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Rh jointly from the Royal Society in 1893. From 1874 to 1884 van't Hoff's attention was mainly given to the law of mass-action, and he established the theorem known by his name, which connects quantitative displacement of equilibrium with change of temperature. From 1885 to 1895 he was engaged on the theory of solutions, and developing the analogy between dilute solutions and gases he showed that the osmotic pressure of a solution has the same value as the pressure that solute would exert if it were contained as a gas in the same volume as is occupied by the solution. From 1885 he published the Zeitschrift für physikalische Chemie, in collaboration with Professor W. Ostwald of Leipzig.

VAN WERT, a city and the county-seat of Van Wert county, Ohio, U.S.A., about 28 m. W. by N. of Lima. Pop. (1890) 5512; (1900) 6422 (221 foreign-born); (1910) 7157. Van Wert is served by the Pennsylvania and the Cincinnati Northern railways, and by an interurban electric line. Among the principal buildings are the city hall, the court house, the Brumback Library of Van Wert county (containing 14,650 volumes in 1908), the Home Office Building of the Home Guards of America (a fraternal society incorporated in 1899 and having about 16,000 members in 1910), and the Home Office Building of the Central Manufactures' Insurance Co. Van Wert is situated in a rich agricultural region. It has railway and machine shops and various manufactures. The municipality owns and operates the waterworks. Van Wert was settled about 1840, was incorporated as a town in 1848 and was chartered as a city in 1903. The county and the city were named in honour of Isaac Van Wert (1760-1828), one of the captors of Major John André.

VAPEREAU, LOUIS GUSTAVE (1810-1906), French man of letters and lexicographer, was born at Orleans on the 4th of April 1819. Educated at the École Normale he became a teacher of philosophy, and was entrusted by Victor Cousin with the preparation of his studies on the Pensées of Pascal. Under the empire his republican principles cost him his position, and Vapereau studied for the bar. He practised, however, little or not at all, and after 1870 he was appointed prefect of Cantal (1870) and of Tarn et Garonne (1871-73). From 1877 to 1888 he was inspector-general of public instruction. He was the author of some excellent editions of the classics, and of works on political and social questions, but he is famous for his valuable Dictionnaire universel des contemporains (1858; 6th ed., 1893), brought up to date in 1895 by a supplementary volume. He also drew up a Dictionnaire universel des littérateurs (1876). At the time of his death at Norsang-sur-Orge in 1906, he had been for twenty-six years a regular contributor to L'Illustration, some of his notes written for this journal being collected in 1896 as L'Homme et la vie.

VAPHIO, an ancient site in Laconia, Greece, on the right bank of the Eurotas, some 5 m. S. of Sparta. It is famous for its &ldquo;bee-hive&rdquo; tomb, excavated in 1889 by Dr Tsountas. This consists of a walled approach, or, about 97 ft. long, leading to a vaulted chamber some 33 ft. in diameter, in the floor of which the actual grave was cut. The objects found here and transferred to the National Museum in Athens include a large number of gems and amethyst beads, together with articles in gold, silver, bronze, iron, lead, amber and crystal. But by far the finest of them are two golden cups decorated with scenes in relief, picturing the capture of bulls. These form perhaps the most perfect works of &ldquo;Mycenaean&rdquo; or &ldquo;Minoan&rdquo; art which have survived. It seems likely that the Vaphio cups do not represent a local art but were imported from Crete, which at that early period was far ahead of mainland Greece in artistic development. The tomb, which probably belonged to Amyclae rather than to Pharis, as is commonly stated, is now almost entirely destroyed.

(M. N. T.)

VAPORIZATION. 1. In common language a vapour is a gaseous or elastic fluid, which emanates or evaporates from the surface of a solid or liquid at temperatures below its boiling-point. A volatile liquid or solid is one which evaporates rapidly at ordinary temperatures. It is a matter of common experience that evaporation is accelerated by currents of air, or by the use of an exhaust pump, or by any process which removes the vapour rapidly from the liquid. On the other hand, it is retarded, and finally ceases, if the vapour is allowed to accumulate in a closed space. When this equilibrium state is reached, the space is said to be saturated with the vapour; the density of the vapour is then the maximum which can exist in the presence of the liquid at the temperature of the experiment, and its pressure is called the saturation-pressure. The term vapour-pressure, when used without qualification, is also generally employed to denote the saturation or maximum pressure. Dalton showed that the saturation-pressure of a vapour depends only on the temperature, and is unaffected by the presence of any neutral gas or vapour. This relation has been more accurately verified by many subsequent observers, and the exceptions to it have been minutely studied and elucidated. The saturation-pressure invariably increases rapidly with rise of temperature, according to a regular law which has been the subject of many elaborate investigations. When the vapour-pressure of a liquid becomes equal to the external pressure, bubbles of vapour are freely formed in the interior of the liquid by the familiar process of boiling or ebullition. The temperature at which this occurs under the normal atmospheric pressure of 760 mm. of mercury (reduced to 0° C. and sea-level in latitude 45°) is termed the boiling-point (B.P.) of the liquid, and is usually determined by taking the temperature of the saturated vapour under normal pressure, to avoid error from super heating (see below, 3) of the liquid. If the external pressure remains constant, the temperature will also remain constant, provided that the liquid is pure and that its composition remains unaltered, until the whole is vaporized. If, on the other hand, the liquid is contained in a closed space, it may be made to boil at much lower temperatures by diminishing the pressure; or the temperature of the liquid may be raised considerably above the normal boiling-point, as in the boiler of a steam-engine, if the pressure is raised by preventing the free escape of the vapour. In all cases, if the temperature is given, there is a corresponding equilibrium or saturation pressure of the vapour, and vice versa, in accordance with Dalton's law. It was shown, however, by Cagniard de la Tour (Ann. Chirn. Phys., 1822, 1823) that the temperature and pressure of the liquid could not be raised indefinitely in this manner. By heating liquids in strong glass bulbs with manometers attached, he found that at a certain temperature the meniscus or curved surface separating the liquid from the vapour disappeared, and the bulb became filled with an apparently uniform substance. The temperature at which this mixing of liquid and vapour occurs is definite for each liquid, and is called the critical temperature. La Tour found the critical temperature in the case of water to be 362° C., a result which has been remarkably confirmed by later researches (Cailletet, Ann. Chirn. Phys. 25, p. 519, 1892). In many books of recent years it has been the custom, following a suggestion of Andrews, to restrict the term “gas” to temperatures above the critical temperature, and the term “vapour” to temperatures below. But this is often inconvenient in practice, as there is no sudden change in the gaseous phase at ordinary pressures on passing the critical temperature. It is more convenient to employ the terms “ vapour ” only when discussing the properties of the gaseous phase in relation to the liquid or solid, and to follow the common usage in describing substances like CO2, or even SO2 and NH3, as gases at ordinary temperatures and pressures.

2. Continuity of State.-The form of the isothermal curve, representing the compression of a vapour at constant temperature, consists, as shown in fig. 1, A, of three discontinuous branches. The relation between pressure and volume for an