities of hydrogen seem princi- 

 pally exerted towards inflammable bodies. 

 It unites with sulphur, phosphorus, and 

 carbon, and forms gaseous compounds ; 

 it appears to be capable of dissolving even 

 some of the metals, particularly, iron, zinc, 

 and arsenic. United with nitrogen, it 

 forms one of the alkalies, ammonia : with 

 oxygen, water. It is also a constituent 

 principle of the greater number of the ve- 

 getable and animal products. 



Hydrogen gas may be regarded as a 

 product of some natural operations. It is 

 found collected often in mines, derived 

 probably from the decomposition of water 

 y metals ; it is known to the miners by 

 the name of fire-damp, and is often the 

 cause of accidents, from exploding on the 

 approach of an ignited body. It is also 

 extricated from stagnant water, and from 

 marshy situations, from the slow decom- 

 position of vegetable and animal sub- 

 stances, holding, dissolved in it, carbon, 

 and perhaps also phosphorus and nitro- 

 gen, and forming, as has been supposed 

 with some probability, gases, which ren- 

 der the air of such places unhealthy. From 

 its levity, it has been supposed, that the 

 quantity of it thus produced at the surface 

 of the earth will rise through the atmo- 



sphere, and occupy the higher regions ; 

 and on its presence some of the pheno- 

 mena of meteorology, particularly the 

 sudden appearance of some fiery meteors, 

 have been supposed to depend. Its af- 

 finities have not been ascertained with 

 any precision, as to their relative force. 



HYDROGRAPHY, the art of measur- 

 ing and describing the sea, rivers, lakes, 

 and canals. With regard to the sea, it 

 gives an account of its tides, counter-tides, 

 soundings, bays, gulphs, creeks, &c. ; as 

 also of the rocks, shelves, sands, shallows, 

 promontories, harbours, the distance and 

 bearing of one port from another, with 

 every thing that is remarkable, whether 

 out at sea, or on the coast. 



HYDROLEA, in botany, a genus of the 

 Pentandria Digynia class and order. Na- 

 tural order of Convolvuli, Jussieu. Es- 

 sential character: calyx five-leaved ; co- 

 rolla wheel-shaped ; filaments cordate at 

 the base ; capsule two-celled, two-valved. 

 There are four species. 



HYDROMANCY, a method of divina- 

 tion by water, practised by the ancients in 

 this manner ; they filled a cup or bowl of 

 water ; then fastening a ring to a piece of 

 thread tied to their finger, held it over the 

 water, and repeated a certain form of 

 words, desiring to be satisfied with regard 

 to their inquiry ; and if the question was 

 answered in the affirmative, the ring 

 would strike the sides of the bowl of its 

 own accord. 



HYDROMETER. The best method of 

 weighing equal quantities of corrosive vo- 

 latile fluids, to determine their specific 

 gravities, appears to consist in inclosing 

 them in a bottle with a conical stopper, 

 in the side of which stopper a fine mark 

 is cut with a file. The fluid being poured 

 into the bottle, it is easy to put in the 

 stopper, because the redundant fluid es- 

 capes through the notch, or mark, and 

 may be carefully wiped off. Equal bulks 

 of water and other fluids are by this means 

 weighed to a great degree of accuracy, 

 care being taken to keep the temperature 

 as equal as possible, by avoiding any con- 

 tact ofthe bottle with the hand or other- 

 wise. The bottle itself shews, with much 

 precision, by a rise or fall ofthe liquid in 

 the notch of the stopper, whether any 

 such change has taken place. See GRAV- 

 ITY, specific. 



But as the operation of weighing re- 

 quires considerable attention and steadi- 

 ness, and also a good balance, the float- 

 ing instrument, called the hydrometer, has 



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