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 ACIREALE —ACOUSTICS a share, and “ natural soda ” from California, from Egypt, (kc. (comp. vol. xxii. p. 240) will come in to a certain extent. Nitric acid (comp. vol. xvii. p. 518).—The manufacture of this acid has been very largely extended, on account both of the enormously increased production of explosives for engineering and mining purposes and of the substitution of smokeless powders for the old gunpowder made from saltpetre. But it is still effected by the wasteful process of decomposing sodium nitrate by sulphuric acid, producing as residue “ nitre-cake,” a product of very little value. Several attempts have been made to decompose the nitrate in a more profitable way, e.g., by means of ferric oxide, in which case the soda is converted into the valuable shape of NaOH, and the ferric oxide is recovered as such; but none of these processes have emerged beyond the experimental stage. The old process has been improved on various sides, chiefly by the elaboration of more rational systems of condensation, and by conducting the operation in such a manner that it furnishes at once a maximum of strong and sufficiently pure acid, which does not need to be freed from the lower oxides of nitrogen by the process of “ bleaching”; also by separating the decomposition of sodium nitrate into two stages and conducting it as a continuous process, with the employment of comparatively weak sulphuric acid even for the production of strong nitric acid (the Rhenania-Uebel process). References. — The principal work on acids and alkali is Lunge’s Sulphuric Acid and Alkali, 2nd ed. 3 vols. 1891-96. The same work has also appeared in French (by Lunge and Naville), 3 vols. 1879-81, and in German, 2nd ed. 3 vols. 1893-96. The same author has given a synopsis of the manufacture of sulphuric acid up to 1899 in his article “ Schwefel ” in Muspratt’s Chemie, 4th ed. vii. 1114-1368. Other works are:—JuRisen, Handbuch der Schwefelsdurefabrikation, 1893.—Sorel, Fabrication de Vacide sulfurique, 1887.—Annual Reports on Alkali, <Lr.., Works, from 1864 upwards.—Journal of the Society of Chemical Industry, from 1882. Fischer’s Jahresberichte der chemischen Technologie. Chemische Industrie. Zeitschrift fur angewandte Chemie. (G. L.) AcireaSe, a town and episcopal see of the province of Catania, Sicily, Italy, 9 miles by E. from Catania by rail. It has a school of the industrial arts and sciences. Population, 25,900 (1881); 35,459 (1901). Acland, Sir Henry Wentworth Dyke, Bart. (1815-1900), British medical professor and man of learning, was born 23rd August 1815, and was the fourth son of Sir Thomas Dyke Acland. He was educated at Harrow and Christ Church, was elected Fellow of All Souls’ in 1841, and, following the medical profession, took his Oxford degree of M.D. in 1848, having in 1845 been appointed Lee’s Header in Anatomy. The revival of medical study and the introduction of the study of natural science into Oxford were in great measure due to Sir Henry Acland. He promoted in every way the foundation of laboratories and of the Oxford Museum, formed an extensive series of physiological preparations on the plan of John Hunter, and did far more to overcome the indifference and remove the suspicion generally prevalent when he commenced his labours than could have been achieved by one less generally acceptable from his birth, his amenity, and the elevation of his character. “ To Henry Acland,” says Buskin, “ physiology was an intrusted gospel of which he was the solitary preacher to the heathen.” On the other hand, as has been well observed, his thorough classical training preserved science at Oxford from too abrupt a severance from the humanities. In conjunction with his intimate friend, Dean Liddell, he revolutionized the study of art and archaeology, so that the cultivation of these subjects, for which, as Buskin declared, no one at Oxford cared before their time, began to flourish in the

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University. He published a memoir on the visitation of cholera in 1854, and another on the topography of the Troad ; but his claims to remembrance rest chiefly on his systematic, sedulous, and successful promotion of his favourite objects. He was Badcliffe librarian (1851), Begins Professor of Medicine (1858), and president of the General Medical Council from 1874 to 1887 ; he was also a curator of the University Galleries and of the Bodleian library. He was created a baronet in 1890. He resigned the Regius Professorship in 1894, and died in October 1900. Acoustics.—The original article in the ninth edition of this Encyclopaedia (hereafter referred to as O. A.) contains an account of many of the leading phenomena of acoustics and their elementary theory, so that it is only necessary here to supplement that account by discussing certain theoretical points, and by describing certain phenomena and methods of investigation which have been brought into more prominent notice, or have been discovered, in recent years. The following elementary method of obtaining the velocity of plane waves of longitudinal disturbance in air brings into prominence the fact that the velocity VeIocit of depends on and varies slightly with the excess So°nd.X ° and defect of pressure from the normal or undisturbed value. The method with appropriate modification will give the velocity of transverse disturbance in strings and longitudinal disturbance in rods. We suppose that a disturbance, the same at every point in a plane perpendicular to the direction of propagation, is in some way made and started in the air, and that external forces are applied to every particle in such a manner that the disturbance is constrained to move on with uniform velocity unchanged in form. The force per unit mass can be expressed in terms of the pressures due to the state of strain together with the applied force, and it can also be expressed in terms of the acceleration which is obtained from the condition that the disturbance moves on unchanged in form with constant velocity U. For this implies that the change in velocity of a particle at a given point during a small time dt is equal to the difference in the velocities at a given instant at that point, and at a point a distance Jdt back along the line of propagation. In Fig. 1 let QP represent the displacement curve of the disturbance (O. A. § 12). Let RM represent two points distant Xidt apart along the line of propagation. Let MP, NQ, represent at a given instant the displacements of the particles which were originally at rest at M, K, but which are now displaced in the direction of propagation by amounts proportional to MP and NQ. Let Mm, represent the velocities of the particles. Then the velocity of the M particle will change from Mm to mn in time dt, so that the acceleration a is given Pig- 1by Us - Mm _ tt ~ (1) dt NM since NM = Udi. But, drawing the tangent PT at P, the velocity at P is the rate at which the displacement is growing at P as the displacement curve travels along, and this is evidently at the rate TS per time taken by the disturbance to travel over NM. TS TT TS Then «m:=-37=U.—— (2) dt NM This may be written as - U But we may express the pressure excess at M in terms of the displacement. For if the layer of air originally between M and N were all compressed as it is at P, the surface through N would be displaced forward NT, while that through M would only be displaced forward MP, or there would be a compression TS in length MN. Since the disturbance is purely longitudinal, this implies that there is a diminution of