a history of science-4-第4部分

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nonconformist　academy　at　Warrington；　and　here；　for　six　years；　he　was　able　to　pursue　his　studies　in　chemistry　and　electricity。　In　1766；　while　on　a　visit　to　London；　he　met　Benjamin　Franklin；　at　whose　suggestion　he　published　his　History　of　Electricity。　　From　this　time　on　he　made　steady　progress　in　scientific　investigations；　keeping　up　his　ecclesiastical　duties　at　the　same　time。　In　1780　he　removed　to　Birmingham；　having　there　for　associates　such　scientists　as　James　Watt；　Boulton；　and　Erasmus　Darwin。

Eleven　years　later；　on　the　anniversary　of　the　fall　of　the　Bastile　in　Paris；　a　fanatical　mob；　knowing　Priestley's　sympathies　with　the　French　revolutionists；　attacked　his　house　and　chapel；　burning　both　and　destroying　a　great　number　of　valuable　papers　and　scientific　instruments。　Priestley　and　his　family　escaped　violence　by　flight；　but　his　most　cherished　possessions　were　destroyed；　and　three　years　later　he　quitted　England　forever；　removing　to　the　United　States；　whose　struggle　for　liberty　he　had　championed。　The　last　ten　years　of　his　life　were　spent　at　Northumberland；　Pennsylvania；　where　he　continued　his　scientific　researches。

Early　in　his　scientific　career　Priestley　began　investigations　upon　the　〃fixed　air〃　of　Dr。　Black；　and；　oddly　enough；　he　was　stimulated　to　this　by　the　same　thing　that　had　influenced　Blackthat　is；　his　residence　in　the　immediate　neighborhood　of　a　brewery。　It　was　during　the　course　of　a　series　of　experiments　on　this　and　other　gases　that　he　made　his　greatest　discovery；　that　of　oxygen；　or　〃dephlogisticated　air；〃　as　he　called　it。　The　story　of　this　important　discovery　is　probably　best　told　in　Priestley's　own　words：

〃There　are；　I　believe；　very　few　maxims　in　philosophy　that　have　laid　firmer　hold　upon　the　mind　than　that　air；　meaning　atmospheric　air；　is　a　simple　elementary　substance；　indestructible　and　unalterable；　at　least　as　much　so　as　water　is　supposed　to　be。　　In　the　course　of　my　inquiries　I　was；　however；　soon　satisfied　that　atmospheric　air　is　not　an　unalterable　thing；　for　that；　according　to　my　first　hypothesis；　the　phlogiston　with　which　it　becomes　loaded　from　bodies　burning　in　it；　and　the　animals　breathing　it；　and　various　other　chemical　processes；　so　far　alters　and　depraves　it　as　to　render　it　altogether　unfit　for　inflammation；　respiration；　and　other　purposes　to　which　it　is　subservient；　and　I　had　discovered　that　agitation　in　the　water；　the　process　of　vegetation；　and　probably　other　natural　processes；　restore　it　to　its　original　purity。。。。

〃Having　procured　a　lens　of　twelve　inches　diameter　and　twenty　inches　local　distance；　I　proceeded　with　the　greatest　alacrity；　by　the　help　of　it；　to　discover　what　kind　of　air　a　great　variety　of　substances　would　yield；　putting　them　into　the　vessel；　which　I　filled　with　quicksilver；　and　kept　inverted　in　a　basin　of　the　same　。。。。　With　this　apparatus；　after　a　variety　of　experiments　。。。。　on　the　1st　of　August；　1774；　I　endeavored　to　extract　air　from　mercurius　calcinatus　per　se；　and　I　presently　found　that；　by　means　of　this　lens；　air　was　expelled　from　it　very　readily。　Having　got　about　three　or　four　times　as　much　as　the　bulk　of　my　materials；　I　admitted　water　to　it；　and　found　that　it　was　not　imbibed　by　it。　But　what　surprised　me　more　than　I　can　express　was　that　a　candle　burned　in　this　air　with　a　remarkably　vigorous　flame；　very　much　like　that　enlarged　flame　with　which　a　candle　burns　in　nitrous　oxide；　exposed　to　iron　or　liver　of　sulphur；　but　as　I　had　got　nothing　like　this　remarkable　appearance　from　any　kind　of　air　besides　this　particular　modification　of　vitrous　air；　and　I　knew　no　vitrous　acid　was　used　in　the　preparation　of　mercurius　calcinatus；　I　was　utterly　at　a　loss　to　account　for　it。〃＇4＇


The　〃new　air〃　was；　of　course；　oxygen。　　Priestley　at　once　proceeded　to　examine　it　by　a　long　series　of　careful　experiments；　in　which；　as　will　be　seen；　he　discovered　most　of　the　remarkable　qualities　of　this　gas。　Continuing　his　description　of　these　experiments；　he　says：

〃The　flame　of　the　candle；　besides　being　larger；　burned　with　more　splendor　and　heat　than　in　that　species　of　nitrous　air；　and　a　piece　of　red…hot　wood　sparkled　in　it；　exactly　like　paper　dipped　in　a　solution　of　nitre；　and　it　consumed　very　fast；　an　experiment　that　I　had　never　thought　of　trying　with　dephlogisticated　nitrous　air。

〃。　。　。　I　had　so　little　suspicion　of　the　air　from　the　mercurius　calcinatus；　etc。；　being　wholesome；　that　I　had　not　even　thought　of　applying　it　to　the　test　of　nitrous　air；　but　thinking　（as　my　reader　must　imagine　I　frequently　must　have　done）　on　the　candle　burning　in　it　after　long　agitation　in　water；　it　occurred　to　me　at　last　to　make　the　experiment；　and；　putting　one　measure　of　nitrous　air　to　two　measures　of　this　air；　I　found　not　only　that　it　was　diminished；　but　that　it　was　diminished　quite　as　much　as　common　air；　and　that　the　redness　of　the　mixture　was　likewise　equal　to　a　similar　mixture　of　nitrous　and　common　air。。。。　The　next　day　I　was　more　surprised　than　ever　I　had　been　before　with　finding　that；　after　the　above…mentioned　mixture　of　nitrous　air　and　the　air　from　mercurius　calcinatus　had　stood　all　night；　。　。　。　a　candle　burned　in　it；　even　better　than　in　common　air。〃

A　little　later　Priestley　discovered　that　〃dephlogisticated　air　。　。　。　is　a　principal　element　in　the　composition　of　acids；　and　may　be　extracted　by　means　of　heat　from　many　substances　which　contain　them。。。。　It　is　likewise　produced　by　the　action　of　light　upon　green　vegetables；　and　this　seems　to　be　the　chief　means　employed　to　preserve　the　purity　of　the　atmosphere。〃

This　recognition　of　the　important　part　played　by　oxygen　in　the　atmosphere　led　Priestley　to　make　some　experiments　upon　mice　and　insects；　and　finally　upon　himself；　by　inhalations　of　the　pure　gas。　　〃The　feeling　in　my　lungs；〃　he　said；　〃was　not　sensibly　different　from　that　of　common　air；　but　I　fancied　that　my　breathing　felt　peculiarly　light　and　easy　for　some　time　afterwards。　Who　can　tell　but　that　in　time　this　pure　air　may　become　a　fashionable　article　in　luxury？　。　。　。　Perhaps　we　may　from　these　experiments　see　that　though　pure　dephlogisticated　air　might　be　useful　as　a　medicine；　it　might　not　be　so　proper　for　us　in　the　usual　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；　h