A Dissertation on the Nature of Evaporation and Several Phaenomena of Air, Water, and Boiling Liquors: In a Letter to the Rev. Charles Dodgson, D. D. F. R. S. from the Rev. Hugh Hamilton,

A Dissertation on the Nature of Evaporation and Several Phaenomena of Air, Water, andBoiling Liquors: In a Letter to the Rev. Charles Dodgson, D. D. F. R. S. from the Rev. HughHamilton, D. D. F. R. S. Professor of Natural Philosophy in the University of DublinAuthor(s): Hugh HamiltonSource: Philosophical Transactions (1683-1775), Vol. 55 (1765), pp. 146-181Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/105458 .

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[ 46 ]

XXIII. 24Dif atior or the Xatare of E<ua- psration snd fieveral Phcegomena of Hir, - lEater,: and boilixg tuors: In a Let- ter to the Rev. Charles DodgCon, D D. F. R. S. froan the Res. Hugh Hamilton, D. D. F. R. S. ProfeJJ5or of ffatural PhiloJ>ophy in the Unruerfity of Dublin.

Dear Sir; Read May 16, T Here Nend you, according to pronliSe,

IX65 1 t]]y thougllts on the nature of evaporation, the afcent of ratery vapours, and fotne other phanomena of the atmofphere, in explaining which I have employed a principle that, as far as I can find, is different from what has been hitherto uSed on this occafion, hoping thereby to avoid thofe objec- ions, which fome late writers llave made to the former accounts, that have been given als of theSe phGenome- na. For in all the accounts I have met with) fire or heat, and rarefadion, by which watery vapours are fuppoSed to lecome fpecifically lighter than air, are made to be tlle principal, if not, the only cauSes of their aScent into the atlnofphere. Dodor Nieuwentyt, and fotne others, fuppofed thatthe particles of fire) by adllering to thofe of water, make up molecula, or finall bodies, EpecisScally lighter than air. And Dr. tialley thought, that by the adion of lleat, the particles of water are fortned into holloss hpherules filled with a finer air highly rarefied, fO as to become Epe- cifical]y liglaier than tlle external air. Tlais lafl:

sras



[ I47 ] was the opinion moR commonly receisred, as Doecor Defaguliers tells us- in his differtation on this fubjed, pllbli{hed, in the Philofophical Tranfadaions,: in tlle year 729, in which he examines and refutes thefe two former opinions, and endeavours to eftablifh his own. He aScribes the aScent of aqueous vapours to tlacir being turned into an elailic Ileam:, and always raretSed more than tlle air is, bySthe degrees of heat) to which bodies are uEually fubjeEt in the diffierent feafons of the year.

Thts opinion, I find, has been as ill received by fabSequent writers as the former ones. Mr. Clare, in his treatiSe on the MotioIl of Fluids, has brougllt many objeEtions againR it; as Mr. Rowning has alSo done in his Syllenl of Natural Philofophy not long Elnce publithed; who fays, that the cauSe of the aCcent of vapours has been much dirputed, but not ye-t fettled, by philofophers, and owns that he cal)not think of any true princigle of philoSophy, upon which it may be accounted or

I nlall not nossr repeat the objeEtions made by thoSe gentlemen; but nouR beg leave to add only the tsvo followinb; which, among many otllers that nzight Ilill be urged, they have not taken notice of.

FirA; If heat was the only cauSe of earaporation, water in a clofie xvarm room would evaporate fafter than when expoSed in a colder place mrhere there is a conilant current of air; which is contrary to experience.

Secondly; The evaporatiotl of water is fO far from depending upon its beirlg rarefied by heat, that it is carried on even wllilR water is condeaSed by the

U 2 coldneEs



[ t48 ] coldnefs of the air; for ssrater is gradually con denfed by cold till the moment it freeses, and fince it evaporates even when frosen into hard ice, it mu{t alfo evaporate in all the lefl4er degrees of cold. Now Mr. Boyle having counterpoifed a piece of ice in a fcale, hung it out in a frolly night, and found ne-xt morning that it had loR confiderably of its weight by evaporation. " Who (fays he) would have thought thae lo extremely hard arld cold a body would evaporate fo faI} in the clear air of a freesing night ?" And fince that time others have obServed the fame thing; which fad Seems to be an unanrwerable objedrion to all the accounts in which rarefadrion by heat is made to be the chxef, if not the only caufe of evaporation; and, therefore, we mu have recourfe to fome other prit}ciple to afl410 us in accounting for this phano- menon.

As the auior of nature does not employ in his works a greater variety of cauSes than is abSolutely neceSary; ie is the bufinefs of natural philoSophy to reduce as many phanomena as may be to fome general well known cauSe; and this is to be done by comparing the phanomena together in their Several circumllances, in which, if-they are found to agree, they are th@n to be confidered as effeEts of the fame kind, and aScribed to the fame cauSe. By which means the cauSes, whofe exiRence is already proved, will be rendered more general, and our knowledge more extenSare. Now as the fuEpenfion of the particles of water in air, of falt in the waters of the ocean, and of oier heavy bodies in the fluids that diSolve them feem to be phanomena of the fame kind, we might reafonably fsppofe- that they atiSe froln

the



- I-49 ]

the fame cauSe; atzd that what we ¢011 evaporation is no more than a gradual folution of water in airj -But that I tnay not propofe this meerly as an hypothefis, I {hall endeavour to prove the truth of it, by conf1dering the nature of folution in general, and comparing its properties and effedrs with thofe of evaporation. By folutson we un- derfl:and, lthe anitgng fo gntimately the particles of e body grith thoJe of a #vid, that the wbole Jhall appevr an homogeneovs maJ^s, as tranJ5parent as the fiuid was b- fore J?ch union, and jhall Jto continue hIIJôme external ca?4Jre produces a change. The -nature of folution has been explained by the writers on chymiIlry in tlli8 manner. When the particles of any body furrounded by a fluid are lefs flcrongly attradred by each other than by the fluid, they feparate from each other, and join themSelves to thoSe of the fluid, and remain fufipended therein.

Thus rarious falts are difl*olved in water, eiSential oyls are diSolved in fipirits of wine, gold in aqua regia, mercury, fil7er, and other metals, in other acid Epirits.

And indeed it Seems to be with great appearance of rearon, that the attradrion between the minute particles of bodies (of which we have fo many other inSances) is affilgned as the cauSe of that union between them, which we experience in folutions. The chief properties of which I ffiall now mention, fo far as may be neceR*ary for the purpofe to which I mean to apply them.

In moflc caSes a difl olsting fluid, or meniEruum, as the chymifcs call it, will diSolve or take up orlly a certain proportion of the body immerfed; and -if thenany more of the fame body be added, it wilt

precipitatc



[ Iso ] precipitate or fall to the bottom, and then the fluid is faid to be faturated with the body it has difl olved; yet a fluid, which is faturated with one body, may afterwards diSolve others of difFerent kInds, and keep all their particles fufpended together.

When any meniRruum has entirely difl olved a body, it will continue as tranfparent as it was before. The cauSe of Nhich may bez alEJgned from what Sir lfaac Newton dircourered by experiments; that the particles of bodies muI} be of a :certain fixe, or bwigneSs, to cauSe any refleEtion or refradion of the rays sf light at their furfaces. From whence he gives the reafon why fome bodies are opake, and others tranfparent; and he alSo obServes, that the nloR opake bodies, fuch as metals, being diSolved in an acid menIlruum, and thereby reduced to thesr ulti- mate and fmalleR particles, do not take away the tranfparency of the menRrllum

Hence we may always knov how to dillinguiXh a folution fIom a mixture; for if a body be reduced to powders and thrown into a fluid that will diIfole it, and they are then nlaken fuddenly together) the fluid will continue fomCewhat opake till the folution be efledred, or till what remains undiSolved falls to the bottoln. For in t}1;S cafe the particles are not at firR reduced to tlleir fmalleIt fize, as they are always sn a folution. I tlwink, therefore, we may confider the trallEpalency of an heterogeneous fl.lid (or one that contains in it particles of another body) as the criterion of a true folution; and where that is want- invg, it is only a mixture, as wllen water and air ap pear together in froth, or in a cloud, or a thick mift, it is only a mixture of thofc bodies, and not a folution of eithcrX

This



[ I5I ]

Thusmuchbeing premifed ofthe nature offolutions in general; I proceed to the proof of what I propofed; and in order to this, I ffiall firR Ihew)- that there is a mtltual attradrion between water and air, the fame that we obServe between the par¢iclgs of any two bodies, one rOf which diSo}ves the other. I ffiall then compare, fin Weveral in{lances, the properties and effeds of common folations, with thofe of evaporation, that, from the exaA refemblance between theSe two phanomena, it may appear that they are natural opeZ rations, or cffedcs of a like kind, and therefore to be cxplained upon the fame principle, or aScribed to the fame cauSe. From thence I Shall ffiew how rthe aScent of arapours, and feveral other phanome_ na of the atmofphere, lnay be accounted for. And laR!y, I ffiall add fomething on the rifirxg of Ream from boiling liquors, and Ihew wherein it differs from common evaporation.

I am firll to prove, that there is really all attrac- tive force between the particles of air and water. It is well known that all waters contain a confiderable qalantity of air that retains its elallicity, by means of which it nlay be feparated frotn the water by boiling, and inclllding it in an exhauIted receiver. And Dr. Boerhaave, in his Elements of ChymiIlry, has Ihewn, by an elegant experilnent, that air extricated from water by boiling, and rellored to its common Itate, will occupy a fpace greater than that pofl4efl:ed by the water in svhich it was contained. Now f1nce it is allowed, that the particles of fO heavy a body as gold are fllEpended in squa regia by their attrad:tion towards the particles of that fluid, it feems reafon_ able to fuppofe, that fo light and elaRic a body as air muR be reta.ined under water by a like force, with-

3 out



[ ts2 ] out which it would always a§cend to the furface, and efcape. But that there i8 really fuch an attradrive force between air and wattr, the Iearned and ings nious gentleman 1d metltioned has fully proved bjr the foliowing experiment.

Let an oil flai be filled almoR ftill with water, deprived of its air as much as may be, let;the mouth of it be then Ropped until the neck be immerted in -t veXel of vrater, a bubb-le of xir will then tEcend into the llpper part of the flaSk. Wkets things have llood in this way for fome days, the watr will be found to have abSorbed the whole bubble of ir (if it was not too large) and intirely filled the flaik; but if the bubble was too large, part of it will be left for the water after fo-me time will a-bSorb nt more air, being:-then -fufiiciently faturated.with it. It is obSerarable that a part of the included sir- enters pretty quickly iIlto the water at firk, but whae remains af terwards makes its way in but very Rowly. T- hi6 experiment -Ihews that water, when deprived of its ait, will again dow the air gradually into sts pores, juk in the fame manner that a lwmp cf dry filgar will draw up water into its pores, which will aScend pretty quickly at firll, but -very {lowly after fome time. We have reaSonX therefore, to conclude that there is the fame kind of attraEion between air and watery that there is between water and any dry porous body that will imbibe it.

As water contains a confiderable quantity of air, fo does air contain a good deal of water, even when we think it quite pure and dry, as appears from the moiEure dratvn from it by dry falt of tanar, in fuch quantity as to make the falt beconle intirely fluids

Now



, I53 .

Now f1nce the air is an heterogeneous fluid, contaitling in it particles of another body, and yet retaining a perfeer tranfparency, which is the criterion of a true folution in other cafes, why &ould we not infer froln analogy, that in this cafe alfo it indicates a true folution of water in air? eEpecially when we conElder that thre are hardly any two fluids that may not, bw themfelves, or by the nzeans of fome third body, be fO thoroughly incorporated, that one of them may be properly faid to be difl*olved in the other. But the truth of this will be further confirmed, by comparing the properties of common- folutions with thoSe of evaporation;0 whicll I {hall now do in feareal inikances.

Firfi; when a body is immerfed in a fluid tI<at difl4olves it ; for infiance, a lump of falt in ssrater; zve fee the falt foon begin to dtiffiolve and itnpregnate Witll its particles th-e water that furrounds it) wl;cll will then appear thick and loaded; and if the water le at reR, the folution will pfoceed very Ilosvly; bXut if it Xoe Rirred about, the falt will foon be intirely diflolved. How exaEtly does this correfipond with xwhat Dr. Halley remarked in an experirnellt :made on the evaporation of water in a clore roonl ? Phil. Tranf. N° I92. sC The fame obtervations (fays he) do sc likewiSe ffiew an odd quality in tlle rapours of wa- 's ter, which is that of adhering to the furface that " exhales them, which ehey clothe, as it werel " with a fleece of vapourous air, which once in- , veRing it, tlae vapour rifies afteLvvards in much lers - quantity." Here we fee, that the air, which lay at ret} over the water,- appeared thick, and loaded xvith aqaleous particles; and tllen the evaporation

VOL. L\f. X proceeded



[ 1547 proceeded very Ilowly, juI} as the water tit- 11es about the falt appears thick and loaded, and whiJe it continues at reIl, the falt is diiNalved very flowly He alfo obfierves on ie iiame occafizon that evapo-- ration is waRlyi prontoted by a current of freffi air paQSlng over the exhaling furface; and this I have no doubt happens for thve fame reafon thatfolutiorl is greatl^y E?romoted by agitation, which continu-ally bri-ngs freSh particles of the fluid into contaft with the bodyX it diXolvesX in the place of tlWoWe that have been already faturateds

Se£ondly; into a glaX of clear cold water throw a lump of any kind of falt which is foluble in its and wlacn it has ftood a Iittle ti;me, {hake the glai:sX or: fiir the water gently with a wire, and the water which is faturated with the falt will rife up among the reR:;of the water in curled wreaths, or long ria, which; will render tlle water fomewhat- opake, caufing it to refra40 irl different diredions the light of an objeflc^fieen through it, and it will make the obJed appear to- hav.e a tremlalous motion, and this weill continue until all parts of the wster are equally impregnated vfiththefalt, andX then its tranfparenc%y: will be reRored As the parts of the water, which are impregnateed with the lalt, are of di5erene ders- fxtsefi from the reIt, while th@y are mixing together they muit oc£afion thofe refradsionsa and this apparent tremulous nlotion, which will ceaSe as foon as alS tlze water becomes of the fame denfity. lDhe lrery lame appearances will attend the mising together of any tvo flaids of diffierent denfities, and wich will thoroughly incorporate with eacb othcrO

In



^ *

155 * In like manner when fmoak ar ileanw ifl4uing frotn

the pipe of a boiling ve{Nel firk riKes into the air, it appears in curled wreaths, and renders the air opake,, but as fc)on as it is intirely diEperfed, the tranfparency is reIlored. Thus alfo in a caltn hot fun-(hille days when we look along a moiil piece of-ground, the tir, and any objedt feen through it, appears to have a tremulous motion like that which we obEerve in atl objeft feerl through any two fluids that are mixing together.

Now as the vapours rife here irl great abundance, and the air has but little- motion, thofe parts of it that are much impregnated: wtth the :aqueous particles are mixed gradually wtth the air that is dri-er aud of a dtfferent (lenfity, which will occafion re- fradions of the light, and that apparent tremulous motion jalfR now mentioned. And in th-is cafe the folutlon of water in air (if I may yet call ie fo) is carried on in a mantler vifible to the eye, as it is in other fluids. The fame tremulous undulating motion ts more obfiervable wherl we look in warn} weather through a teleScope, svhich magnifies the vapours floating in ehe-air; and fFom this kind of refradrion the twinkling of the Prars feems to arife, wtth thts difference only, tha the watery reiadcin:g parttcles in the day time are pailryg into a Ilate of folutton, whereas the vapours alrendy difFolved are, by the cold of the night, beginning to precipitate and return into particles large enough to cauSe refradrion;s in the light of the Rars.

Thirdly; Heat promotes, and cold- in {ome mea fure RopsX or checks, both folution and evaporation; very hot water will difl4olve 1t fooner7 and in a greater

X z quantity



[ I56 ]

quantity tlaan cold water; and if a ilrong folutIon of {glt be made in hot water, the srater, when cold, will let go fosne of the falt befUre diSolved, whicll will :;all to the bottom in Emall particles, or fhou into cryRals. Jt1(t fS will water evaporate faRer in warm ttlan in cold air; and the aqueous vapours, fubended in tlle air during the heat of the day, fi11 down at nigllt, aIld form themSelves into drops of d; ory if the night be very wld, appear next morning in a- h. And thus, if ln a hot day, a bottle be filled with any very cold liquor, asnd expoSed to the air, which to u8 feems very dry, a dew will be foon fUrmed on the outilde of the bottle; fbr the air about it, becomiIlgcold, will letgo part of its moifiure

whicll will be attraflteti to ti filrfice of the glafs. Aazd, for the fame reafon, a dew is formed on the infide of the svindows vf a wartn room, which oa their outfide are expolEd to the cold airF And hence we may obServe, that3 as thue cannot be 18 conti nual and copious an evaporatittl-ia cold weatller, the air will tlletl be generally clearer than it is in hot weather.

He.at feems to omotee blution, becauSe it expands bodiesfi and thereby elzlarges their pores, and Itflions the cohefisre attradtion of their particles, b that a lrodt when hot will more eaftly adn:lit a diSolving fluiel into its pores, and its particles not cohering to gether So Aronglsg, as when cold, will more readily gait each otherl and unite themfelves t thc particlu of the fluid by which they are attrafted; and fbr the fitne reuSm hvat wsll alSo promote the evaporation off

i.S4

But



[ I57 ]

Butfourthly; The quantity of a body diSolved, and of a fluid evaporated, in a given time, depends (ceteris paribus) on its quantity of furface. Tbus a body reduced to powder is fooner diSolved than when it is in a folid form. And thus Imoak, or Ream, which is water, reduced to very fmall particles by heat, is much fooner difiperEed, and incorpoo rated with air, than water in its ufual fortnb

Fifthly; Chymilis obferve, that when fea falt, fal ammoniac, or nitre, is digolved in water, or efl4en- tial oyls ill ppirit of wine, fotne degree of cold is produced in the immediate ad: of folution; arld the quicker the folu-ton, the greater is the cold. And by diSlolving pounded ice, or rather fnow, (wlzoSe particles have a greater ftlrface) in fpirit of nitre, a degree of cold has been produced fo great as to freeze quick f1lver. Cold is likewife produced in the ad of evaporation; for if rpirit of wine, or zther, having the rame tenlperature ssith the airs be rubbed lightly with a feather over the ball of a thermome ter, it will fink as the rpirits evaporate, and the quicker they evaporate, the faIler will the thermo- meter fink. And the fame thing will happen if water be uIêd inRead of fpirits, provided its evapc)- ration be promoted by a Ilrong current of air. And thus have I feen ice prodllced meerly by repeated evaporations of ather.

This laI} obServation Ihews a very remarkable agreement between the natures of folution and evaporation How the cold is produced in either caGe I cannot pletend to fay; but I - mu!l beg leave ju1t to apply this faft to accolllat for a tlzing .Nîsll 1 bclieve moR people have taken notice of.



( 58 ] If we rub Hungary watr, or any other volatile

fipirit, oarer our hand, sit will feel ta)uch colder than water, though they be both of the fame temperature, and will both feel equally cold if sve dip our firlger into each. The reafon of which is, that the {pirit evaporating much quieker than tlle water, pro duces tllereby a greater degree of cold; and fo ather, if it be applied in the fame way, will fEel colder tllars any other fipirit- on account of as more fiudden evaporation.

Sixthly; It is known that reXtified Epirit of wine, when well purged of air, will imbite a large bubble of air, in a much «orter time, tllan water will

do; and I-have myfelf experienced the truth of thiss which {hews that there is a Sronger attraftion O£

affinity, as the chymiRs call it, between fpirit of wine and air, than between water and air; and fince thesfpirit evaporates much faRer than the water, I think we may conclude from hence, that the evaporation of fluids- arifes fiom an attradtive- force between their particles and thofe of- air **

¢ As water and fpirit of vvine, are in no degree vifcid, they may evaporate in propertion to the attraEtioll between them and air. But the ca{E is very different in fiuch Auids as are vifcid; for though-l found that oil of olinres, when zurged of air will imbibe a bubble of air almofi as foon as water does, yet the evaporation of the oil is hardly (if at a11) fenfsble in a very long time. The reafon of which mu bes that tbe attrad:tion between air and the oil, is not able to overcome the tenacity of its particles, and feparate them from each other, though it is fufficient to draw xnto the o}l particles of airs whicb harre no attraEtion to each others juR as a fponge draws in water, svith- out hatlng its particles fieparated by the attraEtion of the -water

Seventhly;



[ I59 ] Seventhly; If into any menIlruum vve thraw a

*body which it diSolves, and afterwards add anatherX tto which the menItruum has a greater affinity than it has to *se firPr} it will diSolve the Second body, and let go the firk, thich will be precipitated and fall to the bottom. In like manner, if to well rec- tified rpirit of wine, we add an equal fquantity of £1ear rains or river water, thefe fluids (which incorporate fo readily) having a greater affiXlity to each other than to the air they contai.nt will let go a great part of the air, which will rife to the top, U 17cick in fmall bubbles to the bolttom md fides of the veflêl; frotn -whence I infer, that air is contained in thefe fiuids in the fame m-anner that the particles of a body are cxtailled in a men0ruum that diXolves it; and, therefUre, thatthe air imbibed bythefefluids, is properly fpsakingfi diSolved in them, and confcquently that any fluid which evaporates, or is imbibed by the air} is alfo, properly fpeaking, diXolved in air. And uBon this wnciple we may faya that water is drawn out of the air, by dry falt of tartar, from its having a greater ainity to that falt than to the air.

1 {hould not Shave been fo tedious in comparing together the natures of folution and evaporation, in fs many inSallces, but that it gave fme an opportu- llity, at the fame eime, of explaining fome of the phanomena that I at fillE intended to confider; which explanation.sf I believe, -will be admitted, if J am right in the mean point, I have endeavoured to prove. And really wben we confier howexadrly folueinn and esraporation agree in their feveral appearances, propertiesg and effedce) I think we may bo convinwd thae they ax ssatllral opera.tions of

tle



[ 60 ] tlle falze killd, and that what we call evaporation is nothing more than a gradual Colution of water in air, produced and promoted by the fame means (to wit) attradionX heat, and motion, by which other folutions are effedred.

I {llall now endeavour to account for feveral pha- nonaena of the atmofiphere upon thi-s principle} which will be Rill iurther confirmed, if it be found to atlfwer tbe purpoSe to wPhich it is applied.

The loweR part of the air being prelied by tlle weight of the atmofphere againli the fillrface of tlle water, and continuall) rubbing upon it by its motion,- has thereby an opportunity of attradting and diilolv- ing thoSe particles with which it is in conta2, and ieparating them from the reR of the water. And fince tlle cauSe of folution in this cafe, is the Itronger attraftion of the particles of water. towards air, ehan tosvards each other, thofe tlaat are already diflolved, alld taken up, will be Rill further raifed by the attradion ofthe dry air that lies overthem, aIld thus will diffiuSe themfelves, ri&ng gradually higher and highers and thereby leave the lowell air not fo nluch faturated, but that it will Rill be able to difliolsre, and take up fieth particles of water. And thus ice, or fnow, will evapora;te as well as water, its particles

being attraded and diINolved by the air which is Ilrongly prefl:*ed againLl its furface, for tllough heat promotes both folution and evaporation, yet we do not find that in-either cafe any fenEble degree of it ss abSolute}y neceiry

$ Water, by freezing, is deprivSed of its air, which we fee gathered into bulables tllrough the iceX therefore the fubAance of the ice,, being depriyed or air, will attrad the externa.l alr

* 1I1



r ^1 ] In this manner will squeous vapours aScend Rowlg

into the atmofphere, even when we fuppofe the air almoft at re{t, for I believe it is nearer p&rfeftly fo: but the folution of water in air, and the afcent of rapours, is greatly promoted by the motion of the winds, which bring freh and drier air into the place of that- which may be already faturated and loaded with moiRure, carrying it together with its moiRure into the higher parts of the atmofphere, and difperfing it into all quarters. If we {hould nosr fuppofe the atmorphere to remain always of the fame temperature as to heat and cold, and to have always the fame denfity; when it was once faturated with water, all evaporation would ceafe, and the vapours already raifed would always remain fuEpend ed; for a fluid, while it continues of the fame temperature and denilty, will never let go the particles of a body tha-t it has diSolved. We muR, there fore, confider what are the cauSes which occafioa the air fometimes to part with the water it has diC- folved, and which thereby keep up a continual circulation of vapours. And thefe I Shall {hevv to be the frequent viciEltudes of heat a-nd coldX conden- fation, and rarefacction, to svhich the atmofphere is fubjed.

As to the eieEts of heat and coldX I have already ffiewn that the former promotes, andthe latter checks, or in fome meafure hinders evaporation, as well as

more llrongly than common water does, which is faturated with air. And, on this account, I ffiould think xt probable that sces notwithREanding its hardnefis, will evaporate X faflc as commo3o water.

VOL. LV. Y other



[ I62 ] other-fohxtions; of whzh I gave an- in{tXanGe in the

trapours that are fafipended in the heat of the day, and by the cold of the night are precipitated, and fuffered to coalefce into drops of dew. From the ISow that lies fO long on the tops of mountwins, and from the experience of thofe who have paINed over them, we find that the higher parts of the atmofphere are much colder than the Iower Now, though wspours are firR raifed, and abound moR in the loxver parts of the atmofphere, yet they cannot there form vllemfelves iato clouds, becauSe the heat that helped to difl>olve them- helps alfo to keep them diSolved. But w-he-n they are carried by the winds into the higher parts, where the fame heat is wanting, the cold air will not -be able to keep diMlved all that are carried up, but- muR fuffer fome of them to coaleSce iIitO fmall particles, which {lightly attraftng each other and being inter£nixed with aiir, will form clogds, having the very fame appearance with Ileams or Emoak, which alfo confiRs of fmall particles of waters mixed with -air,: and not yet dilIolved in it. Thefe cIouds, whei} firll formedj will remain fufpended, though they coni10 of water as well as air? becauSe the weight of their particles :will not be able to ourercome the refi{lance they muIt meet with in de- fcending through the air. For when bodies are di- zzinifhed, their quantitits of matter, to which-their weights are proportionaJ, decreafe faIler, or in a greater ratio, than their furfaces, to which -the re- Si(lanee they meet with is proportional; and-, therefore, in very fmall particles, this refiRance may be- collle greater than their weight. The different heights at which clouds are fortned, depend on the quan-

tity



[ t63 ] tity of vapours carried up) and tlle degrees of 11eaVt in the upper parts of the atmofphere; for the vapours will aIways afcend, til} they meet with air fo cold or fo thin, that it is not able to keep diSolsred all that comes up; hence clouds: are generally higher ito funzmer than in winter. When clouds are much increafed by a continued addition of vapours, and their particles are driven clofe together by the force of the winds, they will-run into drops heavy enough to fall down in rain; fometimes the clouds are firo- zen before their particles are gatllered into drops, alzd then fmall pieces of them, being condenfed and made heavier by the cold, fall down- in thin flakes of fnow, which appear to be fragments of a frozerl cloud. But if the particles be fortned into drops before they are frosen, they fall dosarll in hail-iZos?es.

When the air is replete svith rapollrs, and: a cold brecre fprings up, which it often: does frotn the fea) the folution of theCe vapours is cllecked and clouds are formed in the lc)wer parts of the atmoXphere, and compofe what we call a XtZ' or fog. This general-llr happens: in a cold morningX but when tlle iun has l:)een up for fome time, tile urarlzl air again diflolves -tllofe watery particles, and it frequently clears tlp.

In a ho£ fumlner's day7 tile air lying over wet mar&y groulld, is copiouIly faturated with aqueous vapours; but tlle air growing cooler after fun-Set, will not be able to keep all tE}oSe vapours diXolved, but mull let fomfe part of them coalefce into very Emall vifiL)!e particles, that fornz thoSe tAtiSs, eJhicl appear to rife from lzzarllly grounds in a fummer's cvelling. The vapours near the ground, lreing tllore -<lenCe and copious, will bn fil*R aSezted by the cold,

2 azd



[ I64 ]

an;d aftetwards tllofe that are thinner and higher Up5..

fo that the mill will be low at firIt, but will increaIe in height afterwards; but befides, theSe grounds, an(I the water they contain, wtll acquire fuch a heat from the fiJn, t-hat they may retain it for foxne time,- and-- comtalunicate it to the contiguous air, fo that the ra--- pours y continut to rife for fome time after fun-- fet, and will become vifible when they get up a little way in the cooler air ThoSe cold thick morning fogs, I; mentioned juR nowt are often attended- with a very light fmall rain; for we then fee the drops at their firIl formation, and they are filch as are- gene- rally met with in pai1ng over high mou-ntains; rO that t feems the drops of raln are very fmall when firlEv formed in the clouds; but being driven about by the motion of the airt in their defcent, fome of thetn svill probably touch each other, and run into a drop of a larger fise, and the farther they halre to fallX the luore will their fize be increaSed before they >olzze to the grounde And, for this reaSona the-drops, whicla fall from the hiyhu clouds i-n fiummer, are foutld totbe generalliy Iarger tlaan they are in winter, hen the clouds are low. It has been likewife obferved, tllat the drops of rain ale remarkably-large that fall in tlltlnder Ihowers; of which the reaf-on snay bej that tlle liglltning burficing from a cloud,- anci expanding itSelf greatly, will fuddenly rexnove tIae ail from its place. which air, therefore, mu return to its place with great violence, and thereby tlac- wately particles in the clouds will be h:r-ongly agi- tated and daAned againk each-other, bywhich means thest svill form themfelves into larger drops ehan at ;iCr tinses a or3 perhaps it may be faid, that hen-

a cloud



[ I65 ]

a cloud is filled with lightning which is the ne a. the eledcrs matter, the watery particles, like otller eledrifi& bodies, will repel eas:h other, but being Xddenly deprived of tbis repelling matter, will by theirz mutual attraftion mnze together again ̂ rith fome velocitr, and, therefog will run into drops hrger than uI;lal

MThen the wlnd blows fiom tlie butli, it isgenF nlly warm; and comes replete with aqueous vapours, whichit has diSilved; bllt¢oming int acolder climate, it cannot there keep dz¢ fime quantity of vapours diflblved that it did before, and conkqu:ently muCt part with fome of them, and let them precit pitate; and, theref6rt fouthaly winds gtsxerally bring us rain.: Qn the oths hand, when the wind blows from the north? or any point near itj as it is arery cokl, its cannot have diflblved.a great dSeal of aqueous rapours where it vme from; and, therefores coming into a warmer climates it is ready to diilve more;* and, on this account, thefie winds, if they continue 1Qng; are found to be very dry and parclu ing, and are generally attended with fEir weather.

tI'heSe Seem to be the principal effeds of ]eat an cold in caufing the air to diMlve, aud eake ltA or let go, and prxipitat the aqueous vapllrs, and n confequeace of wlllch we Iinetimes peraive cllanges of ehe weather, even when there is no change ill the height of the barometer

E§ut nndenfitioil and rarefidion; will alfo have th.e like effids in promotiE the fiolution Qf water in air,

or in callfing fome part of what has been diiived a) return again into water and precipitte. It feenas reaSonable to fuppefe, tllat denfe -air, in wleich. the

particles



[ I66 3

particles lit vcry near each other, will be better able to- diffilve and keep fu8zended a greater quantity of water, than the fame air when dififed thro' a greater fipace. Butithat this is really fo we have an experimental proof. For when a receiver is partly exhauRed, we lee the rarefied air begin to let go the water it contained, which gathering into fmall particles appears like Ream or fmoak filling to tlle bottoln. In orilwr to prove the fame thing by another experitnent, I took from the air pump a large exhau{ted receiver twenty inches long, havilzg at tlle bottom a braSs plate, with a Itopco;ck in the tniddle of it; when the Rop-cock was opened, the external air, ruMing in violently, and being much rarefied, let go the water it contained, and threw it againIl the other end of the receiver, where it Ruck c)n the glaSs, and co- vered it with a thin dew, whicll I found to increaSe until the receiver was almoIt full of air.

TheSe experiments prove that air, when rarefied, canwot keep as much water diXolved as it does in a more condenid ite. Hence we SllUa conclude, that svhen the atmothere is much faturated with water, alld changes fFom a denfer to a rarer Rate, the higher and colder parts of it eEpecially, will begin to let go fome part of the water dilSolved; which will fornz nev clouds or add to the fize and number of the particles beire formed, and thereby render them more apt to fill down in rain. On the contrary, when the atmofphere changes firom a rarer to a denfer IRate, it will then be able to llop the pre- cipitation of the water, and again diffiolve in the whole, or in part, iotlle of thote clouds that were tottned before, and confequently vvill render ther

particles



[ I67 ]

particles leSs apt to run into drops, and fall down in rain. And thus we generally find, by experience, that the rarefied and condenCed Ilates of the atmorphere are reEpedively attended with rain and fair weather; though this does not happen at all tinzes, for the air, though rareEed, may IlOt the abound much with aqueous vapours, having already parted with a good deal of them; fo likewiSe, when the air is denfe and heavy, it may then l;>e mucl loaded with vapours, which will increafe its weigllt; and indeed it mull be fo after a long continuance of isir weather, fO that we may then have rain even b;efore the atmofiphere changes to a rarer [tate

Upon this principle I think we may account for the changes of the weather, which ufually attend the riI*lng and falling of the mercury in the barometer, better than by faying, that when the airo glONVS rarer and lighter, it cannot by the laws of hydroIlaticks fo well rupport the clouds and vapou:rss ancl therefore muR pelmit them to fall down in drops of rain : for when tlle air grows rarer, although the clouds wqll deSceled illtO a lower and denlEr part of it, yetthey will 13e there fupported; an(l Ido not fee hy their particles {hould be more apt to run ilto drops there, tllan when they were higller up, unlefs tlley received slotne addition from the water :de- pofited among them by the rarefied air, in the tnanner I have juft now Inentioned. For fince tlle air is rarefied gradually, the < cloads call defcend btlt very <06v1y; and, therefore, their particles will nOt

be fo lnuch prefRed together by the refi{tance they -leet svith in their defcent, as they generally are by tslc ;S7inds vvlaich blow upon them.

\Vllen



[ I68 ] When the ttmofiphere is tnuch faturated with wa-

ter, and grows colder or rarer than it was before, we ilsall then pelceive-the lower air begin to part with fome of the water it contains, -which will fall inGen- fibly to ehe grounds or adhere to the walls of houSes, or-other bodies expoSed to it, and make tlsem become datnp alld wet. And if the moillare fettles -on the fmooth furfaces of cold bodies, fuch as mar- ble or other Rones, whtSe pores cannot imbibe itX it nvill cover them Witll a kind of dew, and then thoSe bodies are srulgarly faid toSvev.

At this tinle ehe hygrometer being affetted by tlle nwoiRure will point to wet, and, as we perctive frola thence, that the air is di8?ofed to part with tlle sra- ;ter it contains, we may generally expedc rain. But rhen the air again grows warm or denfe, it will be able again to difl:olve, and take up the water it before 1depofitedfi and the moi0ure on the bodies expoSed *ev it will difappear, the hygrorneter will {point to dry and we may then promiSe ourfelves fair weather.

I obferved before, that, if a bottle be filled with a very cold liquor, and expoSed to the warm air, a dew will foon be formed on its furface by the moi- iture svhich the cold air depofites. Now if we fuppofe this body Itill to retain the fame degree of cold, -whilSt t'ne air paXes over it the dew on its furface will contialially increafe, and run down its fides in fantll Rreams of water This fe-ems to be exadtly the caSeof mountains, whofe tops reach into the colder parts of the atmofphere; and which, therefore, are themfelvescolder than the airisin general. Forwhen the wind blows the lower parts of the atmofphere Xtwhich ar.e the warme and moll replete with va-

pours)



[ I69 ]

pours) againlt the fides of the mountains, being there Ropped in its courfe, it muR neceSarily aCcend and paSs over their tops. This air, therefore, will be confiderably cooled in its progreSs up the fldes and over the tops of the mountains, and conliequently muR let go a great part of the watery vapours it contains; which will be precipitated in dew and moiIRure upon the furface of the mountain, zthere it will foak into the earthy parts, or infinuate itSelf into the chinks arld crevices of the rocks, where lzeing colleEted, it will afterwards break out in Eprings and fountains, and become the fource of rierers, which are known always to take their rife in mountainous countries; and, on this account, we might have fmall fprings and rivers near mountains, although there were neither clouds nor rain. But tlle moi- Rure, which the sir ufually depofites on the mountains, muR be confiderably increafed by the clouds, which-are driven againA them, and accumulated by the winds, for their particles being then preSed togetller will run into fmall drops of rain Belides, t is svell known, that mountains do gatller and retain t}le clollds about them by their attradvive force, in conCeqllence of snrhich sve often fee fotne clouds coninue at reIt on the mountains, whilA th:e others are calr-ied On gently by the wind; hence it is, that countries, in the neighbour]zood of higi; mollntains are the moff l lb)ed to frequent rains.

Thus I have (hewn how the akent of aqtleous va pours, and thcir corlftant circulation, by precipitating again in moi{lure, or drops of rain, will ariSe from the diifiolvingpoweroftheair, influenced by the viciilitude 3Dfheat and cold, larefadion and condellfation. For ve

LrOL. IJV2 Z find, -



[ T70 ]

find, lty experience, that the laeat and cold, whicll we feel, does not either fs much rarefy, or fo much condenfe the air, but tllat the former nN3wy Rill increafe, and the latter leISen the evaporation of fluids. Nor, indeed-, does tlle Rate of the atnlorphere in general, as to rarity and denfity, depend upon the lleat or cold we experience here below. Thefe cauSes, therefiore, according as tlley take place, in different degrees, will occafion thofe various Itates of the atmofphere, in refpedc to drynefs or moiRure, which we experience in the feveral cllanges of tlle weather. To svhich the winds contribute stery much by heating or coolingX condenf1ng or ralefying, tlle diffierent parts of the atmofphere; and, by pro- moting the folution of water in air, as thes mix- thoSe two fluids together, or when the air is already faturated with aqueous vapours, by preffilng togetller the particles in the clouds, aIld thereby caufing them to run into drops. And thus, from the knownproper- ties of folution, we may accollnt in a fatisfadory nzanner for the aScent and circulation of aqueous vapours, and the feveral phaeno-mena of thge atmofiphere arifing from thence, which is a great con6r- tnation of the argument brout,ht to prove that evaporation is only a particular hpecies of folution$;

* Some time after this eX^y had been read at a meeting ot the Royal Society, the author was illformed that the Abbe Stollet (to whofe works he was then anentire lzEranger) had con- lidered evaporation as a kind of folution; and, having lately Iook ed into his leEtures, he finds the Abbe- offers it as a conjed?cure, that the air may perform the office of a folvent and a fpunge in rsg;ard to the bodies it touches; but this he does not prove, nor does he afterwards apply this principle of folutson in ac-

4ountin? for feveral phzllornena that depend on the nature of 4 and}



X t7t ] alld, therefore, that tlley both proc<ed fret-n the filne cavlfe, to svitX the attradion thav obezirls 1:z. tw¢en the miIlute particles of diSerent bodies, atid svhich is the means of calrying on lio tnany ot})Cr operations of nature. And, indeed, upon tlais principle, air f&ems better fitted to be a gelleral /olvent than any fluid we know of, becauSe its particle$, ncat attraAing each other, are more at liberty to unite tllenzrelves to the tninute particles of other bodies which they do attraft, a3:d accordingly we find tlle atmofphere contains in it bodies of all kinds.

The particles of tolatile fpirits, which are fo eafily, and fo much rarefied by heat, feem to cohere very nightly together, and, therefore, may be more readily attradced by the air, and etaporate more quiclily than other fluids. The particles of- odori ferous bodies Reem to be llrongly attraded by the air, as they are lo readily dirperSed through it, and samphor, which is a light volatile body, may be intirely di{Nolved in the air, without leaving any re- mainder The air abounds with vitriolic and other acids, as is plain from the ruRing of iron expofed to it. It aboundsalSo with fulphurous, nitrous, and other inflammable particles, as appears by the frequent meteors kindled in it. - ln Short, the atmofiphere, as

evaporation, but has recourfe to others of a d;fferent kind. Now as the Abbe's works have been fome years publ;{hed, it might be fufpeded that the author of this eXay had borrowed a hint from thence vvithout acknowledging it, which would have been difinaenuous: he, therefore, thinks himfelf obliged to de- clare, what is certainly true, that he has not here reprefented any thing as new, wh}ch he was confciolls had ezer been pro- pofedl by any one before him, even as a conjeEture.

Z 2 Dr.



[ t72 ] t)r. Boerhaave obServes may be confidered as a chaos containing particles of all kinds of matter. And, here it- may be obServed) that the felreral fluids that are diflblvedW in the air will probably afl lR in diIfolving and taking up the fubtile effluvias which are carrIed off froln volatile bodies. The air, we find, is necefl:iry for tlle prefervation of aninzal life; but when it has palEed two or three times through the lungs of an aninzal, it becomes unfit for refpiration, and an animal inclofed in fuch air will rOon expire; whether the air we breathe depofites in our lungs any kind of matter neceIfary to the fupport of-life, I cannot pretend to judge. But I tllillk we may-bo flre tilae one purpofe for which air was deIlgned, is tlze carrying off that moiRures and other perEpirable maeter, whicll confl:antly exhales from ehe lungs STovY as air lofes nothing of its elafiicity, by palding through the lungs, it will Itill coninue fit for fiuch purpotes) in the animal oeconomy, as may be anfwer- ed by the alternate expanfion and colltradtion of the lungs in refipiration. And, thierefbre, I belieare that ir is rendered unfit for refpirationa clzlefly by being

laturated with that moiRure, and other perfpirable n}atter) which it meets tvith in ths lungs, and thereby lofing its -power of di{iolving, alld carrying oJi- any nlore of that kind of matter, which will then contiIlually increaSe and clog- tlle lungs, fo that an allimal inclofed-in fuch air wlll die, perhaps fome- N?llat ill the fatne manner, though 1z0t fo quickly a$ ;t it had lteen drowned.

Does it not fcem probable tllat, in the conIlant swnal quXlck evaporatlon of moiRure iroIn the lungsf totne degree of cold tnay be produced} as it is in

- otlzer



[ T73 ] other evaporations, vyhich, togvther with the freli air taken in, may ferve to cool the lungs, and the blood pa^sng through them * ?

* As air, even when incorporated with water, does not lofe its elallicity, I took it for granted, that it would not become leSs elaflcic aoy paSsng through the lungs of an animal. But being told, that the contrary opinton was held by fome, who fi pofed that airs having pafl ed through the lungs of atl animal became - unfit for refpiration ly lofing its eIallicity, I refolv-ed to try how the fa& was, by t6e followillg experiment. In a receiver eight inches-diameter, and- twelve inches high, having under it a foft piece of oyled leather, I itlcluded a pretty large chicken and tied the receiver clofe dowl-l to the table; through a holetin the top of the receIvter went a glaIi tube, open at both ends, cemented round the hole with wax, the lower end was immerfed in water (tinged blue) which Rood in a glafs under the receiver. In about an hour after the chicken was included, it grew very much diRrefl ed, gaped wide, and breathed with fgreat difliculty, ant in half a-n hour more it feemed almo{} ready to expire; the infide of the receiver was then cotrered with moiOure, which in fiome places ran down in drops. Now if the included air had loPr any of its ela{licity by paSlng through the lungs i this ani- malf it could not have preSed b flcrongly on the water in the glaSs as it did ae 15rk, and then the external air wouId have preScd through the tube, and appeared coming up through thF water in bubbles; but no fuch thing happened, for as foon a8

the receiver was tied down, the water in the tube rofe about one fifth of an inch above the water in the glaSs, and fo continued during the whole time of the experiment, except: that it rofie and fell near one tenth of an inch every time the chiclrea breatb- ed; and thefe vibrations of the water in the tube, I obServed grew llower, and moved through aXSgreater fpace tovvards the latter end of thetime;; which hewed tbat the chicken thea took in more air every time it breathed than it did at firPr. After things had ood thus above an hour and a;half, and thofe who bw tbe experiment were convincedythat the included air hat not 1PA any of its.elakicity, though grown quite unfit for refpiration, the animal being ready to expire in it, I thought it unne- ceSary to confine the chicken any longer, nd it foon recseredi

X Air



[ I74 1 Air is not lefs neceary for t!ze fa3polt of fire

than of animal life, for lSte svill not long colzti- ntle to blll n without a circEliation of fre(h air, w'tlich I iuppofe llappens not from its adding any tlliilg to the pabulutn of file (for that feenls unneceflary) but rather on tbis account. Tlze air itnmediately abollt a body on fire, is heated and made rpecifically lighter tllan the air at fotne diRance frotn it. Thxs hot air mull, tllerefore, aScend aeld carry with it all tlwofe minute particles of diSerent kinds, which are thrown offfrom the burning body, and which other | wife would rell upon its furface, and thereby clog and Rop tlle fubtile vibrations of tlae burninz nautter, in which the nature of fire partly confiIls. Jf, therefore, fire be confined in a cloSe place, where there can be no cilculation of freSh air, tlle air about it being foon faturateds with the particles ariElng frotn the burning nwatter, will not be able to take up any more of the-m, andX tberefore, the fire mut} go out fmothered as it were in its own aiBes. And llenee it is, that fire burns faIler when 0 air is ilrongly blovsrn uporl it; for this air £arries off the aShes as faR as they are formed on tlle fiurface of the burning body, and thereby keeps thofe particles that have juR taken fire quite free from any thing that can impede or clog their vibratory motion This air vvill alfo fpread t-he fire quickly through the fuel, by blowing the particles that are already kindled amongll them that are not, and, perhaps, the motion of the air may prornote thefe fubtile vibrations in the burning matter, by which the fire is propagated through its parts.

T< lough



[ T7S ]

Though the palticles of fluids, in common evaporation, are raifed into tlle atlnof)here, lzy tlae at traAing and diXolving poV.^rer of the airX yet in fome particular cafes vapours will rife illto the air Otl another account; for in fone places the earth often fends forth 11ot ela{lic vapours that ritE into tlae air by nzeans of tlatir elallicity, and carly with tllem mineral particies of-diSerent kinds. Fermentation generates elaRic vapoxars, which expand themSelves into tlle air. And the particles of water, when fufficiently heated, acqui1e a repelling forcea which fepatates them fronl the furface of the water, and throsvs tlaeln upwards into zthe air. - But all thefe vapours foon lofe that elaIlicity by wkich they were firft raiSed, and they are then retained, and kept fuEpended in the air, by the fame power that keep fufpended all thefe vapours that rife without any elaRicity in cotnmon evaporation. That the particles of i:leam, wllich rifes from hot water, are en- dued with a repelling force, appears plainly ̂7;hen water is boiled in a clofe vefl el; for then the Ileam becomes fo exceedingly elaRic, that, unlefs proper caution be uSed, it will bulIt the IlrongeIt veXel In this caSe the boiling water, being Rrongly preSed by the force of the included Ream, conceives a much greater heat, than it will do in an open veXcl; for even when water is boiled in the open air, it is fomewhat hotter when the atmofphere is heavy, than when it is light, which ffiews that preflure, upon boiling svater, illcreafes its heat; the reafon of which we ihall See prefently.

But tlle mo{t renzarkable phtnomenon that at- tends the boiling of water, is thofe large bubbles

which



E I76 ]

which continue to rife from the bottotll szrhilll even the vvater boils, and long after all the air ss driven out of it. Dr. Boerhaave, in his ELements of Che- miIlry, part II. has proved, by fevelal arguluents, tllat thefe -bubbles do not arife from air; and with regard to tlleir produdtion, he feems to be of the falne opinion svitll Stairs (to whoSe work lle refers) tlzat they alife frotn fotlze adcive fires reElding in tlle snratel. AIariotte, whom he a-lSo mentions on this <3ccafiorl, calis there bllbbles fultnirlatiolls and fup- pofes that thev may arife from fonlW faline particles centained in the walter, wilich being heated adt in the fame tnanner witil the aurunz fulminans. I find it is a received opinlon (but how generally I cannot tell) that theie bubbles ale occaE1olaed by fome fubtile elaitic fluid, tranfmitted frotn the fire througll the bottom of the veflel. EIosêvez, I conceive tlaat the fluid 1o lubtile as to paSs readily tllrough the bottom of the veilel, would paSs allo through the vater fo eaiRlyas not diRurb it; and, tllereforeX I lave tor lonle tilz:3e fulpeiced that theSe bubbles are Sortlled only by an elaJ?ic /2eazn) in the tnanner I shall rlout deScribe. T1ze particles on the furface of

t-lle water long befole it boils, will by means of the repelling force, ss hicll the heat intl oduces anzongft then, rife in Reams and will inlinuate themSelves into the air, which ields eafily to them; but thofe particles that ale preiled againflc the bottonl, by the weight of the atmollzhere and of the incutnbent waters will require a great degree of heat to render them fo elattic, that they Ihall Ibe able to overcome this refitlance, and expand themSeleres into a gleater fpace. Now fance heat expands water, and makes

its



[ I77 ] its particles repel each other according to its diffierent degreesa we may reafonably fuppoSe that thefe particIesw from tlleir being in contaer with the bottom of the veilrel, may, at length, acquire fuch a degree of heat as will give Vthem a repelling force fufficient to over- e:ome the prelfure they fuiRain, and expand them filddenly into thofe large bubbles that aScend through the water when It boils violently. I have lately made fome obirvations and experiments, which feem very much to farrour this opinion Thefe bul)bles, which alicend from the bottom} I obSerlred always grow lefs before they reach the furfacet and thofe fmall bubbles, which adhere to the bottomt for fome- time befbre they aScend, often diSappear intirely before- they reach the fulface; which nlews; that when the matter tlley contain, or any part of it, lofes the heat it had at firiS, it is turxied into wa ter. When:water? that has juRboiled, is poured into a glaSs, and fet llnder the receiver of an air punlp, and the air is almoIl drawn out) the water appears to boil more violently than it does on t-he fire; and the bubbles do not all liKe from the bottom, but break out from other parts of the water, eipecially towards the middle, where xre may huppofe it 11otteR. In this caSe no fiubtile fluid can be -fuppofied to rif: through the bottoln of the veSel; but -tlle heatt which the water retains, will give its particles an elaftic force fufficient to overcome the prefl;lre of what little air remains in the receiver) and expand them into bubbles; and that thefe bubbles are compoSed of Ream appears plainRly frore this expe riment, for as roOn as they begin to afcend, the receiver is filied gtith ileam, wthich being-condenfed

V@L. L\zS A a by



- [ 178 ] by th cold, runs plentiSlly down: its fdes in water.- After a veISel of water had boiled till all the air bubbles were driven oue of it, I turned upon its mouth s large glaSs that lay in-the water; the bubbles that Zcended under the glafis remained in the upper part of itX and forced oue the rater it before containeds and then the elaflcic matter in the glaSs overturned it, againk the fide of tlle reXel3 and burRiw out aCcended to the top in one large bubble upon which the Ream on the furface appeared to be much m- creaidx Now this illews that the matter contained in thofie bubbles, which is quite tranfparent, being a rery rare and homogeneous fluid, gappears afterwards like Ream wIaen it is mixed with the air. But I thought I {houlil make a more decifive experiment if I could obServe the efEedrs of very hot Ileam con- veyed under boiling svater. Therefore when an :tolipile had boiled :till the air was all driven Ollt of the water it contained, without taking it oF the fire, I immerfed its pipe into a veffiel of water which-hacl juR been boiledy and immediately the Iteam, which iffiued from the pipe, rofe up in very large bubbles through -the water, and made ie feem to boil vlolenely. TheJl I held a large glaSs of cold water, So that the pipe of tlle boiling tolipile was irnmerfed into it. At filIt none of thefe large bubbles appeared; for tlle Ream being then condenSed by the cold water, :was mixed with it, making a very unco-mmon noiSe; but -as Soon- as the water in the glaSs grew sery hot thPs noife=ceafed, and the {team being no longer condellfed roSe in large bubbles, and rnade the wa-- tf r appear to boil with great vlole-nce.

TheSe



[ I79 ]

->eSe obServations and experiments Seem to diSco- ver to us fully the nature of thofe bubbles that aScend through bc)iling water. And from hellce I think we may leam the reafon why any fluid, in an open veXel, will acquire only a certain degree of heat when it boils, and will not grow hotter afterwards.!; and why different fluids will acquire different degre¢s of heat in bolling ? The parts of the fluid neareR the ittom grow hot at firit, and being then expanded and m-ade lighter, they aScend and change place with the colder and heavier parts (which occabons tbat Xte/Xixe sozion we per in liqaors while tht are growing hotJ; and- thus the hcat of the whole will increafe, until thofe particlesX that- are in contadr with the bottom of the veINcl, acquire fuch a degree of heat as will g;:re them a repelling force able to overcome the weight of :the atmofphere, the weight of the incumbeng fluid, and the tenacity of its particles; and then they will be fuddenly expanded into bubbIes of lleanl, and aScend quickly to the top, without communicating this heat to tlleu furrounding fluid: for as thefe bubbles hav¢ a degree of heat but little- fu- Zrior to that of the fluid, and jut} fufiicient to keep them expanded, if they ̂ rere to lofe this heat, by communicating it to the flalid in their aScent, they would all difappear beire they got to the furfice; or if the whole flllid was to grow as lzot as the bab- bles, it would, like them, be 211 turned into elaltic Ream; and, thereforeX the fluid itSelf cannot grow hotter than when thefe bubbles began to aScend.

That thefe bubbles are really llotter than the other parts of the fluid I found by the following experi-

A a z men-tf



[ I80 ]

ment. A tin veflel, about nitle inches in diameterX was Set on the fires fo that the water on one f1de only boiled violently but the nzotion arifing from thence made all ehe water circulate through the veSel; and,- therefUre, all the water (after the veliel had boiled for fome time) muit have acquired the fame degree- of heat. I then held a nzercurial thermotneter} with Fahrenheitts Scale, under the waeer; whera it was- jult out of the reach of the bubbles, it rofe there no higher than to 2 I I ; but when it was held among the bubbles, where they afeended thickett, it rofe tO 2 1 2 which is uEually reckoned the mean heat of boiling water, though it feems rather to be the heat of the flieam contained irs the bubbles; and, therefore it is fomewhat greater than the heat which water will bear without being turned into lleam. The }weat of the water was not then fo great as it fome times is, for the atmofphere was then very light, the barometer Randing at *9.

From what has been faid, it follows, that the degrees of heat necefl4ary to raife thefie bubbles- in any fluid, and make it boils will be greater as the fluid is more Rrongly preXed, and as ies particles are more tenacious or vifcid-. And this we find is exadcly agreeable to experience. For fpirit of wine, which is a fluid very light, eafily rare9ed, and in no degree vficid, will boil witha lefis heat than water doez. - But rnercury, -w-hoi particles are heavier, and oil or pitch, w:hofe particles are more vifcid, than thofe-of water, will require a nzuch greater degree of heat to Inake it boil than water does And it is knowny that water boiled in a clofe vefliel, where it is firorxg- ly preIled by the- confined :elaRic Ileavm, will become

- much



[ I8I ] much hotter thatl when boiled in the open air; fo that if the clofe veXel ould burRt or be opened fiuddenly, I doubt not but the whole body of the- water would immediately expand itSelf into dream, and fly out of the veffiel with great violence,

I know not whether theSe experiments, and fihe obServations founded upon them, will appear new to you as they did to me; fuch as thcy are, I fub- mit them to your conf}deration; and am,

S I Rs

ViTith great ree&,

- Your mof} obedient - - 0

Humble Servant,

Tranity College, Dubli* Hugh Hamilft9R. Alarch 5 I 765.

SXIV;



Documents

A Dissertation on the Nature of Evaporation and Several Phaenomena of Air, Water, and Boiling Liquors: In a Letter to the Rev. Charles Dodgson, D. D. F. R. S. from the Rev. Hugh Hamilton,