HVDRIADS HYDROGEN. 



21 



from these apertures keeps up, by its reaction, a con- 

 stant rotary motion. 



HYDRI ADS. The Hydriads, in mythology, were 

 n kind of water-nymphs, who danced with the 

 Hamadryads, to the sound of the pipe of Pan. 



HYDRO ; two syllables which occur in a number 

 of scientific words ; derived from the Greek ulu^ 

 water. See the following articles. 



HYDROCEPHALUS. See Dropsy. 



HYDROGEN ; a simple non-metallic body, form- 

 ing acids by its union with chlorine, iodine, and bro- 

 mine, and hence termed an acidifiable body, and 

 producing water by its combination with oxygen, in 

 illusion to which the name hydrogen (from llu^, 

 water) has been applied. The most simple state in 

 which we can procure it is in that of a gas, i. e., in 

 union with caloric, and possibly with electricity and 

 light. To effect this, water is always employed ; and 

 one of the following arrangements is adopted : water 

 in the state of vapour is passed over the metallic iron 

 heated to redness, by adjusting a retort, half filled 

 with this fluid, to one extremity of an iron tube con- 

 taining clean iron wire, and laid across a heated fur- 

 nace, the other extremity having a bent tube, connect- 

 ed with it and dipping under the shelf of a pneumatic 

 cistern ; the water in the retort is made to boil 

 briskly, and the steam to come in contact with the 

 heated iron ; upon which hydrogen gas is copiously 

 disengaged, and collected in the pneumatic appara- 

 tus. Or, slips of sheet zinc, iron filings or turnings, 

 or small iron nails, are introduced into a small gas- 

 bottle with a bent tube, or into a common retort, 

 upon which sulphuric acid, diluted with five or six 

 times its weight of water, is poured ; effervescence 

 ensues, and the escaping gas may be collected in the 

 nsual manner. One troy ounce (480 grains) of zinc 

 gives 356 oz. measures = about 676 cubic inches ; and 

 1 ounce of iron, 41 2 oz. measures z= 782 cubic inches, 

 of hydrogen gas. The hydrogen obtained in these 

 processes is not absolutely pure. The gas evolved 

 during the solution of iron is contaminated by a com- 

 pound formed from hydrogen and the carbon con- 

 tained in the iron. This compound, which is a 

 volatile oil, is removed by transmitting the gas 

 through alcohol. The gas obtained by means of 

 zinc is more free from impurities ; though the small 

 proportion of sulphur and carbon still remaining in 

 the zinc of commerce, gives rise to the same com- 

 pound as in the former case, and also to a little sul- 

 phureted hydrogen. The impurities in this instance 

 are removed by passing the gas through a solution 

 of caustic potash. Thus purified, hydrogen gas has 

 neither taste nor odour ; it is colourless, and the light- 

 est of all ponderable matter known, its specific 

 gravity being 0.068, that of the atmospheric air 

 being 1.000, or about 14 times lighter than common 

 air. This remarkable levity allows it to ascend with 

 the greatest readiness through all liquids and gases, 

 and is the cause of its being employed to fill bal- 

 loons ; which, notwithstanding the weight of the 

 materials of which they are constructed, are sufficient- 

 ly light, compared with the atmosphere, to rise to 

 very great elevations, or until they meet with a me- 

 dium whose density is such as to render them station- 

 ary. (See Aeronautics.) Hydrogen gas is a 

 powerful refractor of light, and has hitherto resisted 

 all attempts to compress it into a liquid. It is 

 sparingly absorbed by water, 100 cubic inches of 

 that liquid dissolving about lg of the gas. It is in- 

 capable of supporting respiration ; nor is it a sup- 

 porter of combustion ; for when a lighted taper is 

 passed up into an inverted glass full of hydrogen gas, 

 it is immediately extinguished. But its most charac- 

 teristic property is that of its inflammability, though, 

 like other combustibles, it requires the aid of a sup- 



porter for enabling its combustion to take place. 

 This is exemplified by bringing a lighted candle or 

 taper to the mouth of a narrow jar, or wide-mouthed 

 bottle, filled with the gas ; it is immediately kindled, 

 but only burns where it is in contact with the air, 

 the combustion going on quietly in successive strata 

 from the orifice to the bottom of the vessel. Mingled 

 with oxygen gas, no action takes place so long as the 

 compound remains cold ; but, on the approach of a 

 flame, the whole is kindled at the same instant ; a 

 flash of light passes through the mixture, followed by 

 a violent explosion. The report is the loudest when 

 the proportions observed in the mixture are two 

 volumes of hydrogen and one of oxygen. The same 

 phenomena take place, though less strikingly, when 

 atmospheric air is substituted for oxygen gas : in the 

 latter case, however, the proportions are two mea- 

 sures of hydrogen to five or six of air. And not only 

 is hydrogen gas inflamed when in contact with air or 

 oxygen gas by the contact of a burning taper, but by 

 a solid body heated to redness, and Ly the electric 

 spark. If a jet of hydrogen be delivered upon re- 

 cently prepared spongy platinum (see Platinum), this 

 metal very quickly becomes red-hot, and then sets 

 fire to the gas. The electric spark ceases to cause 

 detonation when the explosive mixture, formed of 

 two measures of hydrogen to one of oxygen, is 

 diluted with twelve times its volume of air, fourteen 

 of oxygen, or nine of hydrogen, or when it is ex- 

 panded to sixteen times its bulk by diminished pres- 

 sure. Sudden and violent compression, likewise, 

 causes an explosion of the explosive mixture ; appa- 

 rently from the heat emitted during the operation; 

 for an equal degree of condensation, slowly produced, 

 has not the same effect. When the action of heat, 

 the electric spark and spongy platinum no longer 

 cause an explosion, a silent and gradual combination 

 between the gases may still be occasioned by them. 

 Oxygen and hydrogen gases unite slowly with one 

 another when exposed to a temperature above the 

 boiling point of mercury, and below that at which 

 glass begins to appear luminous in the dark. An 

 explosive mixture, diluted with air to too great a de- 

 gree to explode by electricity, is made to unite 

 silently by a succession of electric sparks. Spongy 

 platinum causes them to unite slowly, though mixed 

 with 100 times their bulk of oxygen gas. A very 

 high temperature is excited by the combustion of 

 hydrogen gas, especially when it is burned in oxy- 

 gen gas, as in the compound blow-pipe of doctor 

 Hare. (See Compound Blow-pipe.) Water is the 

 sole product of the combustion of hydrogen a fact 

 first demonstrated by Cavendish, who burned oxygen 

 and hydrogen gases in a dry glass vessel, and obtained 

 a quantity of pure water exactly equal to that of the 

 gases which had disappeared during the experiment. 

 The synthetic proof of the composition of water is 

 obtained also by detonating two measures of hydro- 

 gen, mixed with one of oxygen, in a tube, over the 

 mercurial cistern ; the whole is condensed into water, 

 Lavoisier first exhibited the composition of water 

 analytically, by passing a known quantity of watery 

 vapour over metallic iron heated to redness in a glass 

 tube. Hydrogen gas was disengaged ; the metal in 

 the tube was oxydized ; and the weight of the hydro- 

 gen, added to the increase which the iron had expe- 

 rienced from combining with oxygen, exactly cor- 

 responded to the quantity of water which had been 

 decomposed. Its composition by volume is very 

 clearly shown by galvanism. On resolving water 

 into its elements by this agent, and collecting them 

 in separate vessels, two measures of hydrogen to one 

 of oxygen are obtained ; and, on the other hand, 

 these gases, when inflamed by the electric spark, 

 unite in the exact ratio of one to two, whatever may 



