a history of science-4-第4章

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　healthy　state　of　the　body。〃

This　suggestion　as　to　the　possible　usefulness　of　oxygen　as　a　medicine　was　prophetic。　　A　century　later　the　use　of　oxygen　had　become　a　matter　of　routine　practice　with　many　physicians。　Even　in　Priestley's　own　time　such　men　as　Dr。　John　Hunter　expressed　their　belief　in　its　efficacy　in　certain　conditions；　as　we　shall　see；　but　its　value　in　medicine　was　not　fully　appreciated　until　several　generations　later。

Several　years　after　discovering　oxygen　Priestley　thus　summarized　its　properties：　　〃It　is　this　ingredient　in　the　atmospheric　air　that　enables　it　to　support　combustion　and　animal　life。　By　means　of　it　most　intense　heat　may　be　produced；　and　in　the　purest　of　it　animals　will　live　nearly　five　times　as　long　as　in　an　equal　quantity　of　atmospheric　air。　　In　respiration；　part　of　this　air；　passing　the　membranes　of　the　lungs；　unites　with　the　blood　and　imparts　to　it　its　florid　color；　while　the　remainder；　uniting　with　phlogiston　exhaled　from　venous　blood；　forms　mixed　air。　It　is　dephlogisticated　air　combined　with　water　that　enables　fishes　to　live　in　it。〃＇5＇


KARL　WILHELM　SCHEELE

The　discovery　of　oxygen　was　the　last　but　most　important　blow　to　the　tottering　phlogiston　theory；　though　Priestley　himself　would　not　admit　it。　But　before　considering　the　final　steps　in　the　overthrow　of　Stahl's　famous　theory　and　the　establishment　of　modern　chemistry；　we　must　review　the　work　of　another　great　chemist；　Karl　Wilhelm　Scheele　（1742…1786）；　of　Sweden；　who　discovered　oxygen　quite　independently；　although　later　than　Priestley。　　In　the　matter　of　brilliant　discoveries　in　a　brief　space　of　time　Scheele　probably　eclipsed　all　his　great　contemporaries。　He　had　a　veritable　genius　for　interpreting　chemical　reactions　and　discovering　new　substances；　in　this　respect　rivalling　Priestley　himself。　Unlike　Priestley；　however；　he　planned　all　his　experiments　along　the　lines　of　definite　theories　from　the　beginning；　the　results　obtained　being　the　logical　outcome　of　a　predetermined　plan。

Scheele　was　the　son　of　a　merchant　of　Stralsund；　Pomerania；　which　then　belonged　to　Sweden。　　As　a　boy　in　school　he　showed　so　little　aptitude　for　the　study　of　languages　that　he　was　apprenticed　to　an　apothecary　at　the　age　of　fourteen。　　In　this　work　he　became　at　once　greatly　interested；　and；　when　not　attending　to　his　duties　in　the　dispensary；　he　was　busy　day　and　night　making　experiments　or　studying　books　on　chemistry。　In　1775；　still　employed　as　an　apothecary；　he　moved　to　Stockholm；　and　soon　after　he　sent　to　Bergman；　the　leading　chemist　of　Sweden；　his　first　discoverythat　of　tartaric　acid；　which　he　had　isolated　from　cream　of　tartar。　This　was　the　beginning　of　his　career　of　discovery；　and　from　that　time　on　until　his　death　he　sent　forth　accounts　of　new　discoveries　almost　uninterruptedly。　Meanwhile　he　was　performing　the　duties　of　an　ordinary　apothecary；　and　struggling　against　poverty。　　His　treatise　upon　Air　and　Fire　appeared　in　1777。　　In　this　remarkable　book　he　tells　of　his　discovery　of　oxygen〃empyreal〃　or　〃fire…air；〃　as　he　calls　itwhich　he　seems　to　have　made　independently　and　without　ever　having　heard　of　the　previous　discovery　by　Priestley。　　In　this　book；　also；　he　shows　that　air　is　composed　chiefly　of　oxygen　and　nitrogen　gas。

Early　in　his　experimental　career　Scheele　undertook　the　solution　of　the　composition　of　black　oxide　of　manganese；　a　substance　that　had　long　puzzled　the　chemists。　　He　not　only　succeeded　in　this；　but　incidentally　in　the　course　of　this　series　of　experiments　he　discovered　oxygen；　baryta；　and　chlorine；　the　last　of　far　greater　importance；　at　least　commercially；　than　the　real　object　of　his　search。　　In　speaking　of　the　experiment　in　which　the　discovery　was　made　he　says：

〃When　marine　（hydrochloric）　acid　stood　over　manganese　in　the　cold　it　acquired　a　dark　reddish…brown　color。　As　manganese　does　not　give　any　colorless　solution　without　uniting　with　phlogiston　＇probably　meaning　hydrogen＇；　it　follows　that　marine　acid　can　dissolve　it　without　this　principle。　But　such　a　solution　has　a　blue　or　red　color。　　The　color　is　here　more　brown　than　red；　the　reason　being　that　the　very　finest　portions　of　the　manganese；　which　do　not　sink　so　easily；　swim　in　the　red　solution；　for　without　these　fine　particles　the　solution　is　red；　and　red　mixed　with　black　is　brown。　The　manganese　has　here　attached　itself　so　loosely　to　acidum　salis　that　the　water　can　precipitate　it；　and　this　precipitate　behaves　like　ordinary　manganese。　　When；　now；　the　mixture　of　manganese　and　spiritus　salis　was　set　to　digest；　there　arose　an　effervescence　and　smell　of　aqua　regis。〃＇6＇

The　〃effervescence〃　he　refers　to　was　chlorine；　which　he　proceeded　to　confine　in　a　suitable　vessel　and　examine　more　fully。　　He　described　it　as　having　a　〃quite　characteristically　suffocating　smell；〃　which　was　very　offensive。　He　very　soon　noted　the　decolorizing　or　bleaching　effects　of　this　now　product；　finding　that　it　decolorized　flowers；　vegetables；　and　many　other　substances。

Commercially　this　discovery　of　chlorine　was　of　enormous　importance；　and　the　practical　application　of　this　new　chemical　in　bleaching　cloth　soon　supplanted　the；　old　process　of　croftingthat　is；　bleaching　by　spreading　the　cloth　upon　the　grass。　But　although　Scheele　first　pointed　out　the　bleaching　quality　of　his　newly　discovered　gas；　it　was　the　French　savant；　Berthollet；　who；　acting　upon　Scheele's　discovery　that　the　new　gas　would　decolorize　vegetables　and　flowers；　was　led　to　suspect　that　this　property　might　be　turned　to　account　in　destroying　the　color　of　cloth。　In　1785　he　read　a　paper　before　the　Academy　of　Sciences　of　Paris；　in　which　he　showed　that　bleaching　by　chlorine　was　entirely　satisfactory；　the　color　but　not　the　substance　of　the　cloth　being　affected。　He　had　experimented　previously　and　found　that　the　chlorine　gas　was　soluble　in　water　and　could　thus　be　made　practically　available　for　bleaching　purposes。　　In　1786　James　Watt　examined　specimens　of　the　bleached　cloth　made　by　Berthollet；　and　upon　his　return　to　England　first　instituted　the　process　of　practical　bleaching。　His　process；　however；　was　not　entirely　satisfactory；　and；　after　undergoing　various　modifications　and　improvements；　it　was　finally　made　thoroughly　practicable　by　Mr。　Tennant；　who　hit　upon　a　compound　of　chlorine　and　limethe　chloride　of　limewhich　was　a　comparatively　cheap　chemical　product；　and　answered　the　purpose　better　even　than　chlorine　itself。

To　appreciate　how　momentous　this　discovery　was　to　cloth　manufacturers；　it　should　be　remembered　that　the　old　process　of　bleaching　consumed　an　entire　summer　for　the　whitening　of　a　single　piece　of　linen；　the　new　process　reduced　the　period　to　a　few　hours。　　To　be　sure；　lime　had　been　used　with　fair　success　previous　to　Tennant's　discovery；　but　successful　and　practical　bleaching　by　a　solution　of　chloride　of　lime　was　first　made　possible　by　him　and　through　Scheele's　discovery　of　chlorine。

Until　the　time　of　Scheele　the　great　subject　of　organic　chemistry　had　remained　practically　unexplored；　but　under　the　touch　of　his　marvellous　inventive　genius　new　methods　of　isolating　and　studying　animal　and　vegetable　products　were　introduced；　and　a　large　number　of　acids　and　other　organic　compounds　prepared　that　had　been　hitherto　unknown。　　His　explanations　of　chemical　phenomena　were　based　on　the　phlogiston　theory；　in　which；　like　Priestley；　he　always；　believed。　　Although　in　error　in　this　respect；　he　was；　nevertheless；　able　to　make　his　discoveries　with　extremely　accurate　interpretations。　A　brief　epitome　of　the　list　of　some　of　his　more　important　discoveries　conveys　some　idea；　of　his　fertility　of　mind　as　well　as　his　industry。　　In　1780　he　discovered　lactic　acid；＇7＇　and　showed　that　it　was　the　substance　that　caused　the　acidity　of　sour　milk；　and　in　the　same　year　he　discovered　mucic　acid。　Next　followed　the　discovery　of　tungstic　acid；　and　in　1783　he　added　to　his　list　of　useful　discoveries　that　of　glycerine。　Then　in　rapid　succession　came　his　announcements　of　the　new　vegetable　products　citric；　malic；　oxalic；　and　gallic　acids。　Scheele　not　only　made　the　discoveries；　but　told　the　world　how　he　had　made　themhow　any　chemist　might　have　made　them　if　he　chosefor　he　never　considered　that　he　had　really　discovered　any　substance　until　he　had　made　it；　decomposed　it；　and　made　it　again。

His　experiments　on　Prussian　blue　are　most　interesting；　not　only　because　of　the　enormous　amount　of　work　involved　and　the　skill　he　displayed　in　his　experiments；　but　because　all　the　time　the　chemist　was　handling；　smelling；　and　even　tasting　a　compound　of　one　of　the　most　deadly　poisons；　ignorant　of　the　fact　that　the　substance　was　a　dangerous　one　to　handle。　His　escape　from　injury　seems　almost　miraculous；　for　his　experiments；　which　were　most　elaborate；　extended　over　a　considerable　period　of　time；　during　which　he　seems　to　have　handled　this　chemical　with　impunity。

While　only　forty　years　of　age　and　just　at　the　zenith　of　his　fame；　Scheele　was　stricken　by　a　fatal　illness；　probably　induced　by　his　ceaseless　labor　and　exposure。　　It　is　gratifying　to　know；　however；　that　during　the　last　eight　or　nine　years　of　his　life　he　had　been　less　bound　down　by　pecuniary　difficulties　than　before；　as　Bergman　had　obtained　for　him　an　annual　grant　from　the　Academy。　　But　it　was　characteristic　of　the　man　that；　while　devoting　one…sixth　of　the　amount　of　this　grant　to　his　personal　wants；　the　remaining　five…sixths　was　devoted　to　the　expense　of　his　experiments。


LAVOISIER　AND　THE　FOUNDATION　OF　MODERN　CHEMISTRY

The　time　was　ripe　for　formulating　the　correct　theory　of　chemical　composition：　it　needed　but　the　master　hand　to　mould　the　materials　into　the　proper　shape。　The　discoveries　in　chemistry　during　the　eighteenth　century　had　been　far…reaching　and　revolutionary　in　character。　　A　brief　review　of　these　discoveries　shows　how　completely　they　had　subverted　the　old　ideas　of　chemical　elements　and　chemical　compounds。　　Of　the　four　substances　earth；　air；　fire；　and　water；　for　many　centuries　believed　to　be　elementary　bodies；　not　one　has　stood　the　test　of　the　eighteenth…century　chemists。　Earth　had　long　since　ceased　to　be　regarded　as　an　element；　and　water　and　air　had　suffered　the　same　fate　in　this　century。　　And　now　at　last　fire　itself；　the　last　of　the　four　〃elements〃　and　the　keystone　to　the　phlogiston　arch；　was　shown　to　be　nothing　more　than　one　of　the　manifestations　of　the　new　element；　oxygen；　and　not　〃phlogiston〃　or　any　other　intangible　substance。

In　this　epoch　of　chemical　discoveries　England　had　produced　such　mental　giants　and　pioneers　in　science　as　Black；　Priestley；　and　Cavendish；　Sweden　had　given　the　world　Scheele　and　Bergman；　whose　work；　added　to　that　of　their　English　confreres；　had　laid　the　broad　base　of　chemistry　as　a　science；　but　it　was　for　France　to　produce　a　man　who　gave　the　final　touches　to　the　broad　but　rough　workmanship　of　its　foundation；　and　establish　it　as　the　science　of　modern　chemistry。　　It　was　for　Antoine　Laurent　Lavoisier　（1743…1794）　to　gather　together；　interpret　correctly；　rename；　and　classify　the　wealth　of　facts　that　his　immediate　predecessors　and　contemporaries　had　given　to　the　world。

The　attitude　of　the　mother…countries　towards　these　illustrious　sons　is　an　interesting　piece　of　history。　　Sweden　honored　and　rewarded　Scheele　and　Bergman　for　their　efforts；　England　received　the　intellectuality　of　Cavendish　with　less　appreciation　than　the　Continent；　and　a　fanatical　mob　drove　Pri