a history of science-2-第50节

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ing　the　tube　to　the　other　end；　be　electrified　so　as　to　give　a　spark；　the　fire　continually　running　out　silently　at　the　point。　In　the　dark　you　may　see　it　make　the　same　appearance　as　it　does　in　the　case　before　mentioned。〃＇3＇　Von　Guericke；　Hauksbee；　and　Gray　had　noticed　that　pointed　bodies　attracted　electricity　in　a　peculiar　manner；　but　this　demonstration　of　the　〃drawing　off〃　of　〃electrical　fire〃　was　original　with　Franklin。　Original　also　was　the　theory　that　he　now　suggested；　which　had　at　least　the　merit　of　being　thinkable　even　by　non…philosophical　minds。　It　assumes　that　electricity　is　like　a　fluid；　that　will　flow　along　conductors　and　accumulate　in　proper　receptacles；　very　much　as　ordinary　fluids　do。　This　conception　is　probably　entirely　incorrect；　but　nevertheless　it　is　likely　to　remain　a　popular　one；　at　least　outside　of　scientific　circles；　or　until　something　equally　tangible　is　substituted。

FRANKLIN'S　THEORY　OF　ELECTRICITY　According　to　Franklin's　theory；　electricity　exists　in　all　bodies　as　a　〃common　stock；〃　and　tends　to　seek　and　remain　in　a　state　of　equilibrium；　just　as　fluids　naturally　tend　to　seek　a　level。　But　it　may；　nevertheless；　be　raised　or　lowered；　and　this　equilibrium　be　thus　disturbed。　If　a　body　has　more　electricity　than　its　normal　amount　it　is　said　to　be　POSITIVELY　electrified；　but　if　it　has　less；　it　is　NEGATIVELY　electrified。　An　over…electrified　or　〃plus〃　body　tends　to　give　its　surplus　stock　to　a　body　containing　the　normal　amount；　while　the　〃minus〃　or　under…electrified　body　will　draw　electricity　from　one　containing　the　normal　amount。　Working　along　lines　suggested　by　this　theory；　Franklin　attempted　to　show　that　electricity　is　not　created　by　friction；　but　simply　collected　from　its　diversified　state；　the　rubbed　glass　globe　attracting　a　certain　quantity　of　〃electrical　fire；〃　but　ever　ready　to　give　it　up　to　any　body　that　has　less。　He　explained　the　charged　Leyden　jar　by　showing　that　the　inner　coating　of　tin…foil　received　more　than　the　ordinary　quantity　of　electricity；　and　in　consequence　is　POSITIVELY　electrified；　while　the　outer　coating；　having　the　ordinary　quantity　of　electricity　diminished；　is　electrified　NEGATIVELY。　These　studies　of　the　Leyden　jar；　and　the　studies　of　pieces　of　glass　coated　with　sheet　metal；　led　Franklin　to　invent　his　battery；　constructed　of　eleven　large　glass　plates　coated　with　sheets　of　lead。　With　this　machine；　after　overcoming　some　defects；　he　was　able　to　produce　electrical　manifestations　of　great　forcea　force　that　〃knew　no　bounds；〃　as　he　declared　（〃except　in　the　matter　of　expense　and　of　labor〃）；　and　which　could　be　made　to　exceed　〃the　greatest　know　effects　of　common　lightning。〃　This　reference　to　lightning　would　seem　to　show　Franklin's　belief；　even　at　that　time；　that　lightning　is　electricity。　Many　eminent　observers；　such　as　Hauksbee；　Wall；　Gray；　and　Nollet；　had　noticed　the　resemblance　between　electric　sparks　and　lightning；　but　none　of　these　had　more　than　surmised　that　the　two　might　be　identical。　In　1746；　the　surgeon；　John　Freke；　also　asserted　his　belief　in　this　identity。　Winkler；　shortly　after　this　time；　expressed　the　same　belief；　and；　assuming　that　they　were　the　same；　declared　that　〃there　is　no　proof　that　they　are　of　different　natures〃；　and　still　he　did　not　prove　that　they　were　the　same　nature。

FRANKLIN　INVENTS　THE　LIGHTNING…ROD　Even　before　Franklin　proved　conclusively　the　nature　of　lightning；　his　experiments　in　drawing　off　the　electric　charge　with　points　led　to　some　practical　suggestions　which　resulted　in　the　invention　of　the　lightning…rod。　In　the　letter　of　July；　1750；　which　he　wrote　on　the　subject；　he　gave　careful　instructions　as　to　the　way　in　which　these　rods　might　be　constructed。　In　part　Franklin　wrote：　〃May　not　the　knowledge　of　this　power　of　points　be　of　use　to　mankind　in　preserving　houses；　churches；　ships；　etc。；　from　the　stroke　of　lightning　by　directing　us　to　fix　on　the　highest　parts　of　the　edifices　upright　rods　of　iron　made　sharp　as　a　needle；　and　gilt　to　prevent　rusting；　and　from　the　foot　of　these　rods　a　wire　down　the　outside　of　the　building　into　the　grounds；　or　down　round　one　of　the　shrouds　of　a　ship　and　down　her　side　till　it　reaches　the　water？　Would　not　these　pointed　rods　probably　draw　the　electrical　fire　silently　out　of　a　cloud　before　it　came　nigh　enough　to　strike；　and　thereby　secure　us　from　that　most　sudden　and　terrible　mischief？　〃To　determine　this　question；　whether　the　clouds　that　contain　the　lightning　are　electrified　or　not；　I　propose　an　experiment　to　be　tried　where　it　may　be　done　conveniently。　On　the　top　of　some　high　tower　or　steeple；　place　a　kind　of　sentry…box；　big　enough　to　contain　a　man　and　an　electrical　stand。　From　the　middle　of　the　stand　let　an　iron　rod　rise　and　pass；　bending　out　of　the　door；　and　then　upright　twenty　or　thirty　feet；　pointed　very　sharp　at　the　end。　If　the　electrical　stand　be　kept　clean　and　dry；　a　man　standing　on　it　when　such　clouds　are　passing　low　might　be　electrified　and　afford　sparks；　the　rod　drawing　fire　to　him　from　a　cloud。　If　any　danger　to　the　man　be　apprehended　（though　I　think　there　would　be　none）；　let　him　stand　on　the　floor　of　his　box　and　now　and　then　bring　near　to　the　rod　the　loop　of　a　wire　that　has　one　end　fastened　to　the　leads；　he　holding　it　by　a　wax　handle；　so　the　sparks；　if　the　rod　is　electrified；　will　strike　from　the　rod　to　the　wire　and　not　effect　him。〃＇4＇　Not　satisfied　with　all　the　evidence　that　he　had　collected　pointing　to　the　identity　of　lightning　and　electricity；　he　adds　one　more　striking　and　very　suggestive　piece　of　evidence。　Lightning　was　known　sometimes　to　strike　persons　blind　without　killing　them。　In　experimenting　on　pigeons　and　pullets　with　his　electrical　machine；　Franklin　found　that　a　fowl；　when　not　killed　outright；　was　sometimes　rendered　blind。　The　report　of　these　experiments　were　incorporated　in　this　famous　letter　of　the　Philadelphia　philosopher。　The　attitude　of　the　Royal　Society　towards　this　clearly　stated　letter；　with　its　useful　suggestions；　must　always　remain　as　a　blot　on　the　record　of　this　usually　very　receptive　and　liberal…minded　body。　Far　from　publishing　it　or　receiving　it　at　all；　they　derided　the　whole　matter　as　too　visionary　for　discussion　by　the　society。　How　was　it　possible　that　any　great　scientific　discovery　could　be　made　by　a　self…educated　colonial　newspaper　editor；　who　knew　nothing　of　European　science　except　by　hearsay；　when　all　the　great　scientific　minds　of　Europe　had　failed　to　make　the　discovery？　How　indeed！　And　yet　it　would　seem　that　if　any　of　the　influential　members　of　the　learned　society　had　taken　the　trouble　to　read　over　Franklin's　clearly　stated　letter；　they　could　hardly　have　failed　to　see　that　his　suggestions　were　worthy　of　consideration。　But　at　all　events；　whether　they　did　or　did　not　matters　little。　The　fact　remains　that　they　refused　to　consider　the　paper　seriously　at　the　time；　and　later　on；　when　its　true　value　became　known；　were　obliged　to　acknowledge　their　error　by　a　tardy　report　on　the　already　well…known　document。　But　if　English　scientists　were　cold　in　their　reception　of　Franklin's　theory　and　suggestions；　the　French　scientists　were　not。　Buffon；　perceiving　at　once　the　importance　of　some　of　Franklin's　experiments；　took　steps　to　have　the　famous　letter　translated　into　French；　and　soon　not　only　the　savants；　but　members　of　the　court　and　the　king　himself　were　intensely　interested。　Two　scientists；　De　Lor　and　D'Alibard；　undertook　to　test　the　truth　of　Franklin's　suggestions　as　to　pointed　rods　〃drawing　off　lightning。〃　In　a　garden　near　Paris；　the　latter　erected　a　pointed　iron　rod　fifty　feet　high　and　an　inch　in　diameter。　As　no　thunder…clouds　appeared　for　several　days；　a　guard　was　stationed；　armed　with　an　insulated　brass　wire；　who　was　directed　to　test　the　iron　rods　with　it　in　case　a　storm　came　on　during　D'Alibard's　absence。　The　storm　did　come　on；　and　the　guard；　not　waiting　for　his　employer's　arrival；　seized　the　wire　and　touched　the　rod。　Instantly　there　was　a　report。　Sparks　flew　and　the　guard　received　such　a　shock　that　he　thought　his　time　had　come。　Believing　from　his　outcry　that　he　was　mortally　hurt；　his　friends　rushed　for　a　spiritual　adviser；　who　came　running　through　rain　and　hail　to　administer　the　last　rites；　but　when　he　found　the　guard　still　alive　and　uninjured；　he　turned　his　visit　to　account　by　testing　the　rod　himself　several　times；　and　later　writing　a　report　of　his　experiments　to　M。　d'Alibard。　This　scientist　at　once　reported　the　affair　to　the　French　Academy；　remarking　that　〃Franklin's　idea　was　no　longer　a　conjecture；　but　a　reality。〃

FRANKLIN　PROVES　THAT　LIGHTNING　IS　ELECTRICITY　Europe；　hitherto　somewhat　sceptical　of　Franklin's　views；　was　by　this　time　convinced　of　the　identity　of　lightning　and　electricity。　It　was　now　Franklin's　turn　to　be　sceptical。　To　him　the　fact　that　a　rod；　one　hundred　feet　high；　became　electrified　during　a　storm　did　not　necessarily　prove　that　the　storm…clouds　were　electrified。　A　rod　of　that　length　was　not　really　projected　into　the　cloud；　for　even　a　very　low　thunder…cloud　was　more　than　a　hundred　feet　above　the　ground。　Irrefutable　proof　could　only　be　had；　as　he　saw　it；　by　〃extracting〃　the　lightning　with　something　actually　sent　up　into　the　storm…cloud；　and　to　accomplish　this　Franklin　made　his　silk　kite；　with　which　he　finally　demonstrated　to　his　own　and　the　world's　satisfaction　that　his　theory　was　correct。　Taking　his　kit