in natural philosophy, signifies the conversion of fluids, principally water, into vapour, so that it becomes specifically lighter than the atmosphere.

The theory of evaporation, and formation of vapour by the absorption of heat, is fully discussed under the article Chemistry; it remains only therefore to take notice of some of the most remarkable phenomena attending it. With regard to water, it is generally allowed that it evaporates in every degree of heat above 32° to 212°, which is its boiling point, when it is dissipated in great quantity, and as fast as possible. 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, is found readily to evaporate 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 vapour ascending from the surface of the metal. In like manner the evaporation of water is very sensible in some 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.

Evaporation, according to the experiments of the Abbé Nollet, appears to be promoted by electricity. 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, and by means of it the whole vegetable kingdom is supplied with rain necessary for its support. This effect of evaporation takes place at all times, not only from the surface of the ocean, but of the earth also. Dr Hally, 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 24 hours; and repeating the experiment after a thunder-storm, he found that in such a state an acre parted with above 1900 gallons of water in 12 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 again in the night-time in dew, being absorbed by the same 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 be apt to 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, must of necessity be destroyed; which yet we see is only the case with the most tender grasses, and even that only. only on the most elevated situations, and when most exposed to the sun.

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 in their jars is always the 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 easily deduced from what is said under the article Chemistry; where it is shown that vapour is composed of fire and water united together. The consequence of this is, that wherever there is any quantity of latent heat above $32^\circ$ 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 latent 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 $32^\circ$ Fahrenheit will be carried off. If instead of water, spirit of wine be made use of, which continues to evaporate long after it is cooled to $32^\circ$, a much greater degree of cold may be produced than by the evaporation of mere water; and if instead of spirit of wine, we make use of 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 means of the expensive liquids of ether and spirit of wine, cannot be employed excepting merely for the sake of experiment; but that by the evaporation of water may be applied to very useful purposes in the warm countries; and it has been customary with sailors to cool their casks of liquors by sprinkling them with sea water.

From the theory of evaporation laid down under the article Chemistry, we may easily see the reason why, in a very warm temperature, animal bodies have the power of producing cold. A vapour, called invisible 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 different degrees of atmospheric heat.

For the same reason also we may 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 laid down under the article Chemistry, furnishes us also with a phenomenon of a very curious phenomenon, inexplicable on any other principle, viz. why melting ice will freeze regard to other pieces together more strongly; and, if a considerable degree of heat is not continued for some time, will again consolidate itself into a much harder mass than before. The fact was discovered by Mr Wedgwood in an attempt to connect his clay thermometer with the common mercurial ones. In this attempt he had occasion to repeat an experiment made by Mellers Lavoisier and de la Place, who had measured the heat of bodies by the quantity of ice they are capable of liquefying. These authors observe, that if ice, cooled to any degree below the freezing point, be exposed to a warmer atmosphere, it will be brought up to the freezing point through its whole mass before any part of it begins to liquefy; and that consequently ice, beginning to melt on the surface, will be always exactly at the same temperature, viz. at the freezing point; and that if a heated body be inclosed in a hollow sphere of such ice, the whole of its heat will be occupied in melting it: so that if the ice be defended from external warmth, by surrounding it with other ice in a proper vessel, the weight of the water produced from it will be exactly proportional to the heat which the heated body has lost; or, in other words, will be a true physical measure of the heat. For the experiment, they provide a tin vessel divided by upright concentric partitions into three compartments, one within another. The innermost compartment is a wire-cage for receiving the heated body; the second, surrounding this cage, is filled with pounded ice, to be melted by the heat; and the outermost is filled also with pounded ice, to defend the former from the warmth of the atmosphere. The first of these ice compartments terminates at bottom in a stem like a funnel, through which the water is conveyed off; and the other ice compartment terminates in a separate canal for discharging the water into that ice which is reduced. As soon as the heated body is dropped into the cage, a cover is put on, which goes over both, that and the first ice compartment; which cover is itself a kind of shallow vessel filled with pounded ice, with holes in the bottom for permitting the water to pass from this ice into the second compartment; all the liquefaction that happens in both being only the effect of the heated body. Another cover, with pounded ice, is placed over the whole as a defence from external warmth.

Mr Wedgwood began by satisfying himself that ice did really acquire the temperature of $32^\circ$ throughout its whole substance before it began to melt; but being apprehensive that the pounded ice might imbibe and retain some water amongst it by capillary attraction, he judged it necessary to attend to this circumstance also. Having therefore pounded some ice, he laid it in a conical heap on a plate; and having at hand some water coloured with cochineal, he poured it gently into the plate at some distance from the heap. It rose hastily to the top, and was retained by the mass as by a sponge; nor did any part of it begin to drop till the heat of his hand began to liquefy the mass. He farther observed, that in a conical heap of this kind the water rose two inches and a half in the space of three minutes; and by weighing the water employed, and what remained upon the plate unabsorbed, it appeared that four ounces of ice had taken up and retained one ounce of water. To ascertain this absorbing power of ice more fully, he pressed five ounces of it into a funnel, having first introduced a wooden core, in order to leave a proper cavity in the middle; then taking out the core, and pouring an ounce of water on the ice, he left the whole for half an hour, during which time there ran out only 12 pennyweights and four grains; so that the ice had retained seven pennyweights and 20 grains; nearly one-twelfth of its own weight, and two-fifths of the weight of the water.

Being now convinced that it would be proper to use solid ice instead of that which was pounded, he determined to congeal a quantity of water into one mass by a freezing mixture, and then expose it to the atmosphere till it began to liquefy. His apparatus for this purpose is represented Plate CLXXXIX. A is a large funnel filled with a solid mass of ice. B, a cavity in the middle of this ice, formed part of the way by scraping with a knife, and for the remaining part by boring with a hot iron wire. C, one of the thermometer pieces, serves for the heated body, and rests on a coil of brass-wire; it had been previously burnt with a strong fire, that there might be no danger of its suffering any farther diminution of bulk by being heated again for those experiments. D, a cork stopped in the orifice of the funnel. E, the exterior vessel, having the space between the sides and its included funnel A filled with pounded ice as a defense to the ice in the funnel. F, a cover for this exterior vessel, filled with pounded ice for the same purpose. G, a cover for the funnel, filled also with pounded ice, with perforations in the bottom for allowing the water to pass from this ice down to the funnel. The thermometer piece was heated in boiling water, taken up with a small pair of tongs equally heated, dropped instantly into the cavity B, and the covers put on as expeditiously as possible; the bottom of the funnel being previously corked, that the water might be detained till it should part with all its heat, and likewise to prevent the water from the other ice, which ran down on the outside of the funnel, from mingling with it. After standing about ten minutes the funnel was taken out, wiped dry, and uncorked over a weighed cup; the water that ran out weighed 22 grains. On repeating the experiment the water weighed only 12 grains; and on a third trial, in which the piece was continued much longer in the cavity, the liquid did not amount to three drops. To his surprise Mr Wedgwood also now found the piece frozen to the ice so that it could not easily be got off, though all the ice was at the beginning of the experiment in a thawing state.

On heating the piece again to 60° of his thermometer (1857° Fahrenheit), and throwing some fragments of ice over it, he found that in about half an hour the water amounted to 11 pennyweights. On stopping the funnel, replacing the covers, and leaving the whole about seven hours, he found, that a considerable quantity of water was collected; but it ran out slowly, that he imagined something had stopped the narrow end of the funnel; but on examining the plate of the ice, he found that the fragments he had thrown over the thermometer-piece were entirely frozen together, and in such a form that it was evident they could not have assumed it without fresh water having been superadded and thrown upon them, the cavities between them being partly filled with new ice. This was so strongly cemented, that he could scarcely get it out with the point of a knife, and great part of the coiled wire was found enveloped in the new ice. The passage through the ice to the stem of the funnel, which had been made pretty wide with a thick iron wire, was so nearly shut up, that the slow draining of the water was now very easily accounted for; this draining of the water indeed being the only sign of any passage at all. On taking the ice out of the funnel, and breaking it to examine this canal, he found it almost entirely filled up with ice projecting from the solid mass in crystalline forms, similar in appearance to the crystals we often meet with in the cavities of flints and quartzose stones. A coating of ice was also found on the outside of the funnel perfectly transparent, and of a considerable extent, about the 7/6th of an inch thick; this coating enveloped also a part of the funnel which was not in contact with the surrounding ice, the latter being melted to the distance of an inch from it. Some of the ice being scraped off from the inside of the funnel and applied to the bulb of the thermometer, the mercury sunk from 50° to 32°, and continued at that point till the ice was melted; after which the water being poured off, it rose in a little time to 47°.

Astonished at these appearances, our author determined to repeat the experiment with some pieces of ice he had stored up in a cellar; but on going thither, he found the cask of ice itself in a similar situation to that made use of in his experiments. Though much of it was melted, yet the fragments were frozen together, so that it was with difficulty that any pieces could be broken or got out with an iron spade; and when so broken, it had the appearance of Breccia marble, or plum pudding stone; the fragments having been broken and rammed into the cask with an iron mall. A porcelain cup being laid upon some of this ice about half an hour, in a room whose temperature was 50°, it was found pretty firmly adhering; and when pulled off, the ice exhibited an exact impression of the fluted part of the cup with which it had been in contact; so that the ice must necessarily have been liquefied first, and afterwards congealed. This was several times repeated with the same event. Fragments of the ice were likewise applied to one another, to sponges, pieces of flannel, and linen cloth, both moist and dry; all these in a few seconds began to cohere; and in about a minute were frozen so as to require some force to separate them. After standing an hour, the cohesion was so firm, that on pulling away the fragments of ice from the woollen and sponge, they tore off with them that part of the surface with which they were in contact; though at the same time both the sponge and flannel were filled with water which that very ice had produced.

The power of the congelation was stronger on the sponge and woollen than on linen; and to estimate its force, a piece of ice was applied to a bit of dry flannel weighing two pennyweights and an half, surrounding them at the same time with other ice. After lying together together three quarters of an hour, he found that a weight of five ounces was necessary to separate them, though so much of the ice had liquefied that the weight of the flannel was increased by more than 12 pennyweights. The piece of ice was then weighed, put to the flannel a second time, and left in contact with it for four hours; at the end of which time they were found to firmly frozen together, that 78 ounces were required for their separation, although from 42 pennyweights of the ice 15 more had melted off: the surface of contact was at this time about a square inch. Continuing them in contact for 7 hours longer, they only bore 62 ounces, the ice being diminished to 14 pennyweights, and the surface of contact reduced to about \( \frac{1}{3} \)ths of an inch square.

On trying whether masses of ice apparently solid would absorb water, he found that they did so in considerable quantity; for on heating some of his thermometer pieces, and laying them on pieces of ice, in which they made considerable cavities, he always found the water absorbed as fast as it was produced, leaving both the piece and the cavity dry.

Thus was our author convinced, that, in his experiments, the two seemingly opposite processes of nature, congelation and liquefaction, went on together at the same time, in the same vessel, and even in the same piece of ice. To account for such an extraordinary phenomenon, he had recourse to two different theories. One was, that water, when highly attenuated, and resolved into vapour, may freeze with a less degree of cold than water in its aggregate or grosser form: whence hoar frost is observed on grass, trees, &c., at times when there is no appearance of ice upon water, and when the thermometer is above the freezing point; which seems also to have been the opinion of Boerhaave, as he places the freezing of vapour, or even of water when divided by absorption in a linen cloth, at 33°.

Now (says Mr Wedgewood), as the atmosphere abounds with watery vapour, or water diffused and chemically combined, and must be particularly loaded with it in the neighbourhood of melting ice; as the heated body introduced into the funnel must necessarily convert a portion of the ice or water into vapour; and as ice is known to melt as soon as the heat begins to exceed 32°, or nearly one degree lower than the freezing point of vapour; I think we may from hence deduce pretty satisfactorily all the phenomena I have observed. For it naturally follows from these principles, that vapour may freeze where ice is melting; that the vapour may congeal, even upon the surface of melting ice itself; and that the heat which, according to the ingenious theory of Dr Black, it emits in freezing, may contribute to the further liquefaction of that very ice upon which the new congelation is formed.

I would further observe, that the freezing of water is attended with plentiful evaporation in a close as well as in an open vessel; the vapour in the former condensing into drops on the under side of the cover, which either continue in the form of water, or assume that of ice or a kind of snow, according to circumstances; which evaporation may perhaps be attributed to the heat, that was combined with the water, at this moment rapidly making its escape, and carrying part of the aqueous fluid off with it. We are hence furnished with a fresh and continual source of vapour as well as heat: so that the processes of liquefaction and congelation may go on uninterruptedly together, and even necessarily accompany one another; although, as the freezing must be in an under proportion to the melting, the whole of the ice must ultimately be consumed.

Some other circumstances may be taken notice of in the coating of ice on the outside of the throat of the funnel. Neither the cover of the outer vessel, nor the aperture in its bottom which the stem of the funnel palled through, were air-tight; and the melting of the surrounding ice had left a vacancy about an inch round that part of the funnel on which the crust had formed. As there was therefore a passage for air through the vessel, a circulation of it would probably take place; the cold and dense air in the vessel would descend into the rarer air of the room, then about 50°, and be replaced by air from above. The effect of this circulation and sudden refrigeration of the air will be a condensation of part of the moisture it contains upon the bodies it is in contact with; the throat of the funnel being one of these bodies, must receive its share; and the degree of cold in which the ice thaws being supposed sufficient for the freezing of this moist vapour, the contact, condensation, and freezing, may happen at the same instant. The same principles apply to every instance of condensation that took place in these experiments; and the congelation was evidently strongest in those circumstances where vapour was most abundant, and on those bodies which from their natural or mechanic structure were capacities of the greatest quantity of it; stronger, for instance, on sponge than on woollen, stronger on this than on the closer texture of linen, and far stronger on all of these than on the compact surface of porcelain.

The second theory proposed by our author for solving the phenomena in question is founded entirely on the principles of evaporation. If nevertheless (says he) the principle I have assumed, that water highly attenuated will congeal with a less degree of cold than water in the mass, should not be admitted; another has above been hinted at, which experiments have decidedly established, from which the phenomena may perhaps be equally accounted for, and which, even though the other also is received, must be supposed to concur for some part of the effect: I mean, that evaporation produces cold; both vapour and steam carrying of some proportion of heat from the body which produces them. If therefore evaporation be made to take place upon the surface of ice, the contiguous ice will thereby be rendered colder; and as it is already at the freezing point, the smallest increase of cold will be sufficient for fresh congelation. If ice is producible by evaporation in the East Indies*, where natural ice is never seen, we need not wonder that congelation should take place where the same principle operates amidst actual ice.

It has been observed above, that the heat emitted by the congealing vapour probably unites with and liquefies contiguous portions of ice: but whether the whole, or either of the heat so emitted, or of that originally introduced into the funnel, is thus taken up; how often it may unite with other portions of ice, and be driven out from other new congelations; whether there exists any difference in its chemical affinity or elective attraction to water in different states and the contiguous bodies; whether part of it may not ultimately escape, without performing the office expected from it upon the ice; and to what distance from the evaporating surface the refrigerating power may extend; must be left for further experiments to determine."