BO! 



formefl, afcendfi contiimally and efcapeJ unimpaired through 

 the fluid, which it therefore heaves with violent agitJitior. 



The lame appearance almoft is pindiiccd by removing or 

 even dimiiiifliiiig the atmofpherfc prelTure. Thus, if a tum- 

 bler holding vN'avin water be introduced under the receiver 

 of an air-pump, as the exhauftion proceeds, or the incumbent 

 weiirht is gradually withdrawn, tlie latent portion of air is 

 dlfcTiargcd in a rapid flow of eKpanded bubbles. But this 

 procefb, at fome certain point ot rartfaftion, is fuccetded 

 by the vchcmer.t commotion which conftitutes boding ; and 

 the water, affiiiiiing its invifible form, fills the impcrfeft void 

 with vapour, which betrays its exillence by condeniing 

 aTainfl. llie fidts of the receiver in copious dew. Nor is heat 

 pofitivoly neceifary towards vapori/Latiou, for it only confpires 

 iu aceomphfliing that effed, and fupplies the want or the 

 impel feftion of our means of producing exhauftion. By 

 Jielp of an air-pump of tl'.e bell conilruftion, the coldcil 

 water may be made to boil, nay, ice itftlf could be changed 

 into invifible Itcam. Hence the utter impofTibility of ever 

 obtaining a perfed vacuum, beeaufe the reftraining influence 

 of preflure being entirely removed, the hquid matter un- 

 avoidably prtlentcd would always diflule a thin vapour. 



The oppolite influence of heat and prefTure on the con- 

 ■Ritution of fluids is well exhib'ted by a verj' fimple yet 

 ftriking e>:perinient. Take a large thin phial, and having 

 warmed il gradually to avoid the rilk of cracking the glais, 

 fill it completdy with boiling water, cork it tight, and ex- 

 pofe it to a current of cold air. As the water cools, it 

 ■ncceffarily contrads its volume, and leaving an imperfcd va- 

 <:uity below the neck of the phial, it hence becomes to a 

 .confidercble degree relieved from the load of atmofpheric 

 prelTure. It therefore foon begins again to boil, nay, it 

 will boil more briltcly the falter it cools ; and this fingular 

 appearance, fo contrary to our ufual notions, may continue 

 perhaps for tlie fpace of half an hour, till the water has 

 grown as cold almofl as the temperature of the human body. 

 On the fame principle depcndn the conftrudion of what is 

 called xhc pulfe ghifs : thisconfifls of two balls conncded by 

 a pretty long tube ; one of thefe balls is fdled with coloured 

 water or fpirits of wine, which having been made to boil and 

 expel the air by its vapour, at the fame inftant the point 

 projcding from the other ball is hermetically fealed. As 

 that vapour condenfes with cold, it will leave the included 

 liquid then in a fort of vacuum, and the heat of the hand is 

 then fufficient to caufe it to boil and to flow from one ball 

 into the other. 



if a veffel containing water be placed over a fteady fire, 

 the water will grow continually hotter till it reaches the 

 limit of boiling, after which the regular acctffions of heat 

 are wholly fpent in converting it into fteam. The water 

 therefore remains at the fame pitch of temperature, how- 

 ever fiercely it boils. The only difference is that, with a 

 ilrong fire, it fooner comes to boil, and more quickly boils 

 away. Hence the reafon why a vetf^l full of water, and 

 plunged into the centre of a larger one, which is likewife 

 filled with that fluid, barely acquires the boding heat, but 

 will never adually boil. 



The formation of (learn occafions a prodigious confump- 

 tion of heat ; for if the time be noted in which water, by 

 the adion of a flrong fire, is raifedfrom the limit of freezing 

 to that of boiling, it will be found to require more than five 

 limes longer a fpace to boil entirely away. Thus, a portion 

 of heat correiponding to above 900 degrees by Fahrenheit's 

 fcale, is always conlumed in the ad of boiling, or rather it 

 is trausferred and enters into the compofition of fleam, 

 the gafeous produd. This abforbed heat is as conftantly 

 evolved when luaui condenfes and returns to its hquid form. 



B O I 



Hence in dlflillation a very large refrigerator/ Is required fov 

 condenfing a comparatively fmall quantity of aqueous or 

 fpirituous vapour. Hence too the explication of the fa- 

 miliar remark that fleam fcalds more cruelly than boiling 

 water. 



The heat of boiling water, being fubjed to the influence 

 of the atmofpheric preiRire, is thus not abfolutely fixed. 

 It varies with the variation of the barometer, and decreafes 

 as the mercury defcends. The extent of this fliduation 

 may in our changeable climate amount to five degrees by 

 Fahrenheit's fcale, the fuccelTivc diiTerence of a dtgiee cor- 

 refponding nearly to each twentieth part of the remaining 

 incumbent weight. On the tops of lofty mountains water 

 will boil much fooner than in the plains below. This curi- 

 ous fad has been noticed by feveral travellers, and was par. 

 ticulruly obferved by Saun"ure on the fummit of Mont Blanc. 

 A Hill greater variation would be experienced on the peak 

 of Chimboraco, the highell point of the Andes, where 

 water would boil with a heat fcarcely fuperior to that which 

 is comm )nly alhgned for the boihiig of fpirits of wine. 



It is therefore evident that, under an augmented prefigure, 

 all liquids will more flovvly reach the crilis of ebullition and 

 will then have acquired a more intenfe heat. Thus water 

 may be heated up many degrees above the mean point of 

 boiling, if it be fubjeded to the adion either of condenfed 

 air or of conllned lleam. Such is the principle of Paphi's 

 Dhcjlcr ; which, being nearly filled with water, is Mint per- 

 fectly clofe, and^fet on a good lire. As the fteam fo formed ii 

 prevented from efcaping, it neceffarily concentrates, and ex- 

 ei ting accumulated energy, it by its prodigious compreflioii 

 enables the water c >ntinually to receive additional heat. 

 Nor would this progrefs at all Hop, till the elallicity of the 

 imprifoned vapour comes to furmount every obftacle, anil 

 burils the veffel with terrible explofion. Accidents of that 

 fort are extremely dangerous, and the experiment has confe- 

 quently never been puflied to its utmoil pradicable limits. 

 ^Vhen the fradure takes place, not only the confined fleam 

 is liberated, but the pren"ure being now removed, the excefs 

 of heat inllantaneoufly converts a part or the whole of the 

 water likewife into fleam, which augments the general 

 cffed. This we may perceive in the burfling of a glafs 

 cracker; for the little bafe is fhivercd into atoms, and the 

 water which it contained is entirely difperfed, beating down 

 flat the wick of the candle by the violence of the fudden ex- 

 panfive blafl. 



Hence the boiling heat of a deep cauldron is always rather 

 grrater than that of a (hallow pan. This excefs we might 

 cllimate at neaily one degree of Fahrenheit, for each foot of 

 depth. The heat of ebullition mull alio rife fomewhat 

 higher, if the fleam be not allowed to efcape as fall as it is 

 generated ; for which reafon there may be a flight difference 

 of energy between rapid and flow boiling. Hence by the 

 combined operation oi both thefe caufes, water deeply lodged 

 in the bowels of the eartii or concealed under the dark ted 

 of the ocean, is capable of acquiring the moft intenfe heat 

 from the adion of fubterranean fires ; a principle of which 

 Dr. Hiitton has ingenioufly availed himfelf in framing his 

 Theory of the Earth. 



But the pofition of the boiling point is likewife modified 

 by the influence of chemical attradion. Thus fugar, com- 

 mon fait, and other faliiie fubilanccs, have all of them a 

 tendency to fix water and retard the crifis of its converfion 

 into eladic vapour. Strong brine will not boil until it is 

 heated up feveral degrees above the ordinary limit. Hence 

 a veffel containing frelh water, and immerfed in another 

 which is filled with brine, will gently boil, while the fur- 

 rounding fluid only fimmcrs. On the other hand, the addi- 

 2 tion 



