States, is recommended for its simplicity and convenience. Of cyanide of silver 50 grains are dissolved in 41 grains of hydrochloric acid diluted with a fluid ounce of distilled water; the mixture is shaken in a well stopped vial, and the clear liquor, poured off from the insoluble matter which subsides, is kept in tight bottles excluded from the light. Single equivalents of the acid and cyanide salt are employed; and by their mutual decomposition hydrocyanic acid is obtained in solution, and chloride of silver falls as a precipitate. By this method the acid may always be prepared as wanted-a matter of no little importance in its medicinal applications, in consideration of its liability to spontaneously decompose, and its consequent uncertain composition and strength. The aqueous solutions prepared by the different processes adopted are not uniform in their proportions of anhydrous acid; but their strength ought not to exceed 3 per cent. of pure acid. Various methods are given in the chemical books of ascertaining this strength and the degree of purity. Sulphuric and hydrochloric acids are the most common foreign bodies present. The quantity of real acid is usually determined by the weight of cyanide of silver precipitated on adding nitrate of silver. By the U. S. formula 100 grains of pure acid must accurately saturate 12.7 grains of nitrate of silver dissolved in distilled water, and produce a precipitate of cyanide of silver, which, washed and dried at a temperature not exceeding 212°, shall weigh 10 grains and be wholly soluble in boiling nitric acid. If a residue remain, it is chloride of silver, indicating the presence of hydrochloric acid in the original. Sulphuric acid would be indicated by a precipitate formed on adding chloride of barium to a portion of the acid.—Hydrocyanic acid is well known as one of the most powerful of poisons, destructive to vegetable as well as animal life. Seeds immersed in it lose their germinating power, and the stems of sensitive plants lose their peculiar property by its appli cation. A drop of the anhydrous acid placed on the eye or throat of a dog will cause violent convulsions, soon terminating in death. Its vapor produces similar effects. Its action appears to be upon the heart, to which it is conveyed by the blood. Its medicinal properties were experimented upon by Italian practitioners in 1806. Magendie recommended its employment in diseases of the chest in 1817; and Drs. A. F. Thomson and Elliotson, by their investigations in 1820 and 1821, caused its use to be much extended in England. Dr. Pereira notices the following symptoms attending its use in gradually increased quantities: a peculiar bitter taste, increased secretion of saliva, irritation in the throat, nausea, disordered respiration, pain in the head, giddiness, sometimes faintness, obscurity of vision, and sleepiness. It has been used in pulmonary complaints, asthma, whooping cough, &c., and in violent and painful affections of the stomach unattended with inflammation; also as an anodyne in cancer, tic

douloureux, &c., and externally as a wash in some cutaneous diseases. It should from its uncertain strength be always administered in its minimum dose, and this gradually increased. In case of poisoning by an overdose, the antidote most to be depended upon is either ammonia or chlorine, administered internally in weak aqueous solution, and the vapor inhaled. Chloride of lime may offer itself as a ready means of affording the chlorine solution; and carbonate of ammonia, if at hand, or else the smelling salts, may be used to furnish the ammonia. Affusions of very cold water upon the head and spine have resuscitated animals apparently dead from the effects of hydrocyanic acid. Respiration by artificial means is also recommended. After death and before decomposition has taken place, the presence of hydrocyanic acid is rendered apparent in the blood vessels and also in the brain by its peculiar odor. To obtain the acid, the contents of the stomach should be washed with distilled water and filtered, and the filtrate distilled in a water bath. The product may then be subjected to the various tests given in the chemical works. One lately suggested by Liebig is recommended as also applicable for the estimation of the acid in cherry laurel water and other fluids in which it is present in very small quantity. To the hy drocyanic acid solution, after it is supersaturated with potassa, are added a few drops of chloride of sodium; nitrate of silver is now gradually added, and cyanide of silver is produced and dissolved by the cyanide of potassium, the two forming a double cyanide consisting of an equivalent of each. When a precipitate begins to appear, the amount of hydrocyanic acid that was present is known from the amount of nitrate of silver employed, being in the proportion of 2 equivalents to one of silver. If 85 grains of nitrate were used, this would give 54 of silver, and this or 27 grains would be the quantity of hydrocyanic acid present.

HYDRODYNAMICS. See HYDROMECHANICS. HYDROGEN (Gr. vdwp, water, and yevvaw, to produce), an elementary gaseous body, named from its property of forming water by combining with oxygen. Its symbol is H; chemical equivalent 1; weight compared with air .06926; 100 cubic inches weigh under ordinary pressure and temperature 2.14 grains, being 16 times less than an equal volume of oxygen, and 14.4 times less than air. It was known near the close of the 17th century, and was termed inflammable air from its burning with a flame; it was also called phlogiston, from the supposition of its being the matter of heat. Its real nature was first described by Cavendish in 1766 ("Philosophical Transactions," vol. lvi. p. 144). The gas is not found uncombined, but is readily obtained by decomposing water, of which it constitutes about by weight, the remainder being oxygen. This process is effected very much as metallic oxides are decomposed, some substance being presented to the compound which has a strong affinity for the

D'Entrecasteaux was sent in search of him in 1791, but no information was obtained as to the missing ships. M. Beautemps-Beaupré was principal marine surveyor in this expedition, and published a treatise on nautical surveying, as an appendix to the narrative of the voyage, in 1808. This work, with the exception of a small "Essay on the most Commodious Methods of Marine Surveying," by Alexander Dalrymple (1771), was the first treatise published in a practical shape, and gave an impulse to the cultivation of this branch of their profession by naval men. About this period M. BeautempsBeaupré took charge of the survey of the French coast, and trained a corps of hydrographical engineers, which enabled him to conduct that extensive work in a manner creditable to the nation, and provide competent surveyors for future expeditions. Spain has done much for hydrography, but in another way. The custom of examining officers as to their competency to navigate a vessel before promotion has given a high reputation to its mercantile marine, and nautical information from this class has been exceedingly valuable. At present, to render hydrographical surveys more perfect, professional hydrographers are employed, and hydrography is dependent on geodesy where any extensive surveys are carried on. Great Britain, France, and other nations have availed themselves of their trigonometrical surveys, and in the U. S. coast survey the trigonometrical and hydrographical labors are carried on together; this last is the greatest hydrographical work ever undertaken, and for accuracy and rapidity of execution has never been equalled. (See COAST SURVEY.) Almost every commercial nation has now its hydrographic office, and appropriations are made for surveys, not only of their own coasts, but those of other countries. The admiralty of Great Britain are, however, the most active in collecting and distributing information.

HYDROMECHANICS, that branch of natural philosophy which treats of the mechanics of liquid bodies, or in other words, of their laws of equilibrium and motion. A great diversity prevails in the naming of this science and its two divisions; but by employing the term above given, with hydrostatics and hydrody namics as the titles of the divisions, we adopt a nomenclature exactly corresponding with that of general mechanics, as well as true to the nature of each subject, and the previous usage of the terms themselves. Hydromechanics comprises properly those phenomena of liquids by which these bodies differ from solids or from bodies at large; hence, its foundation is laid in the properties that distinguish the liquid from other states of bodies, viz., the presence of cohesion, with great mobility of parts, and perfect elasticity. I. HYDROSTATICS. Suppose a hollow cylinder of any depth containing liquid, that this liquid could be destitute of weight, and that a movable piston of 100 square inches area exactly covers its upper surface; there would be no pressure of such a liquid itself

on the bottom of the hollow cylinder; but if a downward load or pressure of just 100 lbs. were applied to the piston, it is plain that this would be a pressure of 1 lb. on every square inch of its area; and the liquid not being compressible in any marked degree, and hence not capable of yielding before the piston, this pressure must be imparted downward from layer to layer, and must amount to a force of 1 lb. on each square inch of the base. A piston of 1 square inch area in the base would therefore receive 1 lb. of the pressure thus applied, and in proportion for any area. Again, if in various parts in the bottom, sides, and top of a vessel containing a confined body of water, pistons of equal size be inserted, and a given force applied upon any one of these, it is found that the same amount of pressure precisely is felt by each of the others, no matter what their number. Thus we arrive at the important and fundamental laws of hydrostatics, viz.: 1, that liquids transmit through their mass any pressure applied to them without diminution, so that the pressure is felt equally upon every equal area of the liquid or its enclosure; 2, weight being disregarded, the pressure is proportional to the area of every surface receiving it; 3, it is transmitted equally in all directions, upward, downward, and laterally. These results are simply due to mobility of parts with incompressibility, in effect, of mass. But as a consequence of this equality of pressure on every equal area within a liquid, the remarkable fact follows that if on a piston of 1 square inch area, touching the surface of a confined body of water, any pressure, say 10 lbs., be exerted, another piston of any larger area, say 40 square inches, in contact with the same body of water, will receive the given pressure on every surface of equal size, and consequently feel, and by proper connections transmit to machinery, the original pressure of 10 lbs. multiplied by the increase of area, in this case 400 lbs. In this way a confined body of liquid serves, first, as a convenient means of transmitting power, and, friction not considered, in any direction or to any distance; and secondly, as a means of multiplying the action of a power; so that water has been properly considered as a 7th element of machinery. The principle now explained includes all that is peculiar in the construction and action of Bramah's hydrostatic press; although in this the power applied can be further increased by the use of the lever, &c., or converted into velocity by trains of wheels. This most powerful of existing machines is used for pressing paper, cloth, gunpowder, &c., raising ships in docks, or any ponderous bodies. That employed in raising the immense tubes of the Britannia bridge had pistons whose respective areas were as 1 to 354, and, with a power on the smaller of 3.8 tons to the square inch, exerted a lifting force of 2,622 tons, and was calculated to be capable of throwing water in a vacuum to a height of 5.4 miles.-But the weight of the upper parts of any liquid mass must be sustained by the lower: and hence, the latter receives

a pressure from this cause that is proportional in every case to the depth or perpendicular distance of the part considered below the surface of the liquid. A horizontal square inch of surface at a depth of 1 foot in a liquid mass is pressed upon with the weight of the liquid column resting upon it, viz., nearly lb.; at the depth of 2 feet, by double this, or nearly lb.; and so on. Every particle of the liquid in such square inch is pressed downward with a force due to the weight of the minute column it sustains. But if a liquid be poured into any vessel or reservoir, and no further agitation imparted to it, it soon comes completely to rest; every part is presently in a state of equilibrium; and hence it follows, again, that whatever pressure a given particle or surface within a liquid may receive and exert, whether by the weight of parts over it, or by force applied from without upon a confined body of it, the contiguous parts of the liquid, by reaction, exert against this an equal pressure and in all other directions, upward and laterally. If this were not so, the particles pressed upon must move; and a liquid mass, instead of coming to rest, would be in a state of continued movement within itself. The upward and lateral pressures against a liquid particle or any surface at 2 feet depth, then, are just twice as great as they are at 1 foot depth; and so for any depth whatever. Consequently, though a vessel with perpendicular sides receives on its bottom the only pressure which a solid filling it, as ice, is capable of exerting, yet if the same ice be changed to water, there will be the same pressure as before on the bottom, and additional pressure arising against the sides of the vessel. With the contained solid, the whole tendency to burst the vessel is only equal to the whole weight of the solid; but with the contained liquid, much greater. Hence, again, if a small and a large column of liquid of equal heights meet in a common surface below, of any shape or size, the downward pressures of the two columns being at the same depth equal on any particle in the common surface of the two, the upward or lateral supporting pressures thence arising on the two sides will also be equal, and the two columns must perfectly balance. Liquids, then, balance each other by their pressures, not by their weights; and their pressures are as their perpendicular heights, not as their quantities. This is still true, though one or both the columns or bodies be oblique or irregular, and though the communication be by any set of tubes, not rising above the common sur face, or otherwise. Any quantity of liquid, however small, then, must balance any other, however great, provided the perpendicular heights be the same. Of this principle, commonly termed the "hydrostatic paradox," a familiar illustration is seen in the balancing of the liquid in the body of a coffee pot by the smaller column in the spout; and in the hydrostatic bellows, a few ounces of liquid may be made to balance many pounds of solid matter.

Since the pressure upon any vertical enclosing wall must increase regularly with the depth, it follows that, in a filled cubical reservoir, the average pressure against any side is at half the depth; but this is also the depth of the centre of gravity of that enclosing surface. The whole pressure of the liquid against such vertical side is just half that on the base; and it is therefore proportional to the product of the area of the side into the depth of its centre of gravity below the liquid surface. Thus, the whole pressure received by the sides and bottom of a filled cubical reservoir is 3 times that on the bottom, . ., 3 times the weight of the contained liquid. In case a lateral wall is inclined in either direction, and in case of an inclined or horizontal bottom, the law above given still holds true, viz., the whole pressure on any such surface is equal to that on a horizontal surface having the same area, and whose depth is the perpendicular depth of the centre of gravity of the surface considered below that of the liquid in such reservoir. If a distant body of liquid, as that in a larger reservoir, communicate by pipes with the confined body of liquid under consideration, and have a higher level, the principle is still the same; the depth in this case is that below the highest liquid surface; so that, in every case, the pressure on a surface will be determined by its area and the entire perpendicular depth of its centre of gravity, not by the shape, size, or connections of the containing vessel. From the increase of lateral pressure with depth, or "head of water," follows the necessity of making dams, flood gates, and locks proportionally more strong as they descend below the water surface; and from the relation of pressure to the amount of surface, as well as owing to the principle of the arch, the greater relative strength and economy of cylindrical over prismatic tubes for conveying liquids. The actual pressure on a horizontal square foot at 1 foot depth in pure water, is the weight of a cubic foot of water, viz., 62.8232 lbs.; at a depth of 2 feet, double this; and proportionally for all depths. At great depths, this pressure becomes enormous; and though it is doubtful whether it materially increases the density or lessens the mobility of water, even at the bottom of the deepest parts of the ocean, it produces many other and obvious effects. Sunken ships have the air in the pores of the wood displaced by the water, become relatively heavier than water, and refuse to rise. The side or bottom of a ship being broken in or perforated in any way, water rushes in with a force proportional to the depth of the opening, so that it is usually quite impracticable successfully to oppose a resistance to it from within. The liquid within the bodies of fishes, or in the human body, balances at moderate depths the pressure of the superincumbent mass, and by itself would do so at any depth; but at great depths the delicate membranes or vessels of the body give way owing to compression from without or displacement of their fluids, and hence divers, as well

as fish, can descend to limited depths only; the latter can descend 150 feet, but do not usually more than 100 feet. Pipes for the conveyance of water in cities suffer a pressure on the square inch of about 43 lbs. for every 10 feet of descent below the level of water in the reservoir, and require to be made correspondingly strong. The outward pressure upon the lower third of one side of a filled cubical box is just equal to that upon the upper two thirds; hence, the middle point of a horizontal line at the depth, in this case, will be the point at which a force could be applied from without so as exactly to sustain the whole pressure of the liquid from within, and to keep the side, if detached from the vessel, in exact equilibrium. This point is therefore the centre of pressure, a point that can be found for a surface of any shape or size, and that is of considerable importance in the practical business of opposing support to or confining liquid bodies. From the relation of liquid pressures to depth it follows also that liquids in different vessels, in bodies of any form, or in different parts of the same body, if free to move, can be in equilibrium and at rest only when their several surfaces stand at the same level. In consequence of the equality of upward to downward pressure at any given depth in their mass, liquids exert a buoyant or supporting power on all solids immersed in them. When an egg floats in the middle of a quantity of the lye of ashes, neither rising to the surface nor sinking to the bottom, it is because the density or specific gravity of the egg is just equal to that of the liquid. Any solid displaces its own volume of liquid; if then, in the case given, the density of the former be equal to that of the latter, the downward pressure which acts at the under surface of the egg is just equal to the upward pressure on the same surface due to the weight of the columns of liquid around it reach ing to the same depth, and the egg is in equilibrium and at rest. But suppose the liquid made denser; the surrounding columns now weigh and press more than that containing the egg; the downward pressure at its under surface is less than the upward, and it is pushed up in consequence. If the liquid be made rarer, the egg must sink. This is true of any solid set free or immersed in a liquid; the solid will always sink or rise until the whole downward pressure acting on its under surface, and due to its own weight with or without the addition of a weight of liquid above it, is exactly equal to the whole upward pressure of surrounding columns, and which is greater the lower the body sinks. If the body be of less weight than its own volume of water, it must float; and it will sink until it has displaced a volume of water whose weight is just equal to its own. It is on this principle that the weight of boats or their cargo is found by the amount of displacement they cause within a reservoir constructed for the purpose. All the upward pressures acting upon the bottom of any floating body, as a ship, combine so as to give a single resultant pressure acting vertically

upward; and the point at which this is in effect applied to the floating body is that of the centre of gravity of the previous mass of liquid, now displaced. This is for the floating body the centre of buoyancy; and the degree of support the body receives is termed its buoyancy. Glass or iron floats on quicksilver, but sinks in water; and some kinds of wood which float on water sink in oil or alcohol. The buoyancy of a body may be increased by incorporating or connecting with it bodies or tight spaces, having a certain volume with very small density. Thus, the human body is made more buoyant by an attachment of inflated bags or other contrivances, known as life preservers; empty boxes, termed "camels," are used to lighten ships over shoals, or to raise those that are sunk, being first let down filled with water, attached to the sides of the body to be lifted, and then exhausted by pumping, while air is allowed to enter. This is also the philosophy of the life boat, and of the recently invented diving engine, the nautilus, which rises or sinks in water according as certain chambers are filled with air or water, just as fish ascend and descend by inflating or emptying the air bladder. In fact, it is upon the principle now stated that the materials of wooden or iron ships can be made buoyant; the air filling their hold, like that in a caldron kettle floating on water, is really, so long as the water cannot enter to displace it, incorporate with the solids in a single body, and gives levity to the whole. Floating bodies may have equilibrium of three kinds: 1, neutral, or indifferent equilibrium, when the centre of gravity of the solid is at the same point with the centre of buoyancy, and the solid will rest indifferently in any position; 2, ordinary stable equilibrium, when the centre of gravity is below that of buoyancy, and the body can oscillate about its position, like a pendulum, but does not easily overturn; 3, unstable, when the centre of gravity is above, and the least inclination must overturn the body. But in a body floating as a ship and oscillating by winds or waves, mathematical analysis discovers a third point, above the centre of buoyancy, which, through any ordinary swing of the ship, keeps its place at every moment in a vertical line over the changing centre of the displaced liquid; this is called the meta-centre, and its peculiarity is, that if the centre of gravity of the ship and cargo be kept below it, even though above that of buoyancy, the ship still possesses stable equilibrium, righting itself after ordinary disturbances of position. A ship is thus rather a supported than a suspended body, and yet enjoys a good degree of stability; indeed, if the centre of gravity be too much depressed, the oscillations of the hull are unfavorably increased in sweep. II. HYDRODYNAMICS, or HYDRAULICS. Under this head three general cases are to be considered: that of liquids issuing from orifices; their flow through tubes, or in streams; and the effects of the momentum and impact of liquids, including the case of