EVAPORATION. Vapor is passed off from water, or moisture contained in any matter, in just proportion to the heat applied, and the dryness of the air. Air saturated with mois ture, can take up no more, and then the vapor of evaporation in being passed off, becomes condensed on the surface. The vapor passed off from a liquid, at any temperature, contains more heat than the fluid from which it is formed, and exhalation goes on faster or slower, accord ing to the pressure of the atmosphere. Yet the pressure of the atmosphere does not prevent evaporation, it simply retards it. If the atmos pheric pressure was entirely taken off, water would boil (theoretically) at about the freezing point, while at the sea level, water boils at 212° Fahr. In the vacuum pan, water boils at from 120° to 140° according to the perfect condition of exhaustion. it may seem strange, but never theless it is a fact, that evaporation is a cooling process, that is to say it has the effect of cooling the body from which it passes off. Hence the act of perspiring cools the body, by passing off the heat of the body with, and by the vapor. Ether, evaporating from a body in a draft of warm air, will cause the body even to freeze, and we all know that the earth is sensibly cooled by evaporation from its surface. Hence one of the advantages of drainage, by which the heat of the soil is conserved. If the excess of water must be carried upward by evapitration, in just proportion to the water evaporated will the soil he cooled. Hence cold, and water soaked soils produce semi-aquatic plants not found on warm, aerated soils. Moisture exhaled at a low temperature, is called vapor, and at a high tem perature, steam. In another sense invisible moisture is vapor, and visible moisture is steam. If the air is suddenly cooled over a heated soil, strongly evaporating moisture, it becomes visible as steam. Hence the expression, the steaming earth. Under the common pressure of the atmos phere, and below the temperature at which water boils, evaporation goes on quietly and slowly. In deep mines, which descend below the
level of the sea, water requires a greater heat than 212' to make it boil. But on high moun tainst or districts rising far above the level of the sea, the pressure of the air is lessened, and boiling takes place, as under the influence of the air pump, at lower degrees, according to pressure. The vapors exhaled from a liquid at any tem perature contain more heat than the fluid from which they sprung; and they cease to form whenever the supply of heat into the liquid is stopped. Yet, a thermometer held in the steam proceeding from hot water rises no higher than when placed in the water itself. The additional heat, therefore, contained by the vapor, is latent, and does not become sensible to the thermometer until the vapor condenses. Any quantity of water requires, for its conversion into vapor or steam, five and a half times as much heat as is sufficient to heat it from the freezing point of 32° to the boiling point of 212°. The quantity of heat absorbed by one volume of water in its conversion into steam, is about 1000° Fahr. ; it would be adequate to heat 1,000 volumes of water one degree of the same scale; or to raise one vol ume of boiling water, confined in a non-conduct ing vessel, to 1180°. Were the vessel, charged with water so heated, opened, it would be instan taneously emptied by vaporization, since the whole caloric equivalent to its constitution as steam is present. When, upon the other hand, steam is condensed by contact. with cold sub stances, so much heat is set free as is capable of beating five and a half times its weight of water, from 32° to 212° Fahr. If the supply of heat to a vessel be uniform, five hours and a half will be required to drive off its water in steam, provided one hour was taken in heating the water from the freezing to the boiling point, under the atmospherical pressure. (See Exhalation.)