meteorology-第29章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！




that　though　copper　can　be　melted　the　agent　in　its　case　is　not　water；



but　some　of　the　bodies　that　can　be　melted　by　water　too　such　as



natron　and　salt　cannot　be　softened　in　water：　for　nothing　is　said　to　be



so　affected　unless　the　water　soaks　into　it　and　makes　it　softer。）



Some　things；　on　the　other　hand；　such　as　wool　and　grain；　can　be



softened　by　water　though　they　cannot　be　melted。　Any　body　that　is　to　be



softened　by　water　must　be　of　earth　and　must　have　its　pores　larger　than



the　particles　of　water；　and　the　pores　themselves　must　be　able　to



resist　the　action　of　water；　whereas　bodies　that　can　be　'melted'　by



water　must　have　pores　throughout。



　　（Why　is　it　that　earth　is　both　'melted'　and　softened　by　moisture；



while　natron　is　'melted'　but　not　softened？　Because　natron　is



pervaded　throughout　by　pores　so　that　the　parts　are　immediately　divided



by　the　water；　but　earth　has　also　pores　which　do　not　connect　and　is



therefore　differently　affected　according　as　the　water　enters　by　one　or



the　other　set　of　pores。）



　　Some　bodies　can　be　bent　or　straightened；　like　the　reed　or　the　withy；



some　cannot；　like　pottery　and　stone。　Those　bodies　are　apt　to　be　bent



and　straightened　which　can　change　from　being　curved　to　being



straight　and　from　being　straight　to　being　curved；　and　bending　and



straightening　consist　in　the　change　or　motion　to　the　straight　or　to



a　curve；　for　a　thing　is　said　to　be　in　process　of　being　bent　whether　it



is　being　made　to　assume　a　convex　or　a　concave　shape。　So　bending　is



defined　as　motion　to　the　convex　or　the　concave　without　a　change　of



length。　For　if　we　added　'or　to　the　straight'；　we　should　have　a　thing



bent　and　straight　at　once；　and　it　is　impossible　for　that　which　is



straight　to　be　bent。　And　if　all　bending　is　a　bending　back　or　a　bending



down；　the　former　being　a　change　to　the　convex；　the　latter　to　the



concave；　a　motion　that　leads　to　the　straight　cannot　be　called　bending；



but　bending　and　straightening　are　two　different　things。　These；　then；



are　the　things　that　can；　and　those　that　cannot　be　bent；　and　be



straightened。



　　Some　things　can　be　both　broken　and　comminuted；　others　admit　only　one



or　the　other。　Wood；　for　instance；　can　be　broken　but　not　comminuted；



ice　and　stone　can　be　comminuted　but　not　broken；　while　pottery　may



either　be　comminuted　or　broken。　The　distinction　is　this：　breaking　is　a



division　and　separation　into　large　parts；　comminution　into　parts　of



any　size；　but　there　must　be　more　of　them　than　two。　Now　those　solids



that　have　many　pores　not　communicating　with　one　another　are



comminuible　（for　the　limit　to　their　subdivision　is　set　by　the



pores）；　but　those　whose　pores　stretch　continuously　for　a　long　way



are　breakable；　while　those　which　have　pores　of　both　kinds　are　both



comminuible　and　breakable。



　　Some　things；　e。g。　copper　and　wax；　are　impressible；　others；　e。g。



pottery　and　water；　are　not。　The　process　of　being　impressed　is　the



sinking　of　a　part　of　the　surface　of　a　thing　in　response　to　pressure　or



a　blow；　in　general　to　contact。　Such　bodies　are　either　soft；　like



wax；　where　part　of　the　surface　is　depressed　while　the　rest　remains；　or



hard；　like　copper。　Non…impressible　bodies　are　either　hard；　like



pottery　（its　surface　does　not　give　way　and　sink　in）；　or　liquid；　like



water　（for　though　water　does　give　way　it　is　not　in　a　part　of　it；　for



there　is　a　reciprocal　change　of　place　of　all　its　parts）。　Those



impressibles　that　retain　the　shape　impressed　on　them　and　are　easily



moulded　by　the　hand　are　called　'plastic'；　those　that　are　not　easily



moulded；　such　as　stone　or　wood；　or　are　easily　moulded　but　do　not



retain　the　shape　impressed；　like　wool　or　a　sponge；　are　not　plastic。



The　last　group　are　said　to　be　'squeezable'。　Things　are　'squeezable'



when　they　can　contract　into　themselves　under　pressure；　their　surface



sinking　in　without　being　broken　and　without　the　parts　interchanging



position　as　happens　in　the　case　of　water。　（We　speak　of　pressure　when



there　is　movement　and　the　motor　remains　in　contact　with　the　thing



moved；　of　impact　when　the　movement　is　due　to　the　local　movement　of　the



motor。）　Those　bodies　are　subject　to　squeezing　which　have　empty



pores…empty；　that　is；　of　the　stuff　of　which　the　body　itself



consists…and　that　can　sink　upon　the　void　spaces　within　them；　or　rather



upon　their　pores。　For　sometimes　the　pores　upon　which　a　body　sinks　in



are　not　empty　（a　wet　sponge；　for　instance；　has　its　pores　full）。　But



the　pores；　if　full；　must　be　full　of　something　softer　than　the　body



itself　which　is　to　contract。　Examples　of　things　squeezable　are　the



sponge；　wax；　flesh。　Those　things　are　not　squeezable　which　cannot　be



made　to　contract　upon　their　own　pores　by　pressure；　either　because　they



have　no　pores　or　because　their　pores　are　full　of　something　too　hard。



Thus　iron；　stone；　water　and　all　liquids　are　incapable　of　being



squeezed。



　　Things　are　tractile　when　their　surface　can　be　made　to　elongate；



for　being　drawn　out　is　a　movement　of　the　surface；　remaining



unbroken；　in　the　direction　of　the　mover。　Some　things　are　tractile；



e。g。　hair；　thongs；　sinew；　dough；　birdlime；　and　some　are　not；　e。g。



water；　stone。　Some　things　are　both　tractile　and　squeezable；　e。g。　wool；



in　other　cases　the　two　qualities　do　not　coincide；　phlegm；　for



instance；　is　tractile　but　not　squeezable；　and　a　sponge　squeezable



but　not　tractile。



　　Some　things　are　malleable；　like　copper。　Some　are　not；　like　stone　and



wood。　Things　are　malleable　when　their　surface　can　be　made　to　move　（but



only　in　part）　both　downwards　and　sideways　with　one　and　the　same



blow：　when　this　is　not　possible　a　body　is　not　malleable。　All　malleable



bodies　are　impressible；　but　not　all　impressible　bodies　are



malleable；　e。g。　wood；　though　on　the　whole　the　two　go　together。　Of



squeezable　things　some　are　malleable　and　some　not：　wax　and　mud　are



malleable；　wool　is　not。　Some　things　are　fissile；　e。g。　wood；　some　are



not；　e。g。　potter's　clay。　A　thing　is　fissile　when　it　is　apt　to　divide



in　advance　of　the　instrument　dividing　it；　for　a　body　is　said　to



split　when　it　divides　to　a　further　point　than　that　to　which　the



dividing　instrument　divides　it　and　the　act　of　division　advances：　which



is　not　the　case　with　cutting。　Those　bodies　which　cannot　behave　like



this　are　non…fissile。　Nothing　soft　is　fissile　（by　soft　I　mean



absolutely　soft　and　not　relatively：　for　iron　itself　may　be



relatively　soft）；　nor　are　all　hard　things　fissile；　but　only　such　as



are　neither　liquid　nor　impressible　nor　comminuible。　Such　are　the



bodies　that　have　the　pores　along　which　they　cohere　lengthwise　and



not　crosswise。



　　Those　hard　or　soft　solids　are　apt　to　be　cut　which　do　not　necessarily



either　split　in　advance　of　the　instrument　or　break　into　minute



fragments　when　they　are　being　divided。　Those　that　necessarily　do　so



and　liquids　cannot　be　cut。　Some　things　can　be　both　split　and　cut；　like



wood；　though　generally　it　is　lengthwise　that　a　thing　can　be　split



and　crosswise　that　it　can　be　cut。　For；　a　body　being　divided　into



many　parts　fin　so　far　as　its　unity　is　made　up　of　many　lengths　it　is



apt　to　be　split；　in　so　far　as　it　is　made　up　of　many　breadths　it　is　apt



to　be　cut。



　　A　thing　is　viscous　when；　being　moist　or　soft；　it　is　tractile。　Bodies



owe　this　property　to　the　interlocking　of　their　parts　when　they　are



composed　like　chains；　for　then　they　can　be　drawn　out　to　a　great　length



and　contracted　again。　Bodies　that　are　not　like　this　are　friable。



Bodies　are　compressible　when　they　are　squeezable　and　retain　the



shape　they　have　been　squeezed　into；　incompressible　when　they　are



either　inapt　to　be　squeezed　at　all　or　do　not　retain　the　shape　they



have　been　squeezed　into。



　　Some　bodies　are　combustible　and　some　are　not。　Wood；　wool；　bone　are



combustible；　stone；　ice　are　not。　Bodies　are　combustible　when　their



pores　are　such　as　to　admit　fire　and　their　longitudinal　pores　conta