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Show 68 NOTE XXV.--EVA p 0 RAT ION. . . h I lead ·with viewleJs marcb AqU'atzc ltymp s · you . cb CANTO III. 1. 13. . d V.1pours up the aerral ar . ~be wmge ' . . f moifiure as a chemical men- 11 l·rr 1 . certalll quanttty o d" . . I T HE atmofphere wi c tno ve a . . s appears from the tmtnutton . b I the freeztng pomt, a d I firuum, even when it is muc.h be ow ch greater quanti. ty of water is evaporafti e .u a·n c of ice fufpended in frofiy atf, uta mu l. h . erhaps the univcrfal caufe of ui tty, r f ded in the air by means of heat, :V HC IS P I ·ch is a proof that it will evaporate 1u pen . h 1 [: h t 111 vacuo w 11 · for water is known to boil Wtt. e s e; the; hinders than promotes its evaporation f fi . n vacuo and that the air there ore ra . . fi ned in vacuo by a fmall degree a er I ' Tl quick evaporatiOn occa lu f h {l d in higher degrees of heat. Je . d a ulfe-glafs, which confit1s o an ex au e of heat is agreeably fcen in what IS ter~·e J "th about two thirds of the cavity filled b Of glafs with a bulb at each end o It an WI b "I by the heat of the finger-end tu e . . . · fi ntly feen to 01 with alcohol, in which the fpll:lt IS m a b lb and is condenfed again in the upper r d on a bubble of fieam 111 the lower u r:' app le . bl ·ative coldne s. . . b lb by the leafl: concelva e compat . 1 . 1cipal caufe of evaporatiOn IS u · · that heat IS t 1e pru · k 2- Another circumfiance evmcmg . n reat quantity of heat IS ta en b . nverted mto neam, a g . h that at the time of water emg co be repeatedly dipped 111 et er, or . b dies If a thermometer . b away from the neighbounng o . bl n f . to expedite the cvaporatton Y . d fed to a an o air, k b I ·n reetified fpirit of wme, an expo . h neter will prefently fin e ow J d · f: 1t the t ermm perpetually removing the faturate atr rom 'b. ent bodies at the time of evaporation by freezing. This warmth, taken from the al~ l d [i d into water. Hence the water . . hen the !learn IS con en e h the fieam, is a gam gtven out w I t. and hence the warmt accom- . b d . dill:illation fo foon becomes 10 ' m a worm-tu unng . . ather . . Panying the dercent of ram 111 cold ~e h ,h . the principal caufe of evapor:lt!On, IS . a iliewmg t at eat IS . . hd 3 The third Clrcumnance, - t of the heat IS Wit rawn. • . condenfed when any par that fame of the fieam becomes a gam . h . fiure fucceed the colder north- ~ h fi winds replete Wit mol br b Thus when warmer out -we . (t . lls brick floors, &c. a lOr call winds all bodies that are denfe a~d fub!la~tlal, ~~ on~::m~s precipitated on them, f h ffing air and Its motnure I fame of the heat rom t e pa I ' h . . 1 in this latitude, and are t lence while the north-eafi wm. d s, b ecome warmer on t e1dr adr nv. a winds · !t d are terme rymg · difpofed to take up more mol ure, an r f h r I t" on of many other bodies, as +· b I p incipal Callie o t e tO u I ld Heat feems to e t le r . ' h. l h expofed to fevere co are bl .. l d'!folved m water, w IC 1 w en . h. 1 common f,tlt, or ue Vltno I f h l ft frozen . this I obferved in a p ta d . d to the part 0 t e water a ' r h . precipitate ' or came ' . . h. h f en. the phial was bur t, t e ICC filled with a folution of blue Vltnol w lC was roz ' . NoTE XXV. EVAPORATION. thawed, and a blue column of cupreous vitriol was left fianding upright on the bottom of the broken glaf.~, as defcribcd in note XIX. art. 3· II. Hence water may either be diffolved in air, and may then be called an aerial folution of water; or it may be diffolvcd in the flnid matter of heat, according to the theory of M. Lavoifier, and ml'y then be called fieam. In the former cafe it is probable there are many other vap ours which may precipitate it, as marine acid gas, or fluor acid gas. So a lea line gas and acid gas eli fTolved in air prC'cipitate each other, nitrous gas precipi t:ltes vital air from irs azote, and inflammable gas rnixcd with vital air ignited by an eleClric fpa rk eit her produces or precipitates the water in both of them. Are there any fu~tle exhabions occa fionally diffufed in the atmofphere which may thus caufe rain? x. But as water is perhaps many hundred times more foluble in the fluid mattrr of heat th:.~ n in air, I fuppofe the educlion of this heat, by whatt\•er me:ms it is occa{joned, is the principal c:wfe of dcv::: poration.- Thus if a region of air is brought from a warmer climate, as the S. W. winds, it becomes cooled by its contatl with the earth in this la ti tude, and parts with fo much of its rnoiHme as was ditfolvcd in the quantity of calorique, or heat, which it now lofes, but retains that part which was fufpenckd by its attraction to the particles of air, or by aeri al folution, even in the moll: fevere frofl:s. 2. A fecond immediate caufe of rain is a fircam of N. E. wind defcending from a fup erior current of air, and mixing with the warmer S. W. wind btlow; or the reverie of thi s, viz. a fuperior current of S. W. wind mixing with an inferior one of N. E. wind; in both thefe cafes the whole heaven becomes inltantly clouded, and the moinure contained in tl.l~ S. \i\1. current is precipit:ltcd. Thio caufe of rfevaporation has been ingcniouOy explained by Dr. Hutton in the Tranfacr. of Edinburgh, Vol. I. and feems to :uife from this circumllance; the particles of air of the N. E. wind educe part of the heat from the S. W. wind, and therefore the water which was diffolved by that quantity of !Jcal is precipitated ; all the other part of the water, which was fufpend ed by its attraction to the particles of air, or dilfolved in the remainder of the heat, continues unprecipitatcd. 3· A third method by which a region of air becomes cooled, and in confequence depofits much of its mofiure, is from the mechanical ex pan lion of air, when part of the preffure is taken <.ff. In this cafe the expanded air becomes capable of receiving or attraCting more of the matter of heat into its interll:ices, and the vapour, which was previoufly diffolved ia this heat, is depn{jted, as is feen in the receiver of an air-pump,. which becomes dewy, as the air within becomes expanded by the eduetion of part of it. See note VII. Hence when the mercury in the barometer finks without a change of the wind the air generally becomes colder. See note VII. on elementary Heat. And it is probably from the varying preffure of the inCIJlllbcnt air that in fummer days fm~ll black clouds are often thus fuddcnly produc d, and again foon vamfh. See a paper m Philof. Tranf. Vol. LXXVIII. intitlcd Frigorific Experiments on the Mechanical Expanfion of Air. |
