Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/211

 CONCEPCION—CONDENSATION France, department of Finistere, arrondissement of Quimper, 14 miles south-east of that town. The town occupies a very picturesque situation. The old portion stands on an island, and is surrounded by ramparts, parts of which date from the 15th century. It is an important centre of the sardine and mackerel fisheries, in which in 1899, 555 boats with 2966 men were employed. Lobsters are also largely reared. Population (1881), 5006; (1896), 6343. Concepci6n, a city in Chile, capital of a province and department of the same name, near the mouth of the Bio-Bio, in 36° 49' 37" S. lat. and 73“ 06' W. long., 355 miles south of Santiago. Its altitude above the sea is 40 feet. Its population in 1895 was 39,837, and in 1898 it was 51,781. It is connected with the rest of the country by three lines of railway. It is an episcopal see, has a public school for boys, one for girls, an agricultural school, a seminary, and various other establishments. It publishes four newspapers. It is the principal commercial centre of Southern Chile, large quantities of wheat, flour, and wool being exported from it, through Talcahuano. Concepci6n, or Villa Concepci6n, the principal city in the northern part of Paraguay, with a population, including its suburbs, of 15,000, 125 miles north-north-east of Asuncion, and about 345 feet above sea-level. It has a national college, two public schools, a custom-house, a good harbour, a large market, two banks, a sub-department of the Agricultural Bank, a post office, street railway, and telephone company. It is the point whence most of the yerba mate is exported, and to which importation is made direct from all countries. Across the river is the English missionary station, the territory of which extends many miles into the interior, among the Indians. Concord, a town and village of Middlesex county, Massachusetts, U.S. A., in the north-eastern part of the state, on the Boston and Maine and the Fitchburg (now a part of the Boston and Maine) Railways. Besides its distinction in revolutionary history, the town is known as the home of Emerson, Hawthorne, Thoreau, and other literary men. Population (1880), 3922; 1890, 4427; (1900), 5652. Concord, capital of Merrimac county, Hew Hampshire, U.S.A., and of the state. It is situated in the southern part of the state, in 43° 12' U. lat. and 71° 30' W. long., at an altitude of 252 feet, on Merrimac river, which here is not navigable, but furnishes excellent water-power. The city has an area of 64 square miles, and is on the Boston and Maine Railway. Population (1880), 13,843; (1890), 17,004; (1900), 19,632. Concord, capital of Cabarrus county, North Carolina, U.S.A., in the Piedmont region, on the Rocky river, and on the Southern Railway. Population (1890), 4339; (1900), 7910, including 1789 negroes. Condensation of Gases.—If the volume of a gas continually decreases at a constant temperature, for which an increasing pressure is required, two cases may occur :—(1) The volume may continue to be homogeneously filled. (2) If the substance is contained in a certain volume, and if the pressure has a certain value, the substance may divide into two different phases, each of which is again homogeneous. The value of the temperature T decides which case will occur. The temperature which is the limit above which the space will always be homogeneously filled, and below which the substance divides into two phases, is called the critical temperaCritical ture of the substance. It differs greatly for Uire*6™' different substances, and if we represent it by Tc, the condition for the condensation of a gas is that T must be below Tc. If the substance is divided

OF

GASES

183

into two phases two different cases may occur. The denser phase may be either a liquid or a solid. The limiting temperature for these two cases, at which the division into three phases may occur, is called the triple point. Let us represent it by T3; if the term “ condensation of gases ” is taken in the sense of “ liquefaction of gases ”—which is usually done—the condition for condensation is Tc>T>T3. The opinion sometimes held that for all substances T3 is the same fraction of T (the value being about ),), has decidedly not been rigorously confirmed. Nor is this to be expected on account of the very different form of crystallization which the solid state presents. Thus for C0o, for which Tc = 304° on the absolute scale, and for which we may put T3 = 2160, this fraction is about 0-7; for water it descends down to 0‘42, and for other substances it may be still lower. If we confine ourselves to temperatures between Tc and T3 the gas will pass into a liquid if the pressure is sufficiently increased. When the formation of liquid sets in we call the gas a saturated vapour. If the decrease of volume is continued the gas pressure remains constant till all the vapour has passed into liquid. The invariability of the properties of the phases is in close connexion with the invariability of the pressure (called maximum tension). Throughout the course of the process of condensation these properties remain unchanged, provided the temperature remain constant; only the relative quantity of the two phases changes. Until all the gas has passed into liquid a further decrease of volume will not require increase of pressure. But as soon as the liquefaction is complete a slight decrease of volume will require a great increase of pressure, liquids being but slightly compressible. The pressure required to condense a gas varies with the temperature, becoming higher as the temperature rises. The highest pressure will therefore be found at Tc and the lowest at T3. We shall represent the pressure at Tc by pc. It is called the critical pressure. The pressure at T3 we shall represent by pz. It is called the pressure pressure of the triple point. The values of Tc and pc for different substances will be found at the end of this article. The values of T3 and pz are accurately known only for a few substances. As a rule pz is small, though occasionally it is greater than 1 atmosphere. This is the case with C0.2, and we may in general expect it if the value of T3/Tc is large. In this case there can only be a question of a real boiling-point (under the normal pressure) if the liquid can be supercooled. We may find the value of the pressure of the saturated vapour for each T in a geometrical way by drawing in the theoretical isothermal a straight line parallel to the v axis in such a way, that j J pdv will have the same value whether the straight line or the theoretical isothermal is followed. This construction, given by Maxwell, may be considered as a result of the application of the general rules for co-existing equilibrium, which we owe to Willard Gibbs. The construction derived from the rules of Gibbs is as follows :— Construe the free energy at a constant temperature, i.e., the quantity — J pdv as ordinate, if the abscissa represents v, and determine the inclination of the double tangent. Another construction derived from the rules of Gibbs might be expressed as follows:—Construe the value of pv - Jpdv as ordinate, the abscissa representing p, and determine the point of intersection of two of the three branches of this curve. As an approximate, half - empirical formula for the calculation of the pressure, - log ^ ^ may be used.