when the air is moderately warm: thus, in the great frost of 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.

Water evaporates in every degree of heat from 32° to 212°, which is its boiling point, when it is dissipated in great quantity, and very rapidly. It has also been supposed to evaporate even after its conversion into ice; but some late authors have denied this to be the case. Other liquids, such as spirit of wine or ether, continue to evaporate long after they have been cooled down to the freezing point of water; nor is there any experiment by which it has yet been discovered at what degree their evaporation ceases. Even quicksilver, to appearance a much more heavy and sluggish fluid, and which does not boil without applying almost three times the heat necessary to make water boil, readily evaporates when the pressure of the atmosphere is taken off; and hence the empty parts of barometrical tubes, where the instruments were made with great accuracy and the tubes perfectly exhausted, have been covered with mercurial globules, owing to an invisible vapor ascending from the surface of the metal. In like manner the evaporation of water is very perceptible in scme experiments with the air pump. Dr. Priestley found that, where moisture was carefully excluded from his apparatus, he was never able to produce such a quantity of inflammable air by heating charcoal, as when a little quantity of water was admitted, by moistening the leather on which the receiver stood. Nor is the elasticity of this kind of steam altogether imperceptible; for, in the barometer above-mentioned, the accuracy of the instrument was considerably affected by the steam of the mercury ascending from it, and occupying the void space in the upper part of the glass tube.

There are 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 800 and 900 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, Contemplation 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 the 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 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.

Dr. Desaguliers, dissatisfied with the hypotheses of Drs. Nieuwentyt and Halley, proposes another in the Philosophical Transactions, No. 407. 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 vapor from water, even in winter, whilst it condenses the air; and thus the particles of water are converted into vapor 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, acccording to their relative gravities. The particles of this vapor 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 theory, by Mr. Clare in his Treatise of the Motion of Fluids, and by Mr. Rowning in his system of Philosophy; 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 less 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 shown, 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. 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 vapors 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 this 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 1747, suggested a similar hypothesis. He observes, that the 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 electrified, 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 vapor 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. M. Le Roi, of the Academy 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 shows, that water undergoes 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; 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 degrees 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 vapor raised in the night is less than that in 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, M. Le Roi answers, 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 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; agreeably to an experiment of Dr. Irving, who says, that in an exhausted receiver water rises in vapor 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 mois and foggy, its powers of solution are diminished and it becomes lighter in proportion as its wate is deposited: and, thirdly, when it is completely precipitated, which may happen either by a slowe process, when the dissolved water falls in drizzling rain, or by a more sudden process, when it descends in brisk showers.

Dr. Halley, whose theory is mentioned above, has furnished some experiments on the evaporation of water, the result of which is; 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 eight inches diameter, evaporate at the rate of six ounces in twenty-four hours: whence by a calculus he finds that, in such circumstances, the water evaporates one-tenth of an inch deep in twelve hours: which quantity, he observes, will be found abundantly sufficient to furnish all the rains, springs, dews, &c. By this experiment, every ten square inches of surface of the water yield in vapor, per day, a cubic inch of water and each square foot, half a wine pint; every space of four feet square, a gallon; a mile square, 6914 tuns; and a square degree, of sixtynine English miles, will evaporate 33,000,000 tuns a day; and the whole Mediterranean, computed to contain 160 square degrees, at least 5,280,000,000 tuns each day. Philosophical Transactions, No. 189, or Abridgment, vol. II, p. 108. 2. A surface of eight square inches, evaporated purely by the natural warmth of the weather, without either wind or sun, in the course of a whole year, 16,292 grains of water, or sixty-four cubic inches; consequently, the depth of water thus evaporated in one year

amounts to eight inches. But this being too little to answer the experiment of the French, who found that it rained nineteen inches of water in one year at Paris; or those of Mr. Townley, who found the annual quantity of rain in Lancashire above forty 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 forty-eight inches; and Dr. Dobson states the same for Liverpool at thirty-six inches and three quarters. Philosophical Transactions, vol. LXVII, p. 252. 3. The effect of the wind is very considerable, on a double account; for the same observations show a very odd quality in the vapors of water, viz. that of adhering and hanging to the surface that exhaled them, which they clothe as it were with a fleece of vaporous air; which once investing the vapor, it afterwards rises in much less quantity. Whence, the quantity of water lost in twenty-four 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 vapor; and this, even though the experiment was made in a place as close from the wind as could be contrived. Add, that the fleece of water, hanging to the surface of waters in still weather, is the occasion of very strange appearances, by the refraction of the vapors 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 show that the evaporations in May, June, July, and August, which are nearly equal, are about three times as great as those in November, December, January, and February. Philosophical Transactions, No. 212, or Abridgment, vol. II., p. 110.

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 the 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 vapors 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 vapors 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 ot different heights, according to the quantity of vapor 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 vapors, 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 hail-stones. When the air is replete with vapors, 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 watery particles. Southerly winds commonly bring rain, because, being warm and replete with aqueous vapors, 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 vapor 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, produce contrary effects in the solution of water, and the consequent ascent or fall of vapors. Several experiments prove that air, when rarefied, cannot keep so much water dissolved as it does in a condensed state; and therefore when the atmosphere is saturated with water, and changes from a denser to a rarer state, the high and cold 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 find that the rarefied and condensed states of the atmosphere are respectively attended with rain or fair weather.

In the Transactions of the American Philosophical Society, vol. III., p. 125, there is an inge nious paper on evaporation, by Dr. Wistar. It is there shown, that evaporation arises when the moist body is warmer than the medium it is en

closed in. And, on the contrary, it acquires moisture from the air, when the body is colder. This carrying off, and acquiring of moisture, it is shown, is by the passage of heat out of the body, or into it.

Evaporation, according to the experiments of the abbé Nollet, appears to be promoted by electricity. See ELECTRICITY, Index. The conclusions drawn from them are, 1. Electricity augments the natural evaporation of fluids; all that were tried, excepting mercury and oil, being found to suffer a considerable diminution, greater than what could be ascribed to any other cause. 2. Electricity augments the evaporation of those fluids the most which are found most readily to evaporate spontaneously: the volatile spirit of sal ammoniac suffering a greater loss than spirit of wine or oil of turpentine, these two more than common water, and water more than vinegar or a solution of nitre. 3. The effects seemed always to be greatest when the vessels containing the fluids were non-electrics. 4. The increased evaporation was more considerable when the vessel which contained the liquor was more open; but the effects did not increase in proportion to the apertures. 5. Electricity was also found to increase the evaporation from solid bodies, and of consequence to augment the insensible perspiration of animals.

Evaporation is one of the great natural processes, by means of which the whole vegetable kingdom is supplied with rain necessary for its support. This evaporation takes place at all times, not only from the surface of the ocean, but of the earth also. Dr. Halley, by an experiment with a pan of water kept in the heat of our summer sun, found that as much water might be reasonably supposed to evaporate from the surface of the Mediterranean Sea as would be sufficient to supply all the rivers which run into it. Dr. Watson, in his Chemical Essays, has shown that the evaporation is not less considerable from the surface of the land than from that of the sea. By inverting a glass vessel on the ground, in the time of a considerable drought, he found that even then about 1600 gallons of water were raised from an acre in twenty-four hours; and, repeating the experiment after a thunder-shower, he found that in such a state an acre parted with above 1900 gallons of water in twelve hours. This evaporation is carried on not only from the ground itself, but from the leaves of trees, grass, &c., with which it is covered; and great part of the water thus raised falls down in the night-time in dew, being absorbed by the vegetables which yielded it before. Thus the earth is not so soon exhausted of water, even for a little way below the surface, as we might imagine from the quantity raised by evaporation for if all that was raised by the sun's heat during the time of a long drought, left the earth not to return to it for perhaps five or six weeks, the whole vegetable kingdom, at least such as do not strike their roots very deeply into the ground, would of necessity be destroyed; which yet we see is only the case with the most tender grass, and even that only on the most elevated situations, and when most exposed to the sun. Dr. Brownrigg, in his Art of making common Salt, p. 189, fixes the evaporation of some parts of England at 73-8 inches

during 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 four years, was found to be only 18-88 inches. Without evaporation there would be no dews nor showers; without which,' as Dr. Derham observes, the trees and plants would languish and die with perpetual drought. but are hereby made verdant and flourishing, gay and ornamental: so that (as the psalmist says, Psalm lxv. 12, 13), The little hills rejoice on every side; the pastures are clothed with flocks; and the valleys shout for joy; they also sing.'-Physico-Theology, p. 22.

Another great use of the natural evaporation is to cool the earth, and prevent its being too much heated by the sun. This property of producing cold by evaporation has been but lately observed by chemists, though it has long been employed by those who knew not the reason of their doing So. It has been observed at Aleppo, in Syria, that the water is always coolest when the weather is most warm and the power of the sun excessive. The heats in that part of the world are sometimes almost intolerable; and at that time the evaporation from the outside of the jars, which are made of porous clay, is very copious; and in proportion to the quantity of water evaporated from without is the degree of cold in the liquor within. The reason of this is, that vapor is composed of fire and water united. The consequence is that, wherever there is any quantity of latent heat above 32° of Fahrenheit contained in any body, the water in contact with the surface, or contained in the pores of the body, will gradually absorb it, and, converting it into lateut heat, will thus be rendered specifically lighter than the common atmosphere and fly off into it. Thus part of the sensible heat of the body will be carried off; and, as subsequent quantities of water always fly off with more and more of the sensible heat, it is plain that, by continued evaporation of water, almost all the sensible heat above 32° of Fahrenheit will be carried off. If, instead of water, spirit of wine be used, a much greater degree of cold may be produced than by the evaporation of mere water; and if, instead of spirit of wine, we use ether, which is still more volatile than spirit of wine, an excessive degree of cold, scarcely inferior to that which congeals mercury, may be produced. This method of producing cold, by the expensive liquids of ether and spirit of wine, cannot be employed excepting merely for experiment: but that by the evaporation of water may be applied to very useful purposes in warm countries; and it has been customary with seamen to cool their casks of liquors by sprinkling them with sea water. From the theory of evaporation laid down, we may easily see the reason why, in a very warm temperature, animal bodies have the power of producing cold. A vapor, called insensible perspiration, continually issues from the bodies of animals, from human bodies especially, which, carrying off great quantities of their sensible heat, enables them, according to its quantity, to preserve the same temperature in many differen degrees of atmospherical heat. For the same reason, also, we see why the continual sprinkling with cold water is so very powerful in depriving

the human body of the heat necessary for the support of life, even though the temperature of the water should not be below what can be easily borne. It has already been shown that, by the evaporation of water, a degree of cold not much inferior to that of freezing water may be produced; and consequently, by continual sprinkling of the body with water, the whole might in time be reduced to nearly the degree of cold in which water freezes. But this is what no human body can bear: and hence we may understand why storms of rain and snow are often fatal; and likewise why, in cases of shipwreck, people have died by being exposed for a few hours to the spray of the sea. The theory of the evaporation of water furnishes us also with a solution of a very curious phenomenon, inexplicable on any other principle, viz. why melting ice will freeze other pieces together more strongly; 2nd, if a considerable degree of heat is not continued for some time, will again consolidate itself into a much harder mass than before. See ICE.

EVAPORATOR, an apparatus calculated for expediting the process of evaporation; a model of which was presented by the inventor, Mr. Browne of Derby, to the Society for the Encouragement of Arts, &c., who conferred on him their gold medal. See Mr. Browne's communication, inserted in the Society's volume for

EVATES, or EUBATES, a branch of the Druids. Strabo divides the British and Gaulish philosophers into three sects; bards, evates, and druids. He adds that the bards were the poets and musicians; the evates, the priests and naturalists; and the druids, moralists as well as naturalists. See DRUIDS.

EVAUX, a town of France, in the department of Creuse, and ci-devant province of Marche: near which there is a mineral spring and baths; eighteen miles N. N. E. of Aubusson, and twentyfive and a half east of Guerat. Long. 2° 35′ E., lat. 46° 13′ N.

EVAUX, or EVAON, a town in the department of La Creuse, France. It is situated on an eminence, and in the neighbourhood are warm salt and sulphur baths. Here are also several ancient monuments of art. Population 2100; twenty

Ive miles east of Guerat.

EUBA, or Abantis, in ancient geography, an oblong island, stretching out between Attica and Thessaly. See ABANTIS. It is now called Negropont, from its principal town, anciently called Chalcis.

EUCERA, in entomology, a genus of hymenoptera, proposed by Scopoli, and adopted by various late writers. Under this denomination are comprehended those of the Linnæan genus

apis which have the mandibles horny, incurvated, acute, and not dentated; jaw elongated, and membranaceous at the tip; lip horny; tongue inflected, and seven-cleft; antennæ cylindrical, quinquefid in the male, often exceeding the length of the body; the abdomen short and downy; female armed with a sting.

EUCHARIST, n. s. Fr. eucharistie; Span. EUCHARISTIC AL, adj. § Port. Ital. and Lat. eucharistia ; Gr. ευχαριστια from εν well and χαρις, thanks. The sacrament of the Lord's Supper; any formal act of thanksgiving: the adjective follows these senses.

Himself did better like of common bread to be used in the eucharist. Hooker.

Some receive the sacrament as a means to procure great graces and blessings, others as an eucharist and an office of thanksgiving for what they have received. Taylor.

The latter part was eucharistical, which began at the Browne. breaking and blessing of the bread.

It would not be amiss to put it into the eucharistical part of our daily devotions: we praise thee, O God, Ray. for our limbs and senses.

EUCHARIST properly signifies giving thanks; the word uxapsia literally importing thanks giving; being formed of vç, good, and xaps, thanks. This sacrament was instituted by Christ himself, and the participation of it is called communion. As to the manner of celebrating the eucharist among some ancient Christians, after the customary oblations were made, the deacon brought water to the bishops and presbyters, standing round the table, to wash their hands; according to the passage of the Psalmist, ‘I will wash my hands in innocency, so will I compass thy altar, O Lord.' Then the deacon cried out aloud Mutually embrace and kiss each other;' which being done, the whole congregation prayed for the universal peace and welfare of the church, for the tranquillity and repose of the world, for the prosperity of the age, for wholesome weather, and for all ranks and degrees of men. After this followed mutual salutations of the minister and people; and then the bishop or presbyter, having sanctified the elements by a solemn benediction, he brake the bread and delivered it to the deacon, who distributed it to the communicants, and after that the cup. Their sacramental wine was usually diluted or mixed with water. During the administration they sang hymns and psalms; and, having concluded with prayer and thanksgiving, the people saluted each other with a kiss of peace, and departed.

EUCHITÆ, or EUCHITES. Gr. Evxirai, from vxn, prayer, a sect of ancient heretics who were first formed into a religious body towards the end of the fourth century, though their chief doctrines and discipline subsisted in Syria, Egypt, and other eastern countries before the birth of Christ. They were thus called because they prayed without ceasing; imagining_that prayer alone was sufficient to save them. They founded their sect on the words of St. Paul, 1 Thess. v. 17, 'Pray without ceasing.' They were also called Enthusiasts and Messalians; a term of Hebrew origin, denoting the same as Euchites. They were a sort of mystics who imagined, according to the oriental notion, that two souls re