HYDROGEN. 



HYDROGEN. A chemical element, which 

 derives its name from two Greek words that 

 signify "a generator of water," because it is 

 one of the constituents of that fluid, which is 

 always formed when hydrogen gas is burned 

 in combination with atmospheric air, or with 

 oxygen gas. It is known to us, in its simplest 

 form, only in the state of gas, and is speedily 

 fatal to animal life when it is breathed unmixed 

 with atmospheric air. It is, however, a com- 

 ponent of animal matters, and it forms a very 

 essential part in the economy of vegetable sub- 

 stances, in which it is always found. Thus 

 sugar contains 6-90 per cent, of hydrogen ; 

 gum, 6-93 ; bee's wax, 12-672 ; wood of the oak, 

 5-69 ; wheat starch, 6-77; acetic acid (the acid 

 of vinegar), 6-35 per cent. It is regarded as 

 an element, because it has resisted every at- 

 tempt to decompose it. It is the lightest of all 

 ponderable matter, 100 cubic inches weigh- 

 ing only 2-15 grains. No known degree of 

 cold has been able to condense it to a liquid. 

 It cannot support combustion, but is combusti- 

 ble in conjunction with atmospheric air. It 

 constitutes one-ninth of the weight of water, 

 a substance essential to vegetation, and which 

 plants are supposed to have the power of de- 

 composing. Under such circumstances, Liebig 

 asserts that 8-04 parts of hydrogen unite with 

 100 parts of carbonic acid to form woody fibre, 

 whilst the oxygen is separated in the gaseous 

 state. (Or^miir ('lion. p. 63.) Most vegetable 

 structures contain hydrogen in the form of 

 water, but the hvdrogen essential to this consti- 

 tution cannot exist in the form of water. That 

 hydrogen gas exerts a considerable influence 

 upon the leaves of plants, was first noticed by Dr. 

 Priestley. Sennebier found that plants which 

 lose their green colour in the dark, preserve it 

 under those circumstances, if a small portion 

 of hvdrogen gas is present in the atmosphere in 

 which they are placed; and Dr. Ingenhous no- 

 ticed that its presence, when they are growing 

 in the light, renders their colour of a deeper 

 preen (Ann. He Chem. vol. iii. p. 57) ; and, again, 

 M. Humboldt has noticed that the Poa annim, 

 Trtfnlinm ttrvense, and other plants growing in 

 the galleries of coal mines, preserve their green 

 colour, although vegetating in the dark, and 

 that, in such situations, the atmosphere con- 

 tains a proportion of hydrogen gas. 



When applied to the roots of plants in mo- 

 derate proportion, the influence of hydrogen 

 gas is evidently beneficial in all those situa- 

 tions where this gas is evolved, as in drains, 

 stagnant waters, dung-hills ; and the vegetation 

 growing over such places is uncommonly rank 

 and luxuriant. The gas observed to arise by 

 the agitation of the mud of stagnant pools is 

 the same gas employed for the purposes of 

 illumination, or carburetted hydrogen gas, a 

 peculiar gas composed of carbon 0-416; hy- 

 drogen 0-0694. In the process of putrefaction, 

 a quantity of water exactly corresponding to 

 that of the hydrogen, is formed by the extrac- 

 tion of oxygen from the air; while all the oxy- 

 gen of the organic matter is returned to the 

 atmosphere in the form of carbonic acid. Now 

 the process of vegetable assimilation consists 

 in the extraction of hydrogen from water, and 

 carbon from the carbonic acid ; hence the ad- 



HYSSOP. 



' vantage of decomposing vegetable matter to 



I living plants. A small portion|of carburetted 



hydrogen gas in the atmosphere, or in the soil 



I of plants, certainly therefore promotes their 



vegetation ; but like pure hydrogen gas, \vhen 



it constitutes their entire atmosphere, it de- 



: stroys them. (Thomson's Chem. vol. iv. p. 347.) 



1 See GASES, THEIU USE TO VEGETATION-. 



HYGROMETER (Gr. J>=c, moist, and ^rrgoy, 

 measure). Aninstrumentwhich indicates the de- 

 gree of moisture or vapour present in the atmo- 

 sphere, or its relative degrees of dampness and 

 dryness. Hygrometers are of several forms, 

 and a rude hygrometer is easily made by means 

 of a long hair, or strip of leather, or cat-gut, 

 suspended from a peg, kept in its upright po- 

 sition by a slight weight: these, by their very 

 sensible contractions and expansion according 

 to the humidity of the air, indicate, by an at- 

 tached scale, its variations. Hygrometers of 

 this kind, however, are defective, from the 

 irregularity of their action, and the impossibi- 

 lity of comparing them with each other, their 

 alteration by time, and other circumstances. 

 These disadvantages gave rise to the construc- 

 tion, by Professor Daniel, of that now common- 

 ly used in England, and called by his name. 

 This instrument is, however, somewhat com- 

 plex and costly, requiring considerable skill in 

 its use. It will doubtless be superseded by 

 other contrivances for ascertaining the precise 

 quantity of moisture in air, one of which, in- 

 vented by Prof. A. D. Bache, of Philadelphia, has 

 been already described under the head of DEW. 



The dew-point is the temperature imme- 

 diately below 'hat of the vapour contained in 

 the surrounding atmosphere : the difference, 

 however, between the dew-point and that of 

 the vapour is so slight, that for all ordinary 

 purposes they may be considered the same. 



The temperature of the invisible vapour of 

 the atmosphere being thus ascertained (for it 

 differs materially from the temperature of the 

 air in which it is contained), it is easy to cal- 

 culate the force it exerts as thus existing in 

 the state of steam, and the weight of a given 

 bulk of it. Tables have been formed showing 

 the proportion of watery vapour in each cubic 

 foot of atmospheric air corresponding to every 

 degree of the dew-point. 



HYSSOP, COMMON (Hyssopus offirinalis, 

 probably from the Hebrew). There are three 

 varieties, distinguished by the colour of the 

 flowers, the white, red, and blue ; the last of 

 which is most commonly cultivated. It is a 

 perennial, native of Siberia. The root is 

 knobbed, woody, fibrous ; the stem about two 

 feet high, quadrangular, erect, branching. 



A dry soil is the one most appropriate fof 

 hyssop. If it is grown on a rich or wet one, 

 it becomes luxuriant; but, from a deficiency 

 of woody matter, is generally destroyed by the 

 frost, as well as rendered less aromatic and 

 powerful in its medicinal qualities. It is pro- 

 pagated by seed and slips of the branches and 

 young shoots, as well as by offsets. The seed 

 may be sown from the close of February until 

 the end of May. Rooted offsets may be planted 

 in March, April, August, and September; cut- 

 tings of the branches in April and May, ami 

 slips of young shoots in June or July. 



651 



