meteorology-第20章

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fluids　which　have　no　sediment　or　only　a　little；　except　where　this



absence　of　sediment　is　due　to　viscosity。　For　in　some　cases；　in　oil　and



pitch　for　instance；　it　is　the　viscosity　which　prevents　any　sediment



from　appearing。



　　It　is　always　a　process　of　heating　or　cooling　that　dries　things；



but　the　agent　in　both　cases　is　heat；　either　internal　or　external。



For　even　when　things　are　dried　by　cooling；　like　a　garment；　where　the



moisture　exists　separately　it　is　the　internal　heat　that　dries　them。　It



carries　off　the　moisture　in　the　shape　of　vapour　（if　there　is　not　too



much　of　it）；　being　itself　driven　out　by　the　surrounding　cold。　So



everything　is　dried；　as　we　have　said；　by　a　process　either　of　heating



or　cooling；　but　the　agent　is　always　heat；　either　internal　or　external；



carrying　off　the　moisture　in　vapour。　By　external　heat　I　mean　as



where　things　are　boiled：　by　internal　where　the　heat　breathes　out　and



takes　away　and　uses　up　its　moisture。　So　much　for　drying。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　6







　　Liquefaction　is；　first；　condensation　into　water；　second；　the　melting



of　a　solidified　body。　The　first；　condensation；　is　due　to　the　cooling



of　vapour：　what　melting　is　will　appear　from　the　account　of



solidification。



　　Whatever　solidifies　is　either　water　or　a　mixture　of　earth　and　water；



and　the　agent　is　either　dry　heat　or　cold。　Hence　those　of　the　bodies



solidified　by　heat　or　cold　which　are　soluble　at　all　are　dissolved　by



their　opposites。　Bodies　solidified　by　the　dry…hot　are　dissolved　by



water；　which　is　the　moist…cold；　while　bodies　solidified　by　cold　are



dissolved　by　fire；　which　is　hot。　Some　things　seem　to　be　solidified



by　water；　e。g。　boiled　honey；　but　really　it　is　not　the　water　but　the



cold　in　the　water　which　effects　the　solidification。　Aqueous　bodies　are



not　solidified　by　fire：　for　it　is　fire　that　dissolves　them；　and　the



same　cause　in　the　same　relation　cannot　have　opposite　effects　upon



the　same　thing。　Again；　water　solidifies　owing　to　the　departure　of



heat；　so　it　will　clearly　be　dissolved　by　the　entry　into　it　of　heat：



cold；　therefore；　must　be　the　agent　in　solidifying　it。



　　Hence　aqueous　bodies　do　not　thicken　when　they　solidify；　for



thickening　occurs　when　the　moisture　goes　off　and　the　dry　matter



comes　together；　but　water　is　the　only　liquid　that　does　not　thicken。



Those　bodies　that　are　made　up　of　both　earth　and　water　are　solidified



both　by　fire　and　by　cold　and　in　either　case　are　thickened。　The



operation　of　the　two　is　in　a　way　the　same　and　in　a　way　different。　Heat



acts　by　drawing　off　the　moisture；　and　as　the　moisture　goes　off　in



vapour　the　dry　matter　thickens　and　collects。　Cold　acts　by　driving



out　the　heat；　which　is　accompanied　by　the　moisture　as　this　goes　off　in



vapour　with　it。　Bodies　that　are　soft　but　not　liquid　do　not　thicken　but



solidify　when　the　moisture　leaves　them；　e。g。　potter's　clay　in



process　of　baking：　but　those　mixed　bodies　that　are　liquid　thicken



besides　solidifying；　like　milk。　Those　bodies　which　have　first　been



thickened　or　hardened　by　cold　often　begin　by　becoming　moist：　thus



potter's　clay　at　first　in　the　process　of　baking　steams　and　grows



softer；　and　is　liable　to　distortion　in　the　ovens　for　that　reason。



　　Now　of　the　bodies　solidified　by　cold　which　are　made　up　both　of　earth



and　water　but　in　which　the　earth　preponderates；　those　which　solidify



by　the　departure　of　heat　melt　by　heat　when　it　enters　into　them



again；　this　is　the　case　with　frozen　mud。　But　those　which　solidify　by



refrigeration；　where　all　the　moisture　has　gone　off　in　vapour　with



the　heat；　like　iron　and　horn；　cannot　be　dissolved　except　by



excessive　heat；　but　they　can　be　softened…though　manufactured　iron　does



melt；　to　the　point　of　becoming　fluid　and　then　solidifying　again。



This　is　how　steel　is　made。　The　dross　sinks　to　the　bottom　and　is



purged　away：　when　this　has　been　done　often　and　the　metal　is　pure　we



have　steel。　The　process　is　not　repeated　often　because　the　purification



of　the　metal　involves　great　waste　and　loss　of　weight。　But　the　iron



that　has　less　dross　is　the　better　iron。　The　stone　pyrimachus；　too；



melts　and　forms　into　drops　and　becomes　fluid；　after　having　been　in　a



fluid　state　it　solidifies　and　becomes　hard　again。　Millstones；　too；



melt　and　become　fluid：　when　the　fluid　mass　begins　to　solidify　it　is



black　but　its　consistency　comes　to　be　like　that　of　lime。　and　earth；



too



　　Of　the　bodies　which　are　solidified　by　dry　heat　some　are　insoluble；



others　are　dissolved　by　liquid。　Pottery　and　some　kinds　of　stone　that



are　formed　out　of　earth　burnt　up　by　fire；　such　as　millstones；　cannot



be　dissolved。　Natron　and　salt　are　soluble　by　liquid；　but　not　all



liquid　but　only　such　as　is　cold。　Hence　water　and　any　of　its



varieties　melt　them；　but　oil　does　not。　For　the　opposite　of　the　dry…hot



is　the　cold…moist　and　what　the　one　solidified　the　other　will　dissolve；



and　so　opposites　will　have　opposite　effects。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　7







　　If　a　body　contains　more　water　than　earth　fire　only　thickens　it：　if



it　contains　more　earth　fire　solidifies　it。　Hence　natron　and　salt　and



stone　and　potter's　clay　must　contain　more　earth。



　　The　nature　of　oil　presents　the　greatest　problem。　If　water



preponderated　in　it；　cold　ought　to　solidify　it；　if　earth



preponderated；　then　fire　ought　to　do　so。　Actually　neither



solidifies；　but　both　thicken　it。　The　reason　is　that　it　is　full　of



air　（hence　it　floats　on　the　top　of　water；　since　air　tends　to　rise）。



Cold　thickens　it　by　turning　the　air　in　it　into　water；　for　any



mixture　of　oil　and　water　is　thicker　than　either。　Fire　and　the　lapse　of



time　thicken　and　whiten　it。　The　whitening　follows　on　the　evaporation



of　any　water　that　may　have　been　in　it；　the　is　due　to　the　change　of　the



air　into　water　as　the　heat　in　the　oil　is　dissipated。　The　effect　in



both　cases　is　the　same　and　the　cause　is　the　same；　but　the　manner　of



its　operation　is　different。　Both　heat　and　cold　thicken　it；　but　neither



dries　it　（neither　the　sun　nor　cold　dries　oil）；　not　only　because　it



is　glutinous　but　because　it　contains　air。　Its　glutinous　nature



prevents　it　from　giving　off　vapour　and　so　fire　does　not　dry　it　or　boil



it　off。



　　Those　bodies　which　are　made　up　of　earth　and　water　may　be



classified　according　to　the　preponderance　of　either。　There　is　a　kind



of　wine；　for　instance；　which　both　solidifies　and　thickens　by　boiling…I



mean；　must。　All　bodies　of　this　kind　lose　their　water　as　they　That　it



is　their　water　may　be　seen　from　the　fact　that　the　vapour　from　them



condenses　into　water　when　collected。　So　wherever　some　sediment　is　left



this　is　of　the　nature　of　earth。　Some　of　these　bodies；　as　we　have　said；



are　also　thickened　and　dried　by　cold。　For　cold　not　only　solidifies　but



also　dries　water；　and　thickens　things　by　turning　air　into　water。



（Solidifying；　as　we　have　said；　is　a　form　of　drying。）　Now　those



things　that　are　not　thickened　by　cold；　but　solidified；　belong　rather



to　water；　e。g。。　wine；　urine；　vinegar；　lye；　whey。　But　those　things　that



are　thickened　（not　by　evaporation　due　to　fire）　are　made　up　either　of



earth　or　of　water　and　air：　honey　of　earth；　while　oil　contains　air。



Milk　and　blood；　too；　are　made　up　of　both　water　and　earth；　though　earth



generally　predominates　in　them。　So；　too；　are　the　liquids　out　of



which　natron　and　salt　are　formed；　and　stones　are　also　formed　from　some



mixtures　of　this　kind。　Hence；　if　the　whey　has　not　been　separated；　it



burns　away　if　you　boil　it　over　a　fire。　But　the　earthy　element　in



milk　can　also　be　coagulated　by　the　help　of　fig…juice；　if　you　boil　it



in　a　certain　way　as　doctors　do　when　they　treat　it　with　fig…juice；



and　this　is　how　the　whey　and　the　cheese　are　commonly　separated。



Whey；　once　separated；　does　not　thicken；　as　the　milk　did；　but　boils



away　like　water。　Sometimes；　however；　there　is　little　or　no　cheese　in



milk；　and　such　milk　is　not　nutritive　and　is　more　like　water。　The



case　of　blood　is　similar：　cold　dries　and　so　solidifies　it。　Those　kinds



of　blood　that　do　not　solidify；　like　that　of　the　stag；　belong　rather　to



water　and　are　very　cold。　Hence　they　contain　no　fibres：　for　the



fibres　are　of　earth　and　solid；　and　blood　from　which　they　have　been



removed　does　not　solidify。　This　is　because　it　cannot　dry；　for　what



remains　is　water；　just　as　what　remains　of　milk　when　cheese　has　been



removed　is　water。　The　fact　that　diseased　blood　will　not　solidify　is



evidence　of　the　same　thing；　for　such　blood　is　of　the　nature　of　serum



and　that　is　phlegm　and　water；　the　nature　of　the　animal　having　failed



to　get　the　better　of　it　and　digest　it。



　　Some　of　these　bodies　are　soluble；　e。g。　natron；　some　insoluble；



e。g。　pottery：　of　the　latter；　some；　like　horn；　can　be　softened　by　heat；



others；　like　pottery　and　stone；　cannot。　The　reason　is　that　opposite



causes　have　opposite　effects：　consequently；　if　solidification　is　due



to　two　causes；　the　cold　and　the　dry；　solution　must　be　due　to　the　hot



and　the　moist；　that　is；　to　fire　and　to　water　（these　being



opposites）：　water　dissolving　what　was　solidified　by　fire　alone；　fire



what　was　solidified　by　cold　alone。　Consequently；　if　any　things



happen　to　be　solidified　by　the　action　of　both；　these　are　least　apt



to　be　soluble。　Such　a　case　we　find　where　things　have　been　heated　and



are　then　solidified　by　cold。　When　the　heat　in　leaving　them　has



caused　most　of　the　moisture　to　evaporate；　the　cold　so　compacts　these



bodies　together　again　as　to　leave　no　entrance　even　for　moisture。



Therefore　heat　does　not　dissolve　them　（for　it　only　dissolves　those



bodies　that　are　solidified　by　cold　alone）；　nor　does　water　（for　it　does



not　dissolve　what　cold　solidifies；　but　only　what　is　solidified　by



dry　heat）。　But　iron　is　melted　by　heat　and　solidified　by　cold。　Wood



consists　of　earth　and　air　and　is　therefore　combustible　but　cannot　be



melted　or　softened　by　heat。　（For　the　same　reason　it　floats　in



water…all　except　ebony。　This　does　not；　for　other　kinds　of　wood　contain



a　preponderance　of　air；　but　in　black　ebony　the　air　has　escaped　and



so　earth　preponderates　in　it。）　Pottery　consists　of　earth　alone　because



it　solidified　gradually　in　the　process　of　drying。　Water　cannot　get



into　it；　for　the　pores　were　only　large　enough　to　admit　of　vapour



escaping：　and　seeing　that　fire　solidified　it；　that　cannot　dissolve



it　either。



　　So　solidification　and　melting；　their　causes；　and　the　kinds　of



subjects　in　which　they　occur　have　been　described。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　8







　　All　this　makes　it　clear　that　bodies　are　formed　by　heat　and　cold



and　that　these　agents　operate　by　thickening　and　solidifying。　It　is



because　these　qualities　fashion　bodies　that　we　find　heat　in　all　of



them；　and　in　some　cold　in　so　far　as　heat　is　absent。　These　qualities；



then；　are　present　as　active；　and　the