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LEFT HYDRAULIC ENGINES 95 HYDROAEROPLANE sidered as a pseudo-salt. Thus, with ox- ide of sodium water forms the compound NaOH.O, or hydrate of soda, which is quite a different body to the simple NaO ; in fact, such is the attraction existing between the two bodies that they cannot be separated by the strongest heat. HYDRAULIC ENGINES, engines of which the motive power is water under pressure. In principle they do not dif- fer essentially from steam engines. The water acts by difference of pres- sure — i. e., it is admitted at a high pres- sure at the beginning of the stroke, and exhausted at a low pressure at the end of the stroke, thus giving a reciprocat- ing motion to the plunger. They may also be rotary. As water is much more dense than steam, the induction and eduction pipes have to be larger and have no abrupt angles. The eduction valve is required to open promptly at the end of the stroke, and the induction valve must not close till the stroke is com- pleted. In most of the hydraulic^ en- gines relief valves or other expedients are used to prevent these concussions. Since hydraulic engines work under much greater pressure they can be made jnuch smaller than steam engines. A common form is the three-cylinder sin- gle acting engine; in each cylinder works a plunger; water is admitted by valves behind the plungers and forces them out; at the conclusion of the out- stroke the pressure water-supply is cut off, and the exhaust valve opened, al- lowing the plunger to push the water out of the cylinder on the return stroke, and so on. Hydraulic engines have been intro- duced in some special industries in large foundries, in the working of cranes and other heavy machinery, and also for draining of mines. Hydraulic machines in a small form are used for many industrial purposes in the operating of printing presses, cir- cular saws, lathes, etc. The water head is obtained from a reservoir or accumu- lator or, in cities where considerable pressure is constantly supplied, through the water mains. HYDRAULIC LIFT, an apparatus on the principle of the hydraulic press, caused by means of a lever to draw up a chain which passes over sets of pulleys, and is thence conducted by leading pul- leys over a jib. The weight is by this arrangement raised many times the stroke of the ram. See Elevator. HYDRAULIC MINING, a system of mining in which the force of a jet of water is used to sluice down a bed of auriferous gravel or earth, which is passed through sluices to detain the par* tides of gold. HYDRAULIC RAM, a machine by which the fall of a column of water in a tube is caused to elevate a portion of itself to a height greater than that of its source. HYDRAULICS, that part of niechun- ical science which has to do with con- ducting, raising, and confining water or supplying it as a motive power. It is sometimes called hydro-mechanics, and is subdivided into hydrostatics and hy- drodynamics. HYDRAZINES (hi'dra-zlns), in chem- istry, H;N — NH2. Hydrazine is not known in a free state. Its derivatives are obtained by the reduction of nitro- soamines by zinc dust and acetic acid, (CH3)2N:NO + 4H' (dimethyl-nitroso- mine= (CHohN — NH2+HOH (di- methylhydrazine) ; also formed by the action of nascent hydrogen on nitroso- diethyl urea and diethyl hydrazine urea. HYDRIODIC ACID, in chemistry, hydric iodide, hydrogen iodide, HI. Hy- driodic acid is a colorless gas forming in the air. At a pressure of four at- mospheres at 0° it Is condensed to a liquid. It can be frozen at ordinary temperature by a mixture of ether and solid C02 at a temperature of 55°. HYDRIODIC ETHER, or IODIDE or ETHYL, a colorless uninflammable liquid composed of iodine and ethyl, of a sharp, pungent taste, and a penetrating ethereal odor; sp. gr. 1.94; boiling point 148° F.; sp. gr. of vapor 5.4. Formula C.HJ. At a red heat it is decomposed, giving off the purple vapors which are peculiar to iodine. Besides its use in the chemical laboratory as a reagent, it has attracted the attention of physicians, es- pecially in the United States and Eng- land, as a remedial agent, to be admin- istered by inhalation, in many cases in which the use of iodine is indicated. It is given in doses of 12 or 15 drops, in- haled from a napkin or sponge. In these doses, it is a gentle stimulant, and anti- spasmodic, but in larger quantities, and when inhaled for a considerable time, it becomes a powerful anesthetic agent. HYDROAEROPLANE, a type of air- plane so constructed that it may start from and alight upon water. There are two types of hydroaeroplanes, those in which pontoons are used for the landing gear, and those which have a boatlike understructure. The hydroaeroplane is usually a large, heavily constructed plane, capable of carrying heavy loads,