Evaporation

“ We have here, then, two processes—one of evaporation, another of condensation; and these, by an express and admirable provision, have a constant tendency to limit each other's operations. Evaporation is increased by heat, but it produces cold in proportion to the rapidity with which it is carried on. Condensation is produced by cold, and at the same time heat is liberated. ”

“ The process of evaporation is one of the most important in the catalogue of nature's dynamics. Without it there would be no verdure on the hills, no trees on the plains, no fields of waving grain, and no animal life upon the land surface of the globe. Evaporation is nature's method of irrigation, and the system is inaugurated on a grand scale, so that there are but few neglected spots upon the face of the earth which moisture, carried up from the great reservoirs of water, does not reach. The rate of evaporation, other things being equal, depends upon the extent of surface ”

“ Cooling Effect of Evaporation. We have seen that warming a gas increases its volume. This expansion is due to the increased motion of the warmed molecules. Now the molecules that escape from a liquid when it evaporates are naturally the fastest moving ones, i.e., the hottest ones. The molecules remaining are the slower moving ones or colder molecules. The liquid therefore becomes colder as it evaporates, unless it is heated. This explains why water evaporating on the surface of our bodies cools us. In evaporating, the water is continually losing its warm, fast moving molecules. ”

“ It is found that, under ordinary circumstances, evaporation is confined to the surface of the heated liquid, and is therefore slower or quicker, in proportion to the extension of that surface. Hence has arisen the adoption of wide, shallow vessels for containing fluids during their exposure to heat for this purpose. Evaporation proceeds most rapidly when a current of air (especially hot and dry air) is made to pass over the surface of the fluid; as, in this ease, the vapour is prevented from resting upon the surface, and impeding the process by its pressure. ”

“ Water absorbs more heat by elevation of temperature than any other substance. In passing from the solid to the liquid state, it absorbs a vast amount of heat without any elevation of temperature. The same thing occurs in the conversion of water into steam or vapor by evaporation. In the evaporation of one pound of water, as much heat is absorbed, or rendered latent, as would suffice to raise nearly a thousand pounds of water one degree in temperature. This heat is abstracted from surrounding objects; and, hence, evaporation is one of the most powerful means of producing cold. ”

“ Every considerable addition to its water wells up, and soaks its very surface; and that which is added after it is already brim full, must flow off over the surface, or lie in puddles upon it. Evaporation is a slow process, and it becomes more and more slow as the level of the water recedes from the surface, and is sheltered, by the overlying earth, from the action of sun and wind. ”

“ If currents of air are continually removing the saturated atmosphere from above the surface of a liquid, evaporation takes place most rapidly, since new portions of air, capable of absorbing moisture, are presented to it. ”

“ Another circumstance evincing that heat is the principal cause of evaporation is that at the time of water being converted into steam, a great quantity of heat is taken away from the neighbouring bodies. If a thermometer be repeatedly dipped in ether, or in rectified spirit of wine, and exposed to a blast of air, to expedite the evaporation by perpetually removing the saturated air from it, the thermometer will presently sink below freezing. ”

“ The daily evaporation of moisture into the atmosphere keeps the atmosphere more or less full of water vapor; but the atmosphere can hold only a definite amount of vapor at a given temperature, and as soon as it contains the maximum amount for that temperature, further evaporation ceases. If clothes are hung out on a damp, murky day they do not dry, because the air contains all the moisture it can hold, and the moisture in the clothes has no chance to evaporate. ”

“ The proportion constantly varies. Evaporation is not a result of accident; it seems an established law that the air shall constantly absorb vapour until it has reached the maximum that it can hold. Experiments have been tried, in which dry air has been pressed upon the surface of water with great force, but no degree of pressure could prevent the formation of vapour. ”

“ By the process of evaporation, it is transformed into vapor and is carried over the land by currents of air. As it comes into contact with colder currents, condensation ensues and then precipitation, and our raindrop descends to earth once more. ”

“ The third essay, ‘On Evaporation,’ showed the quantity of water evaporated in a given time to be strictly proportional to the force of aqueous vapour at the same temperature, and to be the same in air as in vacuo. The fourth, ‘On the Expansion of Gases by Heat,’ announced the law (arrived at almost simultaneously by Gay-Lussac) ‘that all elastic fluids expand the same quantity by heat’ (p. 537) . ”

“ The cold which follows evaporation is caused by a loss of the heat which is required for evaporation, and which passes off with the vapor, as a solution, in the atmosphere; and as heat leaves the body to aid evaporation, it is evident that that body cannot be cooled by the process, below the dew-point at which evaporation ceases. The popular notion that a body may be cooled almost to the freezing-point, in a hot Summer day, by the action of heat alone, is, then, erroneous. ”

“ The liquid saturates the walls of this vessel and evaporates after the manner in which it would in the plant, excepting for the modifications induced by light and by the incidental structural features of the plant. The exposure of the instrument during any given set of conditions for any period gives data from which the actual evaporation may be calculated. Comparison with measured areas of leaf-surface shows that departures from the normal evaporation are made by the plant, and the departure may be expressed as the relative transpiration. ”

“ Here it is rendered latent by the forces of evaporation, and in this form, having fulfilled its office in the economy of the ocean, it passes off into the air, there to enter, in mysterious ways, upon the performance of its manifold tasks.
890. Actinic processes.—As evaporation goes on by day or night, the upper stratum is rendered heavier by reason of both the heat and the fresh water borne away by evaporation; the upper water having been thus rendered both Salter and cooler, has its specific gravity increased so much the more. ”

“ The diminution of evaporation is not the only mode in which under-draining affects the temperature. The increased effective hygroscopicity of the soil increases its absorbent action, and the condensation of atmospheric vapor thus produced is attended with the manifestation of heat. ”

“ Thus, in order to know the evaporative power of the air, we have only to determine the vapor pressure of the air and to remember that the rate of evaporation is in proportion to this pressure, i.e.: when the vapor pressure is high the evaporation will be slow and the eggs remain too wet, and when the vapor pressure is low the eggs will be excessively dried out. ”

“ The descent of the coloured liquid in the other stem will mark the diminution of temperature caused by the evaporation of the water from the humid surface. The drier the ambient air is, the more rapidly will the evaporation go on; and the cold produced will be greater. When the air is nearly saturated with moisture, the evaporation goes on slowly; the cold produced is moderate, because the ball regains a large portion of its lost heat from surrounding bodies; and the degree of refrigeration of the ball is an index of the dryness of the air. ”

“ The opposite is CONDENSATION. In meteorology, the term evaporation is usually restricted in use to the change of water vapor from liquid to gas, while SUBLIMATION is used for the change from solid to gas as well as from gas to solid. Energy is lost by an evaporating liquid, and when n. heat is added externally, the liquid always cools. ”

“ We have seen, that evaporation proceeds much more rapidly from a surface of water, as a pond or river, than from a land surface, unless it be fully saturated, and that evaporation from the water exceeds the whole amount of rain, about as much as evaporation from the land falls short of the amount of rain. ”

“ The constant evaporation from the upper surface will prevent dampness to any serious degree in the lower thicknesses of material. For ourselves, we prefer the method which most effectually prevents the escape of heat, while also securing that vapour shall not be unduly condensed. ”

“ In other words, as the heat increases evaporation increases within certain limits and bears some sort of a ratio to it. Evaporation is not confined to water, but as our subject has to deal with atmospheric phenomena we will speak of it only in its relation to aqueous moisture. ”

“ EVAPORATION. I. Solution of water in air; in the matter of heat; pulse-glass. 2. Heat is the principal cause of evaporation; thermometer cooled by evaporation of ether; heat given from steam to the worm-tub; warmth accompanying rain. 3. Steam condensed on the eduction of heat; moisture on cold walls; south-west and north-east winds. 4. Solution of salt and of blue vitriol in the matter of heat. II. Other vapours may precipitate steam and form rain. ”

“ EVAPORATION, is the conversion of fluids, chiefly of water, into vapour which is specifically lighter than the atmosphere.
There is no subject that has occasioned a greater variety of opinions than the theory of evaporation ”

“ EVAPORATION OF LIQUIDS The greatest care must be taken to prevent loss of solutions during processes of evaporation, either from too violent ebullition, from evaporation to dryness and spattering, or from the evolution of gas during the heating. In general, evaporation upon the steam bath is to be preferred to other methods on account of the impossibility of loss by spattering. If the steam baths are well protected from dust, solutions should be left without covers during evaporation ”

“ The same evaporation which thus affects the hands, renders the land cold, when filled with water, every gallon of which thus carried off requires, and actually carries off, as much heat as would raise five and a half gallons of water from the freezing to the boiling point. ”

“ When evaporation from the surface is active and prolonged, waters which had previously gone down to the zone of soil-formation and taken up soluble matter, may rise again to the surface bringing the soluble matter up and leaving it at the surface on evaporation. ”