escort, representing the navy. In 1901 he published a volume of interesting naval reminiscences, entitled A Sailor's Log.
EV′ANSTON. A city in Cook County, Ill., 12 miles north of Chicago, of which it is a popular suburb; on Lake Michigan, and on the Chicago and Northwestern and the Chicago, Milwaukee and Saint Paul railroads (Map: Illinois, E 1). It is beautifully situated on the lake, and is a well-known educational centre, the Northwestern University (q.v.) and the Garrett Biblical Institute being located here. Settled about 1835, Evanston was incorporated about 1890. Its government is administered by a mayor, elected biennially, and a unicameral city council. With the exception of the city treasurer and city clerk, who are chosen by popular election, all municipal officials are nominated by the mayor, with the consent of the council. The city owns and operates its water-works. Population in 1900, 19,259.
EVANSTON. A city and the county-seat of Uinta County, Wyo., in the southwestern part of the State; on the Bear River, and on the Union Pacific Railroad (Map: Wyoming, D 5). It is the seat of the State Institution for the Insane. The city is an agricultural and stock-raising and coal-mining district, and has some industrial establishments, the principal of which are railroad repair-shops. Oil has been found in the vicinity. Population, in 1890, 1995; in 1900, 2110.
EVANSVILLE. A city, port of entry, and county-seat of Vanderburg County, Ind., 180 miles southwest of Indianapolis; on the Ohio River, and on the Evansville and Terre Haute; the Louisville, Evansville and Saint Louis; the Louisville and Nashville, and other railroads (Map: Indiana, B 4). Coal abounds in the vicinity, and the city has a large trade in coal, timber, grain, pork, tobacco, and flour. The industrial plants include foundries and machine-shops, flouring-mills, furniture-factories, plow-works, and establishments for the manufacture of liquors, cotton and woolen goods, leather, etc. Among the more prominent buildings are the custom-house, court-house, city hall, United States Marine Hospital, State Hospital for the Insane, Evans (Temperance) Hall, and Willard Library. Other features of interest are Cooks, Mesker, and Garvin parks. The government is administered under a charter of 1893, which provides for a mayor, elected every four years, and a city council, the majority of the members of which are elected by wards, and the rest at large. All other officials, excepting the clerk of the city, who is chosen by the council, are appointed by the executive. The annual income of the city is about $940,000; annual expenditures, about $700,000. The principal items of expense are $50,000 for the police department, $55,000 for the fire department, and $170,000 for schools. The water-works are owned by the city, the yearly cost of operation being about $30,000. Evansville founded in 1816 by General Robert M. Evans, became the county-seat in 1819, and with a population of 4000 was incorporated as a city in 1847. Population in 1890, 50,750; in 1900, 59,007.
EVAPORATION (Lat. evaporatio, from evaporare, to evaporate, from e, away + vaporare, to emit vapor, from vapor, vapor). The change of state from solid or liquid to gaseous. All substances emit vapors, and those which at ordinary temperatures evaporate rapidly are said to be volatile. If the vessel in which a liquid is allowed to evaporate be open, the vapor will diffuse through the air, and evaporation will go on until there is no more liquid left. But if a liquid is placed in an air-tight vessel, evaporation goes on until the vapor escaping from the liquid has attained a certain definite pressure within the vessel, and then a dynamic equilibrium is established; that is to say, evaporation will not really cease, but for every amount of liquid evaporated an equal amount of vapor will be condensed, and, as a result, the amount of vapor, and hence the pressure, will remain constant. This constant pressure, measuring the vapor-tension of the liquid, depends entirely on two factors—the nature of the liquid and the temperature at which it is kept. At the same temperature, different liquids have different vapor-tensions, and when the temperature is raised, the vapor-tension of each is increased by an amount depending, again, upon the nature of the liquid. When the vapor of a substance has attained its full pressure, it is said to be saturated. The full vapor-tension corresponding to a certain temperature is not attained instantaneously, since evaporation requires time. The rate of evaporation depends on a large number of circumstances, some of which are more or less accidental, i.e. have nothing to do with the nature of the liquid experimented upon. Thus the size, shape, and material of the vessel in which evaporation is allowed to take place have a considerable influence on the rate. Other factors are the temperature of the liquid itself, the density of the atmosphere above its surface, and the magnitude of the surface exposed, shallow vessels being employed when rapid and copious evaporation is required. In case the liquid is a mixture or a solution, the rate of evaporation depends greatly upon its composition and upon the nature of the several constituents.
The Heat of Evaporation. The temperature of pure boiling water remains constant; yet heat must be continually supplied to it in order that evaporation may go on; the heat does not appear as sensible heat, but is required to produce the change of state. The heat required to convert one gram of a substance into vapor is called its heat of evaporation; that required to convert one ‘mol’ into vapor is called the molecular heat of evaporation. (By a ‘mol,’ or gram-molecule, is meant a number of grams equal to the chemical reacting weight of the substance; thus a mol of water is 18 grams, 18 being the molecular weight of water, H2O.) The number of substances whose heats of evaporation have been actually determined is as yet rather small. On the other hand, the laws of thermodynamics lead to methods that permit of calculating the heat of evaporation of a substance if certain other numerical data in connection with the substance are known. Thus, if the volume occupied by one gram of a liquid is V′, and the volume occupied by one gram of a saturated vapor is V, the ‘absolute temperature’ being T (by the absolute temperature is meant the ordinary centigrade temperature increased by 273; thus the normal boiling-temperature of water on (be absolute scale is 100° + 273° = 373°), and if the increase in the vapor-pressure
