, the act of dissipating the humidity of a body in fumes or vapour; differing from exhalation, which is properly a dispersion of dry particles issuing from a body.

Evaporation is usually produced by heat, and by the change of air: thus, common salt is formed by evaporating all the humidity in the brine or salt water; which evaporation is either performed by the heat of the sun, as in the salt-works on the sea-coast, &c; or by means of fire, as at the salt-springs, &c: and it is well known how useful a brisk wind is in drying wet clothes, or the surface of the ground; while in a calm, still atmosphere, they dry extreme slowly.

But, though Evaporation be generally considered as an effect of the heat and motion of the air, yet M. Gauteron, in the Memoires de l'Acad. des Scienc. an. 1705, shews, that a quite opposite cause may have the same effect, and that sluids lose more of their parts in the severest frost than when the air is moderately warm: thus, in the great frost of the year 1708, he found that the greater the cold, the more considerable the evaporation; and that ice itself lost full as much as the warmer liquors that did not freeze.

There are indeed few subjects of philosophical investigation that have occasioned a greater variety of opinion than the theory of Evaporation, or of the ascent of water in such a fluid as air, between 8 and 9 hundred times lighter than itself, to different heights according to the different densities of the air; in which case it must be specifically lighter than the air through which it ascends. The Cartesians account for it by supposing, that by the action of the sun upon the water, small particles of the water are formed into hollow spheres and filled with the materia subtilis, which renders them specifically lighter than the ambient air, so that they are buoyed up by it.

Dr. Nieuwentyt, in his Religious Philosopher, cont. 19, and several others, have alleged, that the sun emits particles of fire which adhere to those of water, and form moleculæ, or small bodies, lighter than an equal bulk of air, which consequently ascend till they come to a height where the air is of the same specific gravity with themselves; and that these particles being separated from the fire with which they are incorporated, coalesce and descend in dew or rain.

Dr. Halley has advanced another hypothesis, which has been more generally received: he imagined, that by the action of the sun on the surface of the water, the aqueous particles are formed into hollow spherules, that are filled with a finer air highly rarefied, so as to become specifically lighter than the external air. Philos. Trans. number 192, or Abr. vol. 2, p. 126.

Dr. Desaguliers, dissatisfied with these two hypotheses, proposes another in the Philos. Trans. number 407, or Abr. vol. 7, pa. 61. See also his Course of | Experimental Philosophy, vol. 2, p. 336. He supposes that heat acts more powerfully on water than on common air; that the same degree of heat which rarefies air two-thirds, will rarefy water near 14,000 times; and that a very small degree of heat will raise a steam or vapour from water, even in winter, whilst it condenses the air; and thus the particles of water are converted into vapour by being made to repel each other strongly, and, deriving electricity from the particles of air to which they are contiguous, are repelled by them and by each other, so as to form a fluid which, being lighter than the air, rises in it, according to their relative gravities. The particles of this vapour retain their repellent force for a considerable time, till, by some diminution of the density of the air in which they float, they are precipitated downwards, and brought within the sphere of each other's attraction of cohesion, and so join again into drops of water.

Many objections have been urged against this opinion, by Mr. Clare in his Treatise of the Motion of Fluids, pa. 294, and by Mr. Rowning in his System of Philosophy, part 2, diss. 6; to which Dr. Hamilton has added the two following, viz, that if heat were the only cause of evaporation, water would evaporate faster in a warm close room, than when exposed in a colder place, where there is a constant current of air; which is contrary to experience; and that the evaporation of water is so far from depending on its being rarefied by heat, that it is carried on even whilst water is condensed by the coldness of the air, till it freezes; and since it evaporates even when frozen into hard ice, it must also evaporate in all the lesser degrees of cold. And therefore heat does not seem to be the principal, much less the only cause of Evaporation.

Others have more successfully accounted for the phenomena of Evaporation on another principle, viz that of solution; and shewn, from a variety of experiments, that what we call Evaporation, is nothing more than a gradual solution of water in air, produced and supported by the same means, viz, attraction, heat, and motion, by which other solutions are effected.

It seems the Abbé Nollet first started this opinion, though without much pursuing it, in his Leçons de Physique Experimentale, first published in 1743: he offers it as a conjecture, that the air of the atmosphere serves as a solvent or sponge, with regard to the bodies that encompass it, and receives into its pores the vapours and exhalations that are detached from the masses to which they belong in a fluid state; and he accounts for their ascent on the same principles with the ascent of liquors in capillary tubes. On his hypothesis, the condensation of the air contributes, like the squeezing of a sponge, to their descent.

Dr. Franklin, in a paper of Philosophical and Meteorological Observations, Conjectures and Suppositions, delivered to the Royal Society about the year 1747, and read in 1756, suggested a similar hypothesis: he observes, that air and water mutually attract each other; and hence he concludes, that water will dissolve in air, as salt in water; every particle of air assuming one or more particles of water; and when too much is added, it precipitates in rain. But as there is not the same contiguity between the particles of air as of water, the solution of water in air is not carried on without a motion of the air, so as to cause a fresh accession of dry particles. A small degree of heat so weakens the cohesion of the particles of water, that those on the surface easily quit it, and adhere to the particles of air: a greater degree of heat is necessary to break the cohesion between water and air; for its particles being by heat repelled to a greater distance from each other, thereby more easily keep the particles of water that are annexed to them from running into cohesions that would obstruct, refract, or reflect the heat: and hence it happens that when we breathe in warm air, though the same quantity of moisture may be taken up from the lungs as when we breathe in cold air, yet that moisture is not so visible. On these principles he accounts for the production and different appearances of fogs, mists, and clouds. He adds, that if the particles of water bring electrical fire when they attach themselves to air, the repulsion between the particles of water electrisied, joins with the natural repulsion of the air to force its particles to a greater distance, so that the air being more dilated, rises and carries up with it the water; which mutual repulsion of the particles of air is increased by a mixture of common fire in the particles of water. When air, loaded with surrounding particles of water, is compressed by adverse winds, or by being driven against mountains, &c, or condensed by taking away the fire that assisted it in expanding, the particles will approach one another, and the air with its water will descend as a dew; or if the water surrounding one particle of air come in contact with the water surrounding another, they coalesce and form a drop, producing rain; and since it is a well-known fact, that vapour is a good conductor of electricity, as well as of common fire, it is reasonable to conclude with Mr. Henley, that Evaporation is one great cause of the clouds becoming at times surcharged with this fluid. Philos. Trans. vol. 67, pa. 134. See also vol. 55, p. 182, or Franklin's Letters and Papers on Philosophical Subjects, p. 42 &c, and pa. 182, ed. 1769.

M. le Roi, of the Acad. of Sciences at Paris, has also advanced the same opinion, and supported it by a variety of facts and observations in the Memoirs for the year 1751. He shews, that water does undergo in the air a real dissolution, forming with it a transparent mixture, and possessing the same properties with the solutions of most salts in water; and that the two principal causes which promote the solution of water in the air, are heat and wind; that the hotter the air is, within a certain limit, the more water it will dissolve; and that at a certain degree of heat the air will be saturated with water; and by determining at different times the degree of the air's saturation, he estimates the influence of those causes on which the quantity depends that is suspended in the air in a state of solution. Accordingly, the air, heated by evaporating substances to which it is contiguous, becomes more rare and light, rises and gives way to a denser air; and, by being thus removed, contributes to accelerate the Evaporation. The fixed air contained in the internal parts of evaporating bodies, put into action by heat, seems also to increase their Evaporation. The wind is another cause of the increase of Evaporation, chiefly by changing and renewing the air which immediately encompasses the evaporating substances; and from the consideration | of these two causes combined, it appears why the quantity of vapour raised in the night is less than that of the day, since the air is then both less heated and less agitated. To the objection urged against this hypothesis, on account of the Evaporation of water in a vacuum, this ingenious writer replies, that the water itself contains a great quantity of air, which gradually disengages itself, and causes the Evaporation; and that it is impossible that a space containing water which evaporates should remain perfectly free from air. To this objection a late writer, Dr. Dobson of Liverpool, replies, that though air appears, by unquestionable experiments, to be a chemical solvent of water, and as such, is to be considered as one cause of its Evaporation, heat is another cause, acting without the intervention of air, and producing a copious Evaporation in an exhausted receiver; agrecably to an experiment of Dr. Irving, who says, that in an exhausted receiver water rises in vapour more copiously at 180° of Fahrenheit's thermometer, than in the open air at 212°, its boiling point. Dr. Dobson farther adds, that water may exist in air in three different states; in a state of perfect solution, when the air will be clear, dry, and heavy, and its powers of solution still active; in a state of beginning precipitation, when it becomes moist and foggy, its powers of solution are diminished, and it becomes lighter in proportion as its water is deposited; and also, when it is completely precipitated, which may happen either by a slower process, when the dissolved water falls in a drizzling rain, or by a more sudden process, when it descends in brisk showers. Philos. Trans. vol. 67, p. 257, and Phipps's Voyage towards the North Pole, p. 211.

Dr. Hamilton, professor of philosophy in the university of Dublin, transmitted to the Royal Society in 1765, a long Dissertation on the nature of Evaporation, in which he proposes and establishes this theory of solution; and though other writers had been prior in their conjectures, and even in their reasoning on this subject, Dr. Hamilton assures us, that he has not represented any thing as new which he was conscious had ever been proposed by any one before him, even as a conjecture. Dr. hamilton having evinced the agreement between Solution and Evaporation, concludes, that Evaporation is nothing more than a gradual solution of water in air, produced and promoted by attraction, heat, and motion, just as other solutions are effected.

To account for the ascent of aqueous vapours into the atmosphere, this ingenious writer observes, that the lowest part of the air being pressed by the weight of the upper against the surface of the water, and continually rubbing upon it by its motion, attracts and dissolves those particles with which it is in contact, and separates them from the rest of the water. And since the cause of solution in this case is the stronger attraction of the particles of water towards the air, than towards each other, those that are already dissolved and taken up, will be still farther raised by the attraction of the dry air that lies over them, and thus will diffuse themselves, rising gradually higher and higher, and so leave the lowest air not so much saturated but that it will still be able to dissolve and take up fresh particles of water; which process is greatly promoted by the motion of the wind. When the vapours are thus raised and carried by the winds into the higher and colder parts of the atmosphere, some of them will coalesce into small particles, which slightly attracting each other, and being intermixed with air, will form clouds; and these clouds will float at different heights, according to the quantity of vapour borne up, and the degree of heat in the upper parts of the atmosphere: and thus clouds are generally higher in summer than in winter. When the clouds are much increased by a continual addition of vapours, and their particles are driven close together by the force of the winds, they will run into drops heavy enough to fall down in rain. If the clouds be frozen before their particles are gathered into drops, small pieces of them being condensed and made heavier by the cold, fall down in thin flakes of snow. When the particles are formed into drops before they are frozen, they become hailstones. When the air is replete with vapours, and a cold breeze springs up, which checks the solution of them, clouds are formed in the lower parts of the atmosphere, and compose a mist or fog, which usually happens in a cold morning, and is dispersed when the sun has warmed the air, and made it capable of dissolving these watry particles. Southerly winds commonly bring rain, because, being warm and replete with aqueous vapours, they are cooled by coming into a colder climate; and therefore they part with some of them, and suffer them to precipitate in rain: whereas northerly winds, being cold, and acquiring additional heat by coming into a warmer climate, are ready to dissolve and receive more vapour than they before contained; and therefore, by long continuance, they are dry and parching, and commonly attended with fair weather.

Changes of the air, with respect to its density and rarity, as well as its heat and cold, will produce contrary effects in the solution of water, and the consequent ascent or fall of vapours. Several experiments prove that air, when raresied, cannot keep so much water dissolved as it does in a more condensed state; and therefore when the atmosphere is saturated with water, and changes from a denser to a rarer state, the high and colder parts of it will let go some of the water before dissolved, forming new clouds, and disposing them to fall down in rain: but a change from a rarer to a denser state will stop the precipitation of the water, and enable the air to dissolve, either in whole or in part, some of those clouds that were formed before, and render their particles less apt to run into drops and fall down in rain: on this account, we generally sind that the rarefied and condensed states of the atmosphere are respectively attended with rain or fair weather. See more on this subject in the Philos. Trans. vol. 55, pa. 146, or Hamilton's Philosophical Essays, p. 33.

Dr. Halley, before mentioned, has furnished some experiments on the Evaporation of water; the result of which is contained in the following articles: 1. That water salted to about the same degree as sea-water, and exposed to a heat equal to that of a summer's day, did, from a circular surface of about 8 inches diameter, evaporate at the rate of 6 ounces in 24 hours: whence by a calculus he finds that, in such circumstances, the<*>water evaporates 1-10th of an inch deep in 12 hours: | which quantity, he observes, will be found abundantly sofficient to furnish all the rains, springs, dews, &c. By this experiment, every 10 square inches of surface of the water yield in vapour per diem a cubic inch of water: and each square foot half a wine pint; every space of 4 feet square, a gallon; a mile square, 6914 tuns; and a square degree, of 69 English miles, will evaporate 33 millions of tuns a day; and the whole Mediterranean, computed to contain 160 square degrees, at least 5280 millions of tuns each day. Philos. Trans. number 189, or Abridg. vol. 2, pa. 108.—2. A surface of 8 square inches, evaporated purely by the natural warmth of the weather, without either wind or sun, in the course of a whole year, 16292 grains of water, or 64 cubic inches; consequently, the depth of water thus evaporated in one year, amounts to 8 inches. But this being too little to answer the experiments of the French, who found that it rained 19 inches of water in one year at Paris; or those of Mr. Townley, who found the annual quantity of rain in Lancashire above 40 inches; he concludes, that the sun and wind contribute more to Evaporation than any internal heat or agitation of the water. In effect, Dr. Halley fixes the annual Evaporation of London at 48 inches; and Dr. Dobson states the same for Liverpool at 36 3/4 inches. Philos. Trans. vol. 67, p. 252.

3. The effect of the wind is very considerable, on a double account; for the same observations shew a very odd quality in the vapours of water, viz, that of adhering and hanging to the surface that exhaled them, which they clothe as it were with a fleece of vapourous air; which once investing the vapour, it afterwards rises in much less quantity. Whence, the quantity of water lost in 24 hours, when the air is very still, was very small, in proportion to what went off when there was a strong gale of wind abroad to dissipate the fleece, and make room for the emission of vapour; and this, even though the experiment was made in a place as close from the wind as could be contrived. Add, that this fleece of water, hanging to the surface of waters in still weather, is the occasion of very strange appearances, by the refraction of the vapours differing from and exceeding that of common air: whence every thing appears raised, as houses like steeples, ships as on land above the water, the land raised, and as it were lifted from the sea, &c.

4. The same experiments shew that the Evaporation in May, June, July, and August, which are nearly equal, are about three times as great as those in the months of November, December, January, and February. Philos. Trans. numb. 212, or Abr. vol. 2, pa. 110.

Dr. Brownrigg, in his Art of making common salt, pa. 189, fixes the Evaporation of some parts of England at 73.8 inches during the months of May, June, July, and August; and the Evaporation of the whole year at more than 140 inches. But the Evaporation of the four summer months at Liverpool, on a medium of 4 years, was found to be only 18.88 inches. Also Dr. Hales calculates the greatest annual Evaporation from the surface of the earth in England at 6.66 inches; and therefore the annual Evaporation from a surface of water, is to the annual Evaporation from the surface of the earth at Liverpool, nearly as 6 to 1. Philos. Trans. vol. 67, ubi supra.

In the Transactions of the American Philosophical Society, vol. 3, pa. 125, there is an ingenious paper on Evaporation, by Dr. Wistar. It is there shewn, that evaporation arises when the moist body is warmer than the medium it is inclosed in. And, on the contrary, it acquires moisture from the air, when the body is the colder. This carrying off, and acquiring of moisture, it is shewn, is by the passage of heat out of the body, or into it.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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EULER (Leonard)