HYDROCYANIC ACID

(Gr. liS^?, and xvx.es, blue.) This highly noxious compound was first discovered in Pru ian blue, and hence called Pru ic acid. It is best obtained by gently heating in a small retort a mixture of three parts of cyanuret of mercury and two of hydrochloric acid: the evolved vapours should be pa ed tlirough a tube containing fragments of marble, in order to absorb any hydrochloric acid that may chance to distil over, and ultimately condensed in a receiver immersed in a freezing mixture. The hydrocyanic acid is a colourle liquid, having a very strong odour, resembling that of bitter almonds; its specific gravity at 45° is 0*7. It boils at SQO, and freezes at 0°. Di olved in a large quantity of water, it imparts to it the smell and taste of laurel or bitter almond water: it is intensely poisonous. It consists of 26 cyanogen -h 1 hydrogen, or of 14 nitrogen 4- 12 carbon + 1 hydrogen; and its equivalent is 27. HYDRODYNAMICS (Gr.^^Sa;?, and ^v)ioL(x,t;, power), is the science which applies the principles of dynamics to determine the conditions of motion or rest m fluid bodies. It is usually divided into two branches; namely, hydrostatics, which explains the laws of the equilibrium, pre ure, and cohesion of fluids; and hydraulics, which explains the laws of their motion, together with the machines in which they are chiefly concerned. Though the term hydrodynamics is sometimes applied generaUy to fluids of all kinds, it is more usually restricted to the non-elastic or incompre ible fluids, as water, mercury, .; in which case the science which treats of the equilibrium of the compre ible and elastic fluids, like air, is called aerostatics, and that which treats of their motion pneumatics Hydrodynamics, thougli a science of immense importance in its applications to the various purposes of life, was not cultivated to any extent by the ancients, all their knowledge of the doctrine of fluids being limited to a few propositions regarding the pre ure and equilibrium of water. Arch i me des, indeed, established the general principles which serve as the foundation of hydrostatics, in his treatise De Ins ident i bus Humido; and Ctesibius and Hero, who flourished at Alexandria about 120 years after Christ, invented the fountain of compre ion, the syphon, and the forcing pump. Julius Front in us, who was inspector of the public fountains at Rome in the reigns of Nerva and Trajan, wrote a work on the Roman aqueducts, and the modes of distributing water then in use; but he appears to have been unacquainted with the law of the velocities of running water depending on the depth. The first modern treatise on hydrodynamics was published in 1639 by Castelli, a disciple of Galileo, entitled Bella Misura dell' Acque Correnti, and contains a satisfactory explanation of various phenomena in the motion of fluids. Torr i cell i discovered the important property that the velocities of fluids i uing through an orifice are as the square roots of the pre ures; Mariotte, in his Traits du Mouvement des Eaux, employed the principle of Torr i cell i, and explainel tlie [s. 585]