Page:The American Cyclopædia (1879) Volume VI.djvu/815

 EVAPORATION" 795 United States marine hospital, an opera house, and several hotels. The United States govern- ment is also erecting a large building to accom- modate the post office, United States courts, and the custom house. The Wabash and Erie canal extends to Toledo, Ohio, a length of 462 m. ; the Evansville and Orawfordsville railroad connects, ma Terre Haute, with the principal railroads of the state ; and the St. Louis and Southeastern railroad affords communication with that city. The Lake Erie, Evansville, and Southwestern railroad is completed to Boon- ville, the county seat of Warrick co. Evans- ville is a place of great commercial activity, and its manufactures are important. In 1871 the number of arrivals and departures of vessels was 2,586 ; value of imports, $169,588 30, the principal items being railroad iron, hardware, and coffee; amount of duties collected, $69,- 771 72. The principal articles of export are grain and flour, pork, salt, whiskey, cement and lime, cotton, hay, dried fruit, tobacco, and railroad iron. For the year ending June 30, 1872, there were 70 vessels of 10,047 tons belonging to the port, of which 56 of 9,046 tons were steamboats, and 14 of 1,001 tons canal boats ; built during the year, 8 steamboats of 721 tons. The total value of manufactures in 1872 was $5,469,763, consisting of furniture, flour and meal, machinery, planing mill pro- ducts, stoves and hollow ware, malt liquors, saddlery and harness, clothing, leather, mar- ble and dressed stone, cotton goods, cooperage, tinware, brick, &c. There are four national banks, with an aggregate capital of $1,950,000. The city is divided into twelve wards. Terms of the United States circuit and district courts are held here. The assessed value of property in 1851 was $1,907,632 ; in 1861, $5,385,675 ; in 1871, $20,528,075 ; taxation in 1871, $265,945. There are nrae large public school buildings belonging to the city, the high school building being the finest in the state. The number of separate departments in 1872 was 49, viz. : a high school, 8 grammar, and 40 primary, hav- ing 73 teachers and 4,583 pupils. The library association has 3,500 volumes. There are four daily newspapers (two German), one tri-week- ly, five weekly, and one semi-monthly. There are 24 churches, viz. : 4 Baptist, 1 Christian, 2 Episcopal, 1 Evangelical German, 3 Lutheran, 6 Methodist, 4 Presbyterian, and 3 Roman Catholic. Evansville was laid out in 1817. EVAPORATION, the dissipation of bodies by the volatile particles at their surface assuming the form of vapors and disappearing in the space around them. Liquids manifest this property most sensibly. Mercury exhibits it at temper- atures exceeding 60 F., as is shown by the in- visible fumes forming an amalgam on the sur- face of a bit of gold leaf, suspended for some days over the surface of the metal; and at temperatures far below this limit the vapor of mercury is present in the vacuum of the upper portions of the tubes of barometers and ther- mometers. Many solid bodies are subject to j it ; camphor, ice, snow, musk, &c., waste away by their particles being taken in invisible va- por into the surrounding atmosphere. It is a part of the process provided by nature for re- storing to the earth, through the medium of the clouds, the waters which have drained from its surface into the sea, and those also held in the soil, or upon the leaves of the forest. Once having performed their office, they are recalled by the process of evaporation, purified by it of their earthy contaminations, and are again poured out for the refreshment of vegetable and animal life. (See ATMOSPHEEE, DEW, HEAT, and ICE.) The following table gives some re- lations between the vapor of water and the liquid : Temperature, Fahrenheit. Volume of vapor contain- ing a unit of water. Mechanical force re- quired to vaporize, in foot pounds. 4 +82 50 100 140 212 250 650,588 182,323 102,670 22,513 7,937 1,696 896 1,395 1,488 1,565 1,722 1,847 2,078 2,192 Evaporation takes place in ordinary tempera- tures only from the surface of objects. It is greater in a warm dry air than when the tem- perature is low, or the atmosphere is already nearly filled with vapor. The more moisture is taken up into the same body of air, the more the process is retarded, until at length it is en- tirely checked: It is renewed by new supplies of dry air. The most favorable natural condi- tions for its rapid action are presented on the Atlantic ocean under the trade winds, which from the hot deserts of Africa blow across to the Cordilleras. The Amazon and the Orinoco are the fruits of the evaporation thus produced. The vapors that are continually ascending from moist surfaces are for the most part invisible, like those exhaled by breathing. Their exist- ence is proved by instruments called hygro- scopes, hygrometers, and psychrometers ; and at times they become visible, as when in clear frosty weather they rise copiously from the sur- face of pools fed by deep springs, and are seen congealed in white clouds, like the vapors of the breath under the same conditions. But unless deprived of their heat they possess the properties of gaseous bodies ; a given bulk of air or of other gases takes up the same quan- tity of a vapor as would be received in a vacant space of the same extent and temperature. This was conclusively proved from the experi- ments of Dr. Dal ton. It results that no more vapor can be received into any space after the weight of that already there amounts to the elastic force of the vapor at the temperature of the surface which generates it; in propor- tion as the air approaches this state of complete saturation it is said to be more moist, and evaporation proceeds more slowly. Increase of temperature adds to the elasticity of the va- por and promotes evaporation; cold reduces