_Prop._ The strong, aqueous, hydrofluoric acid obtained by the action of
oil of vitriol on fluor-spar, is a densely fuming, volatile, colourless
liquid, which boils at 60° F., and remains unfrozen at 4°. It combines
with water so greedily, and evolves so much heat in doing so, as to give
rise to a hissing noise like that produced when a red-hot iron is plunged
into cold water. In a concentrated form it has a specific gravity of
1·060. Brought into contact with animal matter of any kind it instantly
destroys it, the smallest drop on the skin producing a deep and painful
wound; hence the necessity of the greatest care in its preparation. With
the exception of platinum, gold, silver, mercury and lead, hydrofluoric
acid, when diluted, dissolves the metals, the metal when it undergoes
solution, displacing hydrogen. Potassium decomposes the strong acid with
explosion.
In both the gaseous and fluid form hydrofluoric acid is largely consumed
for etching on glass; and this property constitutes one of its most
available and reliable tests. The test may be conveniently applied as
follows:--
Cover a small piece of window glass or a watch glass with a thin layer of
wax, scraping away a very small portion by means of a sharply pointed
instrument, and then expose the glass for a short time to the vapour of
the acid, given off when the materials are heated in a small leaden saucer
or platinum crucible; on removing the wax with a little turpentine, the
marks on the glass caused by the hydrofluoric acid will be distinctly
perceived.
=HYDROFLUOSILIC'IC ACID.= (_{4}HF.SiF_{4}.) FLUORIDE OF SILICON AND
HYDROGEN. _Prep._ From powdered fluor-spar, and siliceous sand or powdered
glass, of each 1 part; concentrated sulphuric acid, 2 parts; mix in a
glass retort, apply a gentle heat, and pass the evolved gas (fluoride of
silicon) into water. Decomposition ensues, silica being deposited in a
gelatinous state, and hydrofluosilicic acid remaining in solution. This
acid liquor, which is a double fluoride of silicon and hydrogen, is used
as a test for barium and potassium, with which it forms nearly insoluble
precipitates.
=HY'DROGEN. (H.)= _Syn._ HYDROGENIUM, L. An elementary body discovered by
Cavendish in 1766. It has been found existing in an uncombined state in
the gases evolved from the solfataras of Iceland. Combined with oxygen, it
constitutes water, and in this form is extensively distributed through
earth, air, and ocean. It is an important constituent of all organised
tissues.
_Prep._ Hydrogen is always obtained for experimental purposes by the
deoxidation of water, by one or other of the following methods:--
1. A tube of iron or porcelain (a gun-barrel, for instance) containing a
quantity of iron turnings or scraps of iron, is fixed across a furnace, so
that its middle portion may be made red-hot; to the one end is attached a
retort or other vessel containing water, and to the other a bent tube
connected with a pneumatic trough or gasometer. The tube being now heated
to redness, and the water in the retort brought into a state of brisk
ebullition, the evolved steam suffers decomposition; the oxygen being
absorbed by the iron, and the hydrogen escaping into the gas receiver.
2. Sulphuric acid (oil of vitriol), diluted with 6 or 8 times its bulk of
water, is poured on granulated zinc (or scraps of iron) placed in a retort
or gas bottle; hydrogen is evolved and is collected, as before.
_Obs._ This is the most convenient method of preparing hydrogen, and the
one usually adopted in the laboratory. To ensure the gas being quite pure,
distilled zinc is employed, and the gas is passed, first through a
concentrated solution of pure potassa, then through a solution of nitrate
of silver, and, lastly, through strong oil of vitriol, or over fragments
of chloride of calcium. When hydrogen is prepared from crude zinc, it has
a slight smell; and when from iron, its odour is often strong and
disagreeable.
_Prop._ Gaseous; colourless; tasteless; odourless (when pure);
combustible; sp. gr. ·06935, being 16 times lighter than oxygen gas, and
14·4 times lighter than atmospheric air. 100 cubic inches, at 60° Fahr.
and 30 inches of the barometer, weigh 2·1371 (say 2·14) gr.; 1 gr.
occupies 46·6 inches. It is very readily inflamed, even by a red-hot wire.
It burns with a scarcely visible flame. Mixed with atmospheric air or
oxygen, it explodes with extreme violence on the approach of flame, or
sudden compression. One measure of hydrogen and 5 of atmospheric air, and
2 of hydrogen and 1 of oxygen, are the proportions that explode with the
greatest violence. The combination of hydrogen and oxygen, when mixed, is
brought about by the heat of a red-hot solid or a flame, by the electric
spark, and by the presence of spongy platinum, the black powder of
platinum, clean platinum foil, and some other substances. A jet of
hydrogen burnt in oxygen gas, or a jet of these gases (mixed) burnt in the
air, with proper precautions, produces a most intense heat. Water absorbs
about 2% by volume of hydrogen.
Hydrogen has recently been liquefied and even solidified.
_Tests._ It is recognised by--its combustibility;--the pale colour of its
flame;--producing water only when burnt in air or oxygen;--extinguishing
the flame of other bodies; and--exploding when mixed with half its volume
of oxygen, and fired.
_Uses, &c._ Pure and uncombined hydrogen is not employed in the arts.
Inhalations of this gas have, however, been occasionally used in medicine.
Dr Beddoes recommended them in phthisis. In combination, the uses of
hydrogen are almost numberless. Combined with oxygen, it forms water; with
chlorine, hydrochloric acid; with fluorine, hydrofluoric acid; with
cyanogen, hydrocyanic acid; with carbon, innumerable hydrocarbons; with
nitrogen, ammonia; with sulphur, sulphuretted hydrogen--in fact, an
enumeration of the valuable compounds which it enters into would occupy
many pages of this work. From its extreme lightness it has been used to
fill balloons, but coal-gas is now commonly employed for this purpose. On
its property of inflaming in contact with spongy platinum is arranged the
popular little instrument for the production of instantaneous light
(DOBEREINER'S LAMP) sold by the philosophical instrument makers. The
chemist avails himself of the great heat developed by its combustion in
oxygen in the formation of the OXYHYDROGEN BLOWPIPE.
Some of the compounds of hydrogen are noticed _below_; the others under
their respective names.
=Hydrogen, Antimo''niuretted.= (SbH_{3}.) _Syn._ ANTIMONETTED HYDROGEN,
HYDRIDE OF ANTIMONY, STIBAMINE; HYDROGENIUM ANTIMONIATUM, L. A gaseous
compound of antimony and hydrogen, prepared by dissolving an alloy of
antimony with a large excess of zinc in hydrochloric or dilute sulphuric
acid. It has never been obtained pure, a variable proportion of free
hydrogen being always present. It burns with a bluish-white flame, giving
rise to dense fumes of teroxide of antimony, and when conducted through a
red-hot tube, or the flame is thrown on a cold surface, as a porcelain
plate, metallic antimony is deposited. This gas is a deadly poison when
inhaled. See ARSENIOUS ACID.
=Hydrogen, Arsen''iuretted.= (AsH_{3}.) _Syn._ ARSENETTED HYDROGEN,
HYDRIDE OF ARSENIC, ARSENAMINE; HYDROGENIUM ARSENIURATUM, L. A gaseous
compound of arsenic and hydrogen.
_Prep._ Arsenide of zinc (made by fusing together equal weights of zinc
and arsenic) is acted upon by strong hydrochloric acid or by sulphuric
acid diluted with three parts of water.
_Obs._ This gas is produced whenever arsenious or arsenic acid, or any of
their salts, is in presence of nascent hydrogen. The properties of
arsenetted hydrogen are fully described in the tests for ARSENIOUS ACID.
This gas is a deadly poison when inhaled.
=Hydrogen, Car'buretted.= _Syn._ CARBONETTED HYDROGEN. This term is
specially applied to two of the numerous compounds of carbon and hydrogen
(CARBIDES OF HYDROGEN, HYDROCARBONS):--
1. =Light Carburetted Hydrogen.= (CH_{4}.) _Syn._ MARSH GAS, FIRE-DAMP,
GAS OF THE ACETATES. This is often abundantly disengaged in coal mines,
and its combustion occasions those fearful explosions which are so
destructive to human life. The mud at the bottom of stagnant pools, on
being stirred, suffers bubbles of gas to escape, which, when collected and
examined, are found to be a mixture of light carburetted hydrogen and
carbonic acid. The latter is easily removed by passing the gas through a
solution of caustic potassa or milk of lime.
_Prep._ (Dumas.) A mixture of acetate of soda (cryst.) and hydrate of
potassa (dry), of each 2 parts, and quicklime (in powder), 3 parts, is
strongly heated in a flask or retort. The gas in a state of absolute
purity is disengaged in great abundance, and may be collected over water.
_Prop._ Colourless; neutral; nearly inodorous; burns with a yellow flame,
producing pure water and carbonic acid; explodes when kindled in contact
with air or oxygen.