Part 198
It is not necessary to dissolve the ferrocyanide in water previous to adding the sulphuric acid, as it readily dissolves in the water during the process of distillation.
Three conditions are important to be observed in the arrangements of the apparatus:--1. The mixture in the retort should not be allowed to spirt over. 2. It should contain but little air. 3. It should present the greatest possible amount of surface to be cooled.
If sulphate of potassium and prussian blue are spirted over into the distillate, this must be carefully rectified over a small quantity of magnesia, chalk, or carbonate of barium.
_b._ _From cyanide of potassium_ (without distillation).--To a solution of nine parts of tartaric acid in sixty parts of water, contained in a well-stoppered bottle nearly filled with it, four parts of pure cyanide of potassium are added; the vessel is shaken, frequently dipped into cold water, and then left in the cold for twelve hours; and the aqueous hydrocyanic acid, which contains but a very small quantity of tartrate of potassium, is poured off from the crystallised tartrate.[351] This acid contains 3·6 per cent. of anhydrous hydrocyanic acid.
[Footnote 351: 'London Med. Surg. Journ.,' vi, 524.]
_c._ _From cyanide of mercury._--126 (accurately weighed) parts of cyanide of mercury are agitated with at least 28 parts of iron filings, in a well-stoppered bottle, containing 49, or rather more, parts of oil of vitriol, diluted with a considerable quantity of water. The agitation is continued until a portion of the liquid taken out is not blackened by sulphuretted hydrogen.
The solution, decanted from the iron and mercury, is then placed in a retort and distilled; the acid coming over at a gentle heat. Excess of iron accelerates the decomposition.
_d._ _From cyanide of silver._--Everitt recommends 200 parts of pure cyanide of silver to be shaken up with 240 parts of hydrochloric acid of specific gravity of 1·129, and when the decomposition is complete, the hydrocyanic acid to be separated from the chloride of silver by decomposition.
This hydrocyanic acid may contain a small quantity of hydrochloric acid, a not very objectionable admixture, since it retards decomposition. It possesses the advantage of definite strength.
_Prop._ The aqueous is very similar in properties to the anhydrous acid, differing in taste, odour, poisonous and combustible properties, according to its degree of concentration. Like the anhydrous, the aqueous acid decomposes, but not so readily; when perfectly pure, becoming brown, and at last black. As before stated, a little free mineral acid assists to preserve it. It should be always kept in a dark place.
_Detection and estimation of hydrocyanic acid and soluble cyanides._--The presence of hydrocyanic acid, indicated by the characteristic smell, which is given off by the contents of the stomach, or of any fluid containing it (provided this is not disguised by any substance of stronger odour) may be confirmed by the following tests:
1. To the filtered suspected fluid add a slight excess of caustic potash, and then a solution containing ferrous and ferric sulphates. If hydrocyanic acid, or a soluble cyanide be present, upon the addition of an excess of hydrochloric acid the liquid turns to a blue colour (more or less intense according to the quantity of acid present), owing to the formation of Prussian blue.
2. Add the suspected fluid solution of nitrate of silver; if hydrocyanic acid be present, a white cyanide of silver is formed, which is nearly insoluble in cold nitric acid, but is soluble in ammonia and cyanide of potash, and which, when heated to redness, gives off the inflammable violet-flamed cyanogen.
3. Acidulate a small quantity of the suspected liquid with a few drops of hydrochloric acid, and place it in a watch-glass; then invert a second watch-glass, moistened with a drop of solution of ammonic sulphide over this. After a few minutes remove the upper watch-glass, and evaporate the liquid to dryness over a water bath; let the dry residue be treated with a drop of a weak solution of ferric chloride. If hydrocyanic acid be present, a blood-red colour is produced, owing to the formation of red ferric hydrocyanide, which may be discharged by chloride of mercury; a reaction which distinguishes it from a similar colour given by meconic acid.
Where large quantities of material have to be examined, it is desirable that the acid should be distilled off by the heat of a water-bath, acidulating the liquid with tartaric acid if it be alkaline. The distillate is then to be tested by any of the above methods.
_Antidotes._--Give a scruple of carbonate of potash dissolved in about an ounce of distilled water, and directly afterwards, ten grains of sulphate of iron, also dissolved in the same quantity of distilled water, to which should be added one drachm of tincture of perchloride of iron. Whilst this is being prepared, and subsequently, apply cold affusion to the head and neck, artificial respiration, and, if practicable, give strong coffee and brandy. A more ready remedy is ammonia, given both internally and applied to the nostrils.
=Hydrocyanic Acid, Diluted.= _Syn._ ACIDUM HYDROCYANICUM DILUTUM. (B. P.) The 'British Pharmacop[oe]ia' gives the following simple directions for the preparation of this acid:--
Dissolve two and a quarter ounces (avoir.) of ferro-cyanide of potassium in ten fluid ounces of distilled water, then add one fluid ounce of sulphuric acid, previously diluted with four fluid ounces of distilled water, and cooled. Put them into a retort and adapt this to a receiver, containing eight fluid ounces of water, which must be kept carefully cold. Distil with a gentle heat until the fluid in the receiver measures seventeen fluid ounces. Add to this three fluid ounces of the water, or as much as may be sufficient to bring the acid to the required strength of two per cent. by weight.
_Prop._ Specific gravity, ·997. 100 grains, or 110 minims, precipitated with a solution of nitrate of silver, give a precipitate of cyanide of silver, which when dried, weighs 10 grains. 270 grains rendered alkaline by liquor sodæ, require a 1000 grain measures of volumetric solution of nitrate of silver, before a permanent precipitate begins to form.
_Antidotes._ See HYDROCYANIC ACID.
=Hydrocyanic Acid, Scheele's.= _Syn._ ACIDUM HYDROCYANICUM SCHEELII. The original process of Scheele does not yield an acid of uniform strength, and is probably never followed. It is therefore impossible to state precisely what is intended when Scheele's acid is prescribed, or to understand why it should be preferred by certain physicians to the British Pharmacop[oe]ia preparation, which is of a known and definite strength. As prepared by different makers it has been found to contain from three to five per cent. of anhydrous acid. The following is Scheele's process:--
Mix two ounces of Prussian blue, with six ounces of red precipitate of mercury, and add six ounces of water. Boil for some minutes, constantly agitating; pour the whole on a filter and wash the residuum on the filter with two ounces of hot water, which is to be added to the filtered liquor. Add to this an ounce and a half of clean iron filings, and three drachms of sulphuric acid; shake well and let it settle; then pour the clear liquor into a retort, and distil a fourth part into a receiver well luted and kept cold.
=HYDROFLUORIC ACID. (H. F.)= _Syn._ HYDRIC FLUORIDE, HYDROGEN FLUORIDE. _Prep._ 1. From _fluor-spar_ (free from silica and metallic sulphides) and oil of vitriol. The fluor-spar being reduced to fine powder and placed in a leaden retort, is mixed with twice its weight of concentrated oil of vitriol, and on applying heat, an acid and highly acid vapour distils over, which condenses to a liquid if passed into a receiver of the same metal, standing in a freezing mixture at a temperature of 4° Fahr. Louyet has shown that the liquid acid, obtained as above, is not (as once believed) anhydrous.
2. From the double fluoride of potassium and hydrogen. Fremy first renders the salt anhydrous by careful drying; and by the subsequent application of a strong heat, expels the equivalent of hydrofluoric acid contained in it; condensing it into a colourless, mobile, very volatile liquid by the application of a freezing mixture of ice and salt.
3. By decomposing plumbic fluoride by dry hydrogen.
_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.
2. =Heavy Carburetted Hydrogen.= (C_{2}H_{4}.) See OLEFIANT GAS.
_Obs._ COAL GAS, OIL GAS, and RESIN GAS, consist, for the most part, of mixtures of these two gaseous hydrocarbons in uncertain proportions, obtained respectively from coal, oil, and resin, by the action of heat, and used for the purposes of illumination. See GAS.
=Hydrogen, Oxides of.= There are two well-defined compounds of hydrogen and oxygen:--
1. =Subox'ide of Hydrogen.= (H_{2}O.) Water (which _see_).
2. =Perox'ide of Hydrogen.= (HO.) _Syn._ HYDROXYL, BINOXIDE OF HYDROGEN, DEUTOXIDE OF H., OXYGENATED WATER; HYDROGENIIBINOXYDUM, L. This singular fluid was discovered by M. Thénard in 1818.
_Prep._ (Odling.) A known quantity of pure hydrochloric acid, diluted with 8 or 10 times its volume of distilled water, is placed in a glass beaker surrounded with ice or a freezing mixture. A quantity of binoxide of barium rather less than sufficient to neutralise the acid is then ground to a fine paste with distilled water, and added gradually to the acid in which it should dissolve without effervescence. Diluted sulphuric acid is next added cautiously, to precipitate the barium, and reproduce hydrochloric acid to act upon a fresh quantity of peroxide. The liquid having been filtered from the insoluble sulphate of baryta, a second proportion of binoxide of barium paste is added gradually, as before. The treatment with sulphuric acid, filtration and addition of binoxide, is repeated 6 or 7 times. Sulphate of silver is then very carefully added, so as exactly to precipitate in the form of chloride of silver the whole of the chlorine. After filtration, pure baryta, first as a paste and then in solution, is cautiously added, to precipitate exactly the sulphuric acid set free from the sulphate of silver. Filtration is again resorted to, and the clear liquid (aqueous solution of peroxide of hydrogen) is placed in a dish over oil of vitriol in vacuo, in order that the water mixed with it may evaporate.
_Prop., &c._ A colourless, transparent, somewhat syrupy liquid, of sp. gr. 1·452. It has a metallic taste, and corrodes the skin. It is easily resolved into oxygen and water. It mixes freely with water, and becomes more permanent by the dilution. It bleaches organic substances, and acts as a powerful oxidating agent. Under certain circumstances, however, it plays the part of a reducing agent. To the chemist, peroxide of hydrogen and its analogue, binoxide of barium, have been of great service as instruments of research. Binoxide of hydrogen has been applied in the arts to restore the blackened lights of paintings which have become darkened by sulphuretted hydrogen; it is also sold by hair-dressers for bleaching human hair.
=Hydrogen, Phosphuret'ted.= See PHOSPHORUS.
=Hydrogen, Sulphides of.= See SULPHUR.
=HYDROMEL.= _Syn._ HYDROMELI, L. An aqueous solution of honey. _Prep._ (P. Cod.) Honey, 2 oz.; boiling water, 32 oz.; dissolve, and strain. A refreshing and slightly laxative drink; in fevers, hoarseness, sore throats, &c.
=HYDROM'ETER.= _Syn._ AREOMETER, GRAVIMETER; HYDROMETRUM, L. An instrument for ascertaining the specific gravities of liquids, and hence their strength, the latter being either in inverse or direct proportion to the former. Hydrometers are of two kinds:--1. Those which are always immersed to the same depth in distilled water, and the liquid to be tried, small weights being used for the purpose, as in FAHRENHEIT'S and NICHOLSON'S hydrometers; and 2nd, those which are suffered to rise or sink freely in the liquid, until they come to a state of rest, as in SYKE'S, BAUMÉ'S, &c. In both cases a correction must be made for any variation in temperature.
Of the two kinds, the first give the most accurate results, and have the great advantage of being applicable to liquids either lighter or heavier than water, but the second are the readier in practice, requiring less time and less skill to use them. The following are those best known:--
BAUMÉ'S HYDROMETER or AREOMETER, which is very generally employed on the Continent, consists of two distinct instruments, the one for liquids heavier than water, the other for liquids lighter than that fluid. The first floats at the 0, or 'zero,' of the scale, in distilled wafer, at the temperature of 58° Fahr., and each degree, marked downwards, indicates a density corresponding to one per cent. of common salt. The hydrometer for liquids lighter than water is poised so that the 0 of the scale is at the bottom of the stem, when it is floating in a solution of 1 oz. of common salt in 9 oz. of water, and the depth to which it sinks in distilled water shows 10°; the space between these fixed points being equally divided, and the graduation continued upwards to the top of the scale.
The temperature at which these instruments were originally adjusted by Baumé was 12·5° Centigrade (54·5° Fahr.). They are now commonly adjusted in this country at 58° or 60° Fahr. Hence arise the discrepancies observable in the published tables of the "correspondence between degrees of Baumé and real specific gravities."
CARTIER'S HYDROMETER, which is much used in France for light liquids, has the same point for the zero of its scale as Baumé's, but its degrees are rather smaller, 30° Baumé being equal to 32° Cartier.
FAHRENHEIT'S HYDROMETER consists of a hollow ball, with a counterpoise below, and a very slender stem above, terminating in a small dish. The middle, or half-length of the stem, is distinguished by a fine line across it. In this instrument every division of the stem is rejected, and it is immersed in all experiments to the middle of the stem, by placing proper weights in the little dish above. Then, as the part immersed is constantly of the same magnitude, and the whole weight of the hydrometer is known, this last weight, added to the weights in the dish, will be equal to the weight of fluid displaced by the instrument, as all writers on hydrostatics prove. And accordingly, the specific gravities for the common form of the tables will be had by the proportion--
_As the whole weight of the hydrometer and its load, when adjusted in distilled water_, is to the _number_ 1000, so is the _whole weight when adjusted in any other fluid, to the number expressing its specific gravity._