ight。　Fire　on　the　other　hand　is　seen　both　in　darkness　and　in　light；　this　double　possibility　follows　necessarily　from　our　theory；　for　it　is　just　fire　that　makes　what　is　potentially　transparent　actually　transparent。　　　The　same　account　holds　also　of　sound　and　smell；　if　the　object　of　either　of　these　senses　is　in　immediate　contact　with　the　organ　no　sensation　is　produced。　In　both　cases　the　object　sets　in　movement　only　what　lies　between；　and　this　in　turn　sets　the　organ　in　movement：　if　what　sounds　or　smells　is　brought　into　immediate　contact　with　the　organ；　no　sensation　will　be　produced。　The　same；　in　spite　of　all　appearances；　applies　also　to　touch　and　taste；　why　there　is　this　apparent　difference　will　be　clear　later。　What　comes　between　in　the　case　of　sounds　is　air；　the　corresponding　medium　in　the　case　of　smell　has　no　name。　But；　corresponding　to　what　is　transparent　in　the　case　of　colour；　there　is　a　quality　found　both　in　air　and　water；　which　serves　as　a　medium　for　what　has　smell…I　say　'in　water'　because　animals　that　live　in　water　as　well　as　those　that　live　on　land　seem　to　possess　the　sense　of　smell；　and　'in　air'　because　man　and　all　other　land　animals　that　breathe；　perceive　smells　only　when　they　breathe　air　in。　The　explanation　of　this　too　will　be　given　later。
  8
  Now　let　us；　to　begin　with；　make　certain　distinctions　about　sound　and　hearing。　　　Sound　may　mean　either　of　two　things　（a）　actual；　and　（b）　potential；　sound。　There　are　certain　things　which；　as　we　say；　'have　no　sound'；　e。g。　sponges　or　wool；　others　which　have；　e。g。　bronze　and　in　general　all　things　which　are　smooth　and　solid…the　latter　are　said　to　have　a　sound　because　they　can　make　a　sound；　i。e。　can　generate　actual　sound　between　themselves　and　the　organ　of　hearing。　　　Actual　sound　requires　for　its　occurrence　（i；　ii）　two　such　bodies　and　（iii）　a　space　between　them；　for　it　is　generated　by　an　impact。　Hence　it　is　impossible　for　one　body　only　to　generate　a　sound…there　must　be　a　body　impinging　and　a　body　impinged　upon；　what　sounds　does　so　by　striking　against　something　else；　and　this　is　impossible　without　a　movement　from　place　to　place。　　　As　we　have　said；　not　all　bodies　can　by　impact　on　one　another　produce　sound；　impact　on　wool　makes　no　sound；　while　the　impact　on　bronze　or　any　body　which　is　smooth　and　hollow　does。　Bronze　gives　out　a　sound　when　struck　because　it　is　smooth；　bodies　which　are　hollow　owing　to　reflection　repeat　the　original　impact　over　and　over　again；　the　body　originally　set　in　movement　being　unable　to　escape　from　the　concavity。　　　Further；　we　must　remark　that　sound　is　heard　both　in　air　and　in　water；　though　less　distinctly　in　the　latter。　Yet　neither　air　nor　water　is　the　principal　cause　of　sound。　What　is　required　for　the　production　of　sound　is　an　impact　of　two　solids　against　one　another　and　against　the　air。　The　latter　condition　is　satisfied　when　the　air　impinged　upon　does　not　retreat　before　the　blow；　i。e。　is　not　dissipated　by　it。　　　That　is　why　it　must　be　struck　with　a　sudden　sharp　blow；　if　it　is　to　sound…the　movement　of　the　whip　must　outrun　the　dispersion　of　the　air；　just　as　one　might　get　in　a　stroke　at　a　heap　or　whirl　of　sand　as　it　was　traveling　rapidly　past。　　　An　echo　occurs；　when；　a　mass　of　air　having　been　unified；　bounded；　and　prevented　from　dissipation　by　the　containing　walls　of　a　vessel；　the　air　originally　struck　by　the　impinging　body　and　set　in　movement　by　it　rebounds　from　this　mass　of　air　like　a　ball　from　a　wall。　It　is　probable　that　in　all　generation　of　sound　echo　takes　place；　though　it　is　frequently　only　indistinctly　heard。　What　happens　here　must　be　analogous　to　what　happens　in　the　case　of　light；　light　is　always　reflected…otherwise　it　would　not　be　diffused　and　outside　what　was　directly　illuminated　by　the　sun　there　would　be　blank　darkness；　but　this　reflected　light　is　not　always　strong　enough；　as　it　is　when　it　is　reflected　from　water；　bronze；　and　other　smooth　bodies；　to　cast　a　shadow；　which　is　the　distinguishing　mark　by　which　we　recognize　light。　　　It　is　rightly　said　that　an　empty　space　plays　the　chief　part　in　the　production　of　hearing；　for　what　people　mean　by　'the　vacuum'　is　the　air；　which　is　what　causes　hearing；　when　that　air　is　set　in　movement　as　one　continuous　mass；　but　owing　to　its　friability　it　emits　no　sound；　being　dissipated　by　impinging　upon　any　surface　which　is　not　smooth。　When　the　surface　on　which　it　impinges　is　quite　smooth；　what　is　produced　by　the　original　impact　is　a　united　mass；　a　result　due　to　the　smoothness　of　the　surface　with　which　the　air　is　in　contact　at　the　other　end。　　　What　has　the　power　of　producing　sound　is　what　has　the　power　of　setting　in　movement　a　single　mass　of　air　which　is　continuous　from　the　impinging　body　up　to　the　organ　of　hearing。　The　organ　of　hearing　is　physically　united　with　air；　and　because　it　is　in　air；　the　air　inside　is　moved　concurrently　with　the　air　outside。　Hence　animals　do　not　hear　with　all　parts　of　their　bodies；　nor　do　all　parts　admit　of　the　entrance　of　air；　for　even　the　part　which　can　be　moved　and　can　sound　has　not　air　everywhere　in　it。　Air　in　itself　is；　owing　to　its　friability；　quite　soundless；　only　when　its　dissipation　is　prevented　is　its　movement　sound。　The　air　in　the　ear　is　built　into　a　chamber　just　to　prevent　this　dissipating　movement；　in　order　that　the　animal　may　accurately　apprehend　all　varieties　of　the　movements　of　the　air　outside。　That　is　why　we　hear　also　in　water；　viz。　because　the　water　cannot　get　into　the　air　chamber　or　even；　owing　to　the　spirals；　into　the　outer　ear。　If　this　does　happen；　hearing　ceases；　as　it　also　does　if　the　tympanic　membrane　is　damaged；　just　as　sight　ceases　if　the　membrane　covering　the　pupil　is　damaged。　It　is　also　a　test　of　deafness　whether　the　ear　does　or　does　not　reverberate　like　a　horn；　the　air　inside　the　ear　has　always　a　movement　of　its　own；　but　the　sound　we　hear　is　always　the　sounding　of　something　else；　not　of　the　organ　itself。　That　is　why　we　say　that　we　hear　with　what　is　empty　and　echoes；　viz。　because　what　we　hear　with　is　a　chamber　which　contains　a　bounded　mass　of　air。　　　Which　is　it　that　'sounds'；　the　striking　body　or　the　struck？　Is　not　the　answer　'it　is　both；　but　each　in　a　different　way'？　Sound　is　a　movement　of　what　can　rebound　from　a　smooth　surface　when　struck　against　it。　As　we　have　explained'　not　everything　sounds　when　it　strikes　or　is　struck；　e。g。　if　one　needle　is　struck　against　another；　neither　emits　any　sound。　In　order；　therefore；　that　sound　may　be　generated；　what　is　struck　must　be　smooth；　to　enable　the　air　to　rebound　and　be　shaken　off　from　it　in　one　piece。　　　The　distinctions　between　different　sounding　bodies　show　themselves　only　in　actual　sound；　as　without　the　help　of　light　colours　remain　invisible；　so　without　the　help　of　actual　sound　the　distinctions　between　acute　and　grave　sounds　remain　inaudible。　Acute　and　grave　are　here　metaphors；　transferred　from　their　proper　sphere；　viz。　that　of　touch；　where　they　mean　respectively　（a）　what　moves　the　sense　much　in　a　short　time；　（b）　what　moves　the　sense　little　in　a　long　time。　Not　that　what　is　sharp　really　moves　fast；　and　what　is　grave；　slowly；　but　that　the　difference　in　the　qualities　of　the　one　and　the　other　movement　is　due　to　their　respective　speeds。　There　seems　to　be　a　sort　of　parallelism　between　what　is　acute　or　grave　to　hearing　and　what　is　sharp　or　blunt　to　touch；　what　is　sharp　as　it　were　stabs；　while　what　is　blunt　pushes；　the　one　producing　its　effect　in　a　short；　the　other　in　a　long　time；　so　that　the　one　is　quick；　the　other　slow。　　　Let　the　foregoing　suffice　as　an　analysis　of　sound。　Voice　is　a　kind　of　sound　characteristic　of　what　has　soul　in　it；　nothing　that　is　without　soul　utters　voice；　it　being　only　by　a　metaphor　that　we　speak　of　the　voice　of　the　flute　or　the　lyre　or　generally　of　what　（being　without　soul）　possesses　the　power　of　producing　a　succession　of　notes　which　differ　in　length　and　pitch　and　timbre。　The　metaphor　is　based　on　the　fact　that　all　these　differences　are　found　also　in　voice。　Many　animals　are　voiceless；　e。g。　all　non…sanuineous　animals　and　among　sanguineous　animals　fish。　This　is　just　what　we　should　expect；　since　voice　is　a　certain　movement　of　air。　The　fish；　like　those　in　the　Achelous；　which　are　said　to　have　voice；　really　make　the　sounds　with　their　gills　or　some　similar　organ。　Voice　is　the　sound　made　by　an　animal；　and　that　with　a　special　organ。　As　we　saw；　everything　that　makes　a　sound　does　so　by　the　impact　of　something　（a）　against　something　else；　（b）　across　a　space；　（c）　filled　with　air；　hence　it　is　only　to　be　expected　that　no　animals　utter　voice　except　those　which　take　in　air。　Once　air　is　inbreathed；　Nature　uses　it　for　two　different　purposes；　as　the　tongue　is　used　both　for　tasting　and　for　articulating；　in　that　case　of　the　two　functions　tasting　is　necessary　for　the　animal's　existence　（hence　it　is　found　more　widely　distributed）；　while　articulate　speech　is　a　luxury　subserving　its　possessor's　well…being；　similarly　in　the　former　case　Nature　employs　the　breath　both　as　an　indispensable　means　to　the　regulation　of　the　inner　temperature　of　the　living　body　and　also　as　the　matter　of　articulate　voice；　in　the　interests　of　its　possessor's　well…being。　Why　its　former　use　is　indispensable　must　be　discussed　elsewhere。　　　The　organ　of　respiration　is　the　windpipe；　and　the　organ　to　which　this　is　related　as　means　to　end　is　the　lungs。　The　lat