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Hydrocyanic.

ating it with ammonia, it should give no precipitate of oxide of iron; sulphureted hydrogen should produce no turbidity in it, which would be the case if arsenic, free chlorine, or sulphurous acid were present; and on dilution with three or four times its bulk of water no white cloud of sulphate of barium should be produced by the addition of chloride of barium."

The presence of hydrochloric acid, or of the soluble chlorides in solution, may be detected by the addition of a few drops of a solution of nitrate of silver, which occasions the formation of a white curdy precipitate of chloride of silver, which is insoluble in nitric acid, but dissolves in a solution of ammonia.

Liquid hydrochloric acid (under the name of spirit of salt) was known to the alchemists. Hydrochloric acid gas was discovered by Priestley in 1772; and Davy, in, 1810, ascertained that it was composed of chlorine and hydrogen.

In many of their properties, the analogous acids, hydrobromic, hydrofluoric, and hydriodic acids resemble hydrochloric acid.

HYDROCO TYLE, a genus of umbelliferous plants, having simple umbels, entire acute petals, and fruit of two flat orbicular carpels, with five more or less distinct threadlike ribs, and no vittæ. The species are numerous, generally more or less aquatic, widely distributed. One only is a native of Britain, H. vulgaris, which grows in marshy places, and is called marsh pennywort from the orbicular leaves, and sometimes whiterot, sheeps-bane, flowk-wort, etc., from a notion that it is injurious to sheep which eat it, causing foot-rot or fluke-worm-effects rather to be ascribed to the marshy situations in which it grows.

HYDROCYAN'IC ACID (HCN or HCy), known also as prussic acid, from its having been first obtained by Scheele, in 1782, from the substance known as Prussian or Berlin blue, is of almost equal interest to the chemist, the physician, and the toxicologist. We shall notice (1) its chemistry, (2) its medicinal value, and (3) its action as a poison, and its antidotes.

1. Its Chemistry.-Pure anhydrous hydrocyanic acid is a limpid volatile fluid, with a specific gravity of 0.697 at 64.4° F. (18° C.). It boils at 79.7° F. (26.5° C.), and solidifies into a crystalline mass at 5° F. (-15° C.). Its volatility is so great that if a drop be allowed to fall on a piece of glass, part of the acid becomes frozen by the cold produced by its own evaporation. It possesses a very penetrating odor, resembling that of peach blossoms or oil of bitter almonds. It burns with a whitish flame, reddens litmus paper slightly (its acid properties being feeble), and is very soluble in water and alcohol. Pure hydrocyanic acid may be kept unchanged if excluded from light, which occasions its decomposition, and the formation of a brown substance known as azulmic acid.

Hydrocyanic acid is readily obtained by distillation from the kernels of bitter' almonds, and many kinds of stone-fruit, from the leaves and flowers of various plants, and from the juice of the tapioca plant (jatropha manihot). Anhydrous hydrocyanic acid is obtained by the reaction of concentrated hydrochloric acid on cyanide of mer

cury.

The preparation of the dilute acid is, however, of much greater practical importance. The London, Edinburgh, Dublin, and United States pharmacopoeias agree in recommending that it should be obtained by the distillation of a mixture of dilute sulphuric acid and ferrocyanide of potassium (known also as prussiate of potash). The distillate should contain nothing but hydrocyanic acid and water, so that, by the addition of more water, we can obtain an acid of any strength we please. Sometimes, however, a second, or even a third distillation is necessary. The dilute acid of the Ph. Lond. contains 2 per cent; that of the Ph. Dub. rather more; that of the U. S. Ph. contains 2 per cent.; while what is known as Scheele's acid is very variable, but averages 4 per cent of the anhydrous acid.

The ordinary tests for hydrocyanic acid are 1, the peculiar odor; 2, the nitrate of silver test-there being formed a white precipitate of cyanide of silver, which is soluble in boiling nitric acid; 3, the formation of Prussian blue, by adding to the fluid under examination a solution of some proto and per-salt of iron, by then saturating with caustic potash, and finally adding an excess of hydrochloric acid; when, if hydrocyanic acid is present, we have a characteristic blue precipitate; 4, the sulphur test, which is the best and most accurate that has yet been discovered. Let the suspected liquid be acidulated with a few drops of hydrochloric acid; place it in a watch-glass, and let a second watch-glass, moistened with a drop of a solution of sulphydrate of ammonia, be inverted over it; after a few minutes, let the upper glass be removed, and the moistened. spot be gently dried. The whitish film which is left may consist merely of sulphur; when hydrocyanic acid is present, it consists of sulphocyanide of ammonia. Let this residue be treated with a drop of a weak solution of perchloride of iron, when, if hydrocyanic acid was present, a blood-red tint is developed, which disappears on the addition of one or two drops of a solution of corrosive sublimate. This is known as Liebig's test.

2. Its Medicinal Uses.-We are indebted to the Italians for the introduction of hydro cyanic acid in the materia medica; and it was first employed at the beginning of the present century. There are no cases in which it is so serviceable as in those affections of the stomach in which pain is a leading symptom, as in gastrodynia, water-brash, and

VII.-24

Hydrodynamics.

in cases of intense vomiting. Hence it is often useful in English cholera, when opium has completely failed. In pulmonary diseases it does not produce the good effects that were formerly ascribed to it; but it is sometimes useful in allaying spasmodic cough. It has been employed with advantage in chronic skin-diseases, to allay pain and irritation. A mixture of two drams of the dilute acid (of 2 per cent strength) with half a pint of rose-water, and half an ounce of rectified spirit, forms a good lotion. When given internally, the average dose is from 3 to 5 minims of the 2 per cent dilute acid, three or four times a day; it must be administered in some milk vehicle, such as simple water, or orange-flower water.

3. As a Poison.-Hydrocyanic acid is one of our most energetic poisons, and is frequently employed both in murder and suicide. When a small poisonous dose (about half a dram of the 2 per cent acid) has been taken, the first symptoms are, weight and pain in the head, with confusion of thought, giddiness, nausea (and sometimes vomiting), a quick pulse, and loss of muscular power. If death result, this is preceded by tetanic spasms and involuntary evacuations. When a large dose has been taken (as from half an ounce to an ounce of the 2 per cent acid), the symptoms may commence instantaneously, and it is seldom that their appearance is delayed beyond one or two minutes. When," says Dr. A. S. Taylor, the patient has been seen at this period, he has been perfectly insensible, the eyes fixed and glistening, the pupils dilated and unaffected by light, the limbs flaccid, the skin cold and covered with a clammy perspiration; there is convulsive respiration at long intervals, and the patient appears dead in the intermediate time; the pulse is imperceptible, and the respiration is slow, deep, gasping, and some times heaving or sobbing." The patient survives for a longer or shorter period, according to the dose. According to Dr. Lonsdale, death has occurred as early as the second, and as late as the forty-fifth minute.

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The parts specifically affected are the brain and the spinal system. The affection of the respiratory system seems to be due to the influence of the acid on those parts of the nervous system from which the respiratory organs derive their nervous power. The immediate cause of death is, in most cases, the obstruction of the respiration; but in some cases, the stoppage of the heart's action.

Where the fatal action is so rapid antidotes are of comparatively little value. Chlo rine, ammonia, cold affusion, and artificial respiration are the most important agents in the treatment. The first two should be used with great caution, and only by the medi cal practitioner. Cold affusion on the head, neck, and down the spine is a valuable remedy, and it is asserted that its efficacy is almost certain when it is employed before the convulsive stage of poisoning is over, and that it is often successful even in the stages of insensibility and paralysis. Artificial respiration (see RESPIRATION, ARTIFI CIAL) should never be omitted. Dr. Pereira states that he once recovered a rabbit by this means only, after the convulsions had ceased, and the animal was apparently dead.

HYDRODYNAMICS treats of the laws of the motion of liquids; the flow of water from orifices and in pipes, canals, and rivers; its oscillations or waves; and its resist ance to bodies moving through it. The term hydraulics is sometimes applied to the same subjects, from the Greek word aulos, a pipe. The application of water as a moving power forms the practical part of the subject.-In what follows, the illustration is taken from the case of water, but the principles established are true of liquids in general.

Efflux.-If three apertures, D, C, E, are made at different heights in the side of a vessel filled with water, the liquid will pour out with greater impetuosity from C than

M

F

C

E

from D, and from E than from C. The velocity does not increase in the simple ratio of the depth. The exact law of dependence is known as the theorem of Torricelli; the demonstration is too abstruse for introduction here, but the law itself is as follows: "Particles of fluid, on issuing from an aperture, possess the same degree of velocity as if they had fallen freely, in vacuo, from a height equal to the dis tance of the surface of the fluid above the center of the aperture." The jet from C, for instance, has the same velocity as if the particles composing it had fallen in vacuo from the level of the liquid to C. Now, the velocity acquired by a body in falling is as the time of the fall; but the space fallen through being as the square of the time, it follows that the velocity acquired is as the square root of the space fallen through. In the first second, a body falls 16 ft., and acquires a velocity of 32 feet. If E, then, is 16 ft. below the level, a jet from E flows at the rate of 32 ft.; and if D is at a depth of 4 ft., the velocity of the jet at D will be half the velocity of that at E, or 16 feet. In general, to

B

K L

find the velocity for any given height, multiply the height by 2×32, and extract the square root of the product. This rule may be expressed by the formula = √2gh, in which a signifies the velocity of the issue, g the velocity given by gravity in a second, or 32 ft., and h the height of the water in the reservoir above the orifice. This last quantity is technically called the head or charge.

That this theory of the efflux of liquids is correct, may be proved by experiment. Let the vessel, MB, have an orifice situated as at o; the water ought to issue with the velocity that a body would acquire in falling from M to the level of o. Now, it is established in the doctrine of projectiles (q.v.), that when a body, is projected vertically upwards with a certain velocity, it ascends to the same height from which it would require to fall in order to acquire that velocity. If the theory, then, is correct, the jet ought to rise to the level of the water in the vessel at M. It is found in reality to fall short of this; but not more than can be accounted for by friction, the resistance of the air, and the water that rests on the top in endeavoring to descend. When the jet receives a very slight inclination, so that the returning water falls down by the side of the ascending, ten in. of head of water may be made to give a jet of nine inches. A stream of water spouting out horizontally, or in any oblique direction, obeys the laws of projectiles, and moves in a parabola; and the range of the jet for any given velocity and angle of direction may be calculated precisely as in projectiles. The range of horizontal jets is readily determined by practical geometry. On AB describe a semicircle; from D, the orifice of the jet, draw DF perpendicular to AB, and make BK equal to twice DF; then it can be proved by the laws of falling bodies and the properties of the circle, that the jet must meet BL in the point K. If BE is equal to AD, the perpendicular EH is equal to DF; and therefore a jet from E will have the same range as that from D. Of all the perpendiculars, CG, drawn from the middle point C, is the greatest; therefore, the jet from C has the longest possible range.

The area of the orifice and the velocity of the flow being known, it is easy to calculate the quantity of water discharged in a given time. Thus, suppose the area to be 1 sq.in., and the velocity 20 ft. a second, it is evident that there issues in a second a cylinder or a prism of water 1 sq.in. in section and 20 ft. long, the content of which is 1×240-240 cubic inches. In any given time, then, as three minutes (=180 seconds), the discharge is 240x180=43,200 cubic inches.

It has as yet been assumed that the water in the vessel or reservoir is kept constantly at the same height, and that thus the velocity is constant. We have now to consider the case of a vessel allowed to empty itself through an orifice at the bottom. As the surface of the water sinks, the velocity of the discharge diminishes or is retarded; and when the vessel is of the same area from top to bottom, it can be proved that the velocity is uniformly retarded. Its motion follows the same law as that of a body projected vertically upwards. Now, when a motion uniformly retarded comes to an end, the space described is just half what the body would have passed over had it gone on uniformly with the velocity it had at the outset. Therefore, when the vessel has emptied itself in the way supposed, the quantity discharged is half what would have been discharged had the velocity been uniform from the beginning.

The "Contraction of the Vein."-When, by means of the area of the opening and the velocity thus determined, we calculate the number of cubic feet or of gallons that ought to flow out in a given time, and then measure the quantity that actually does flow, we find that the actual flow falls short of the theoretical by at least a third. In fact, it is only the central part of the jet, which approaches the opening directly, that has the velocity above stated. The outer particles approach from all sides with less velocity; they jostle one another, as it were, and thus the flow is retarded. In consequence of this want of uniformity in velocity and direction among the component layers of the jet, as they enter the orifice, there takes place what is called a "contraction of the vein" (vena contracta); that is, the jet, after leaving the orifice, tapers, and becomes narrower. The greatest contraction is at a distance from the orifice equal to half its diameter; and there the section of the stream is about two-thirds the area of the opening. It is, in fact, the section of the contracted vein that is to be taken as the real area of the orifice, in calculating by the theory the quantity of water discharged. If the wall of the vessel has considerable thickness, and the orifice is made to widen gradually inwards, in the proportions of the contracted vein, the stream does not suffer contraction, and the area of the orifice where it is narrowest may be taken as the actual area of discharge.

Adjutages.-It has as yet been supposed that the issue is by means of a simple opening or hole in the side or bottom of the vessel; but if the flow takes place through a short tube, the rate of discharge is remarkably affected. Through a simple opening, in a thin plate, the actual discharge is only about 64 per cent of the theoretical: through a cylindrical conducting-tube, or adjutage, as it is called, of like diameter, and whose length is four times its diameter, the discharge is 84 per cent. The effect is still greater if the discharge-tube is made conical both ways, first contracting like the contracted vein, and then widening. The effect of a conducting-tube in increasing the discharge is accounted for by the adhesion of the water to its sides, which widens out the column to a greater area than it would naturally have. It has thus a tendency to form a vacuum in the tube, which acts like suction on the water in the reservoir, and

Hydrography.

increases the quantity discharged. The flow is more free if the orifice is in the bottom of the vessel, than in the side on a level with the bottom. If the discharge tube is made to project inwards beyond the thickness of the walls of the vessel, the velocity is much impeded, owing to the opposing currents produced by the water approaching the opening.

Pipes. When a conduit pipe is of any considerable length, the water issues from it at a velocity less than that due to the head of water in the reservoir, owing to the resistance of friction. With a pipe, for instance, of 14 in. in diameter, and 30 ft. long, the discharge is only one-half what it would be from a simple orifice of the same diameter. The rate of reduction depends upon the diameter of the tube, its length, the bendings it undergoes, etc. The resistance to the flow of water in pipes does not arise properly from friction, as understood of solids, but from the adhesion of the water to the sides of the pipe, and from the cohesion of the watery particles among themselves; it makes little difference, therefore, whether an earthenware pipe, for instance, be glazed or not. Large projections form an obstacle; but mere roughness of surface is filled up by an adhering film of water, which is as good as a glaze. The resistance increases greatly with the narrowness of the pipes. Engineers have formulas, deduced in great part from experiment, for calculating the discharge through pipes of given length and diameter, and with a given head; but the subject is too complicated for introduction here. If water flowed in a conduit pipe without friction or other obstruction, so that its velocity were always equal to that due to the head of water, there would be no lateral or bursting pressure on the walls of the pipe; and if the pipe were pierced, the water would not squirt out. Accordingly, with a short tube or adjutage, which, instead of obstructing, increases the flow, there is not only no lateral outward pressure on the walls of the tube, but there is actually a pressure inwards. If a hole is made in the wall of a cylindrical adjutage and the one end of a small bent tube is inserted in the hole, while its other end is dipped in a vessel of water, the water will be sucked up the tube, showing the tendency that the adjutage has to form a vacuum. But when the velocity of discharge is diminished by the friction of a long pipe, or by any narrowing, bending, or other obstruction in the pipe, then that portion of the pressure of the head of water that is not carried off in the discharge, becomes a bursting pressure on the walls of the pipe. This pressure is unequal at different parts of the pipe. At the end where the water issues free and unobstructed, it is next to nothing, and gradually increases towards the reservoir, where it is equal to the difference between the head of water in the cistern, and the head due to the velocity with which the water is actually flowing in the pipe. The principle now explained accounts for the fact, that pipes often burst or begin to leak on the motion of the water in them being checked or stopped.

Resistance of Water to Bodies moving through it.—This is greatly affected by the shape of the body, which ought to have all its surfaces oblique to the direction of the motion. When a cylinder terminates in front in a hemisphere, the resistance is only one-half what it is when the cylinder terminates in a plane surface at right angles to the axis; and if instead of a hemisphere, the termination is an equilateral cone, the resistance is only one-fourth. If a globe is cut in halves, and a cylinder, whose length and the diameter of whose base are each equal to the diameter of the globe, is fixed between them; this cylinder with hemispherical ends experiences less resistance than the globe alone, the diminution being about one-fifth of the resistance to the globe. Also the resistance increases in a higher ratio than the simple one of the velocity. One part of the resistance arises from the momentum that the body has to give to the water it displaces. Moving at a certain rate, it displaces a certain quantity; moving at twice that rate, it displaces twice the quantity in the same time. But not only does it displace twice the number of particles of water; it also has to displace them with twice the velocity; the pressure of the resistance is thus not merely doubled, but quadrupled or squared. Similarly, when the velocity is tripled, the resistance arising from the simple displacement of water becomes nine times as great. Another part of the resistance of liquids to bodies moving in them is owing to the cohesion of the particles, which have not to be thrown aside merely as separate grains, but to be torn asunder. In addition to this, when the velocity is considerable, the water becomes heaped up in front, and depressed at the other end from not having time to close in behind, thus causing an excess of hydrostatic pressure against the direction of the motion. Owing to the combination of these causes, the real law of the increase of resistance is difficult to investigate, and the results of experiments are not a little discordant. See WATERPOWER; WAVE.

HYDROFLUOR'IC ACID. See FLUORINE.

HYDROFLUOSILICIC ACID. See FLUORINE,

HYDROGEN (symbol H, equiv. 1), so called from the Greek words hydör, water, and gennão, to generate, is an elementary substance, which exists as a colorless and inodorous gas (regarded as permanent till 1878). One of its most striking peculiarities is its specific gravity, it being the lightest of all known bodies. Assuming the weight of a given volume of atmospheric air to be 1, the weight of the same volume of hydrogen under similar conditions is 0.0692; hence hydrogen is 14 times lighter than atmo

739

Hydrography.

spheric air; while, on the other hand, it is 241.573 times lighter than platinum, the
heaviest body known. Its refractive power is greater than that of any other gas, and is
When a lighted
more than 6 times as great as that of atmospheric air. It is combustible; that is to say,
it is capable of combining with oxygen, and developing light and heat.
taper is passed up into an inverted jar of hydrogen, the gas burns quietly with a pale-blue,
scarcely visible flame, and the taper is extinguished. The flame only occurs at the line
If the hydrogen be mixed with air
of junction of the hydrogen and the external air.
or oxygen prior to the application of the taper, the whole mixture is simultaneously
inflamed, and there is a loud explosion, which is most violent when 2 volumes of hydro-
gen are mixed with 1 volume of oxygen, or with 5 volumes of atmospheric air. The
hydrogen and oxygen in these cases combine to form watery vapor or steam, which
suddenly expands from the high temperature attendant on the combustion, but imme-
diately afterwards becomes condensed; this condensation causes a partial vacuum, into
which the surrounding air rushes, and by the collision of its particles produces the
report. At ordinary temperatures, water dissolves rather less than 2 per cent of its
volume of hydrogen. Hydrogen was liquefied for the first time in 1878, and even
solidified (see GASES). Pure hydrogen, though it cannot support life, is not poisonous,
and when mixed with a sufficient quantity of atmospheric air or oxygen, may be breathed
for some time without inconvenience.

Hydrogen does not possess very marked chemical properties. The only substances with which it combines directly at ordinary temperatures are chlorine and oxygen. Hydrogen and chlorine, mixed together, and exposed to direct sunlight, combine with explosion; in diffused daylight, they gradually unite; but in the dark do not act on one another. Hydrogen and oxygen do not combine spontaneously even in direct sunlight, but require the presence of a red-hot solid, of flame, or of spongy platinum.

It is generally stated that hydrogen does not exist naturally in a pure or uncombined state, but Bunsen recognized its presence in variable proportions in the gases evolved from the solfataras of Iceland, and it will probably be detected in other localities where similar geological relations hold good. In combination with oxygen, as water, it not only forms a very considerable part of the earth, and of the atmosphere, but enters largely into the structure of every animal and vegetable organism. It is an essential ingredient of many inflammable minerals, such as coal, amber, and petroleum; and of certain gases, such as marsh gas, ammonia, and hydrosulphuric acid (or sulphureted hydrogen). It likewise enters into the composition of a large number of manufactured substances and products used in the arts, medicine, etc., as for instance, sal-ammoniac, starch, sugar, vinegar, alcohol, olefiant gas, aniline, indigo, morphia, strychnia, hydrocyanic

acid, etc.

There are numerous ways in which hydrogen may be prepared, but the usual and most convenient process is by the action of diluted sulphuric acid on zinc. About half an ounce of granulated zinc is placed in a retort, and a dilute acid, prepared by gradually mixing an ounce of oil of vitriol with six ounces of cold water, is poured on the zinc. Hydrogen gas is rapidly evolved in great abundance, but the first portions should not be collected, since they are mixed with the atmospheric air which was contained in the retort. The rest of the gas may be collected in the ordinary way over water. In this process the zinc takes oxygen from the water, and forms oxide of zinc, which The reaction is shown in the combines with the sulphuric acid, forming sulphate of zinc, which remains in solution, while the hydrogen of the decomposed water escapes. A precisely similar reaction ensues if we use formula, Zn + H2SO4 = ZnSO, +H2. iron in place of zinc, but in this case the gas is generally less pure.

Hydrogen gas, under the name of combustible air, was obtained in the 16th c. by Paracelsus by treating certain metals with dilute acids, and was more or less known to Boyle and others; but Cavendish, in his paper on "Factitious Airs," published in the Transactions of the Royal Society for 1766, was the first to describe accurately the properties of this gas, and the methods of obtaining it; hence he is usually mentioned as its discoverer.

HYDROGEN, BINOXIDE OF (symb. H,O,, equiv. 34), is a colorless liquid of a syrupy consistence, with a specific gravity of 1.45 (water being 1), and a peculiar odor, something like that of very dilute chlorine. It bleaches vegetable colors, and when applied to the tongue or the skin produces a white spot, and excites considerable pain. From the readiness with which it gives off its oxygen, it is a powerful oxidizing agent. The method of preparing it is complicated and difficult. This substance was discovered in 1818 by Thenard, who termed it oxidized water. Dr. B. W. Richardson, an eminent London physician, has lately examined its value (in solution) as a therapeutic agent, and has found it to be of extreme use in whooping-cough, in certain forms of rheumatism, and (as a palliative) in the last stages of consumption.

HYDROGʻRAPHY (Gr. hydor, water, graph-, to write) is a description of the surface waters of the earth, particularly of the bearings of coasts, of currents, soundings, islands, shoals, etc., and of anything the knowledge of which may be useful for purposes of navigation. It consequently includes the construction of charts, maps, etc., in which these particulars are detailed. It is, in fact, to the sea what geography is to the land. The first step in the erection of hydrography into a science was made in the

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