a history of science-2-第38章

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ed　that　the　success　we　had　before　depended　upon　the　lines　that　supported　the　line　of　communication　being　silk；　and　not　upon　their　being　small；　as　before　trial　I　had　imagined　it　might　be；　the　same　effect　happening　here　as　it　did　when　the　line　that　is　to　convey　the　electric　virtue　is　supported　by　pack…thread。〃　Soon　after　this　Gray　and　his　host　suspended　a　pack…thread　six　hundred　and　sixty…six　feet　long　on　poles　across　a　field；　these　poles　being　slightly　inclined　so　that　the　thread　could　be　suspended　from　the　top　by　small　silk　cords；　thus　securing　the　necessary　insulation。　This　pack…thread　line；　suspended　upon　poles　along　which　Gray　was　able　to　transmit　the　electricity；　is　very　suggestive　of　the　modern　telegraph；　but　the　idea　of　signalling　or　making　use　of　it　for　communicating　in　any　way　seems　not　to　have　occurred　to　any　one　at　that　time。　Even　the　successors　of　Gray　who　constructed　lines　some　thousands　of　feet　long　made　no　attempt　to　use　them　for　anything　but　experimental　purposessimply　to　test　the　distances　that　the　current　could　be　sent。　Nevertheless；　Gray　should　probably　be　credited　with　the　discovery　of　two　of　the　most　important　properties　of　electricitythat　it　can　be　conducted　and　insulated；　although；　as　we　have　seen；　Gilbert　and　Von　Guericke　had　an　inkling　of　both　these　properties。

EXPERIMENTS　OF　CISTERNAY　DUFAY　So　far　England　had　produced　the　two　foremost　workers　in　electricity。　It　was　now　France's　turn　to　take　a　hand；　and；　through　the　efforts　of　Charles　Francois　de　Cisternay　Dufay；　to　advance　the　science　of　electricity　very　materially。　Dufay　was　a　highly　educated　savant；　who　had　been　soldier　and　diplomat　betimes；　but　whose　versatility　and　ability　as　a　scientist　is　shown　by　the　fact　that　he　was　the　only　man　who　had　ever　contributed　to　the　annals　of　the　academy　investigations　in　every　one　of　the　six　subjects　admitted　by　that　institution　as　worthy　of　recognition。　Dufay　upheld　his　reputation　in　this　new　field　of　science；　making　many　discoveries　and　correcting　many　mistakes　of　former　observers。　In　this　work　also　he　proved　himself　a　great　diplomat　by　remaining　on　terms　of　intimate　friendship　with　Dr。　Graya　thing　that　few　people　were　able　to　do。　Almost　his　first　step　was　to　overthrow　the　belief　that　certain　bodies　are　〃electrics〃　and　others　〃non…electrics〃that　is；　that　some　substances　when　rubbed　show　certain　peculiarities　in　attracting　pieces　of　paper　and　foil　which　others　do　not。　Dufay　proved　that　all　bodies　possess　this　quality　in　a　certain　degree。　〃I　have　found　that　all　bodies　（metallic；　soft；　or　fluid　ones　excepted）；〃　he　says；　〃may　be　made　electric　by　first　heating　them　more　or　less　and　then　rubbing　them　on　any　sort　of　cloth。　So　that　all　kinds　of　stones；　as　well　precious　as　common；　all　kinds　of　wood；　and；　in　general；　everything　that　I　have　made　trial　of；　became　electric　by　beating　and　rubbing；　except　such　bodies　as　grow　soft　by　beat；　as　the　gums；　which　dissolve　in　water；　glue；　and　such　like　substances。　'Tis　also　to　be　remarked　that　the　hardest　stones　or　marbles　require　more　chafing　or　heating　than　others；　and　that　the　same　rule　obtains　with　regard　to　the　woods；　so　that　box；　lignum　vitae；　and　such　others　must　be　chafed　almost　to　the　degree　of　browning；　whereas　fir；　lime…tree；　and　cork　require　but　a　moderate　heat。　〃Having　read　in　one　of　Mr。　Gray's　letters　that　water　may　be　made　electrical　by　holding　the　excited　glass　tube　near　it　（a　dish　of　water　being　fixed　to　a　stand　and　that　set　on　a　plate　of　glass；　or　on　the　brim　of　a　drinking…glass；　previously　chafed；　or　otherwise　warmed）；　I　have　found；　upon　trial；　that　the　same　thing　happened　to　all　bodies　without　exception；　whether　solid　or　fluid；　and　that　for　that　purpose　'twas　sufficient　to　set　them　on　a　glass　stand　slightly　warmed；　or　only　dried；　and　then　by　bringing　the　tube　near　them　they　immediately　became　electrical。　I　made　this　experiment　with　ice；　with　a　lighted　wood…coal；　and　with　everything　that　came　into　my　mind；　and　I　constantly　remarked　that　such　bodies　of　themselves　as　were　least　electrical　had　the　greatest　degree　of　electricity　communicated　to　them　at　the　approval　of　the　glass　tube。〃　His　next　important　discovery　was　that　colors　had　nothing　to　do　with　the　conduction　of　electricity。　〃Mr。　Gray　says；　towards　the　end　of　one　of　his　letters；〃　he　writes；　〃that　bodies　attract　more　or　less　according　to　their　colors。　This　led　me　to　make　several　very　singular　experiments。　I　took　nine　silk　ribbons　of　equal　size；　one　white；　one　black；　and　the　other　seven　of　the　seven　primitive　colors；　and　having　hung　them　all　in　order　in　the　same　line；　and　then　bringing　the　tube　near　them；　the　black　one　was　first　attracted；　the　white　one　next；　and　others　in　order　successively　to　the　red　one；　which　was　attracted　least；　and　the　last　of　them　all。　I　afterwards　cut　out　nine　square　pieces　of　gauze　of　the　same　colors　with　the　ribbons；　and　having　put　them　one　after　another　on　a　hoop　of　wood；　with　leaf…gold　under　them；　the　leaf…gold　was　attracted　through　all　the　colored　pieces　of　gauze；　but　not　through　the　white　or　black。　This　inclined　me　first　to　think　that　colors　contribute　much　to　electricity；　but　three　experiments　convinced　me　to　the　contrary。　The　first；　that　by　warming　the　pieces　of　gauze　neither　the　black　nor　white　pieces　obstructed　the　action　of　the　electrical　tube　more　than　those　of　the　other　colors。　In　like　manner；　the　ribbons　being　warmed；　the　black　and　white　are　not　more　strongly　attracted　than　the　rest。　The　second　is；　the　gauzes　and　ribbons　being　wetted；　the　ribbons　are　all　attracted　equally；　and　all　the　pieces　of　gauze　equally　intercept　the　action　of　electric　bodies。　The　third　is；　that　the　colors　of　a　prism　being　thrown　on　a　white　gauze；　there　appear　no　differences　of　attraction。　Whence　it　proceeds　that　this　difference　proceeds；　not　from　the　color；　as　a　color；　but　from　the　substances　that　are　employed　in　the　dyeing。　For　when　I　colored　ribbons　by　rubbing　them　with　charcoal；　carmine；　and　such　other　substances；　the　differences　no　longer　proved　the　same。〃　In　connection　with　his　experiments　with　his　thread　suspended　on　glass　poles；　Dufay　noted　that　a　certain　amount　of　the　current　is　lost；　being　given　off　to　the　surrounding　air。　He　recommended；　therefore；　that　the　cords　experimented　with　be　wrapped　with　some　non…conductorthat　it　should　be　〃insulated〃　（〃isolee〃）；　as　he　said；　first　making　use　of　this　term。

DUFAY　DISCOVERS　VITREOUS　AND　RESINOUS　ELECTRICITY　It　has　been　shown　in　an　earlier　chapter　how　Von　Guericke　discovered　that　light　substances　like　feathers；　after　being　attracted　to　the　sulphur…ball　electric…machine；　were　repelled　by　it　until　they　touched　some　object。　Von　Guericke　noted　this；　but　failed　to　explain　it　satisfactorily。　Dufay；　repeating　Von　Guericke's　experiments；　found　that　if；　while　the　excited　tube　or　sulphur　ball　is　driving　the　repelled　feather　before　it；　the　ball　be　touched　or　rubbed　anew；　the　feather　comes　to　it　again；　and　is　repelled　alternately；　as；　the　hand　touches　the　ball；　or　is　withdrawn。　From　this　he　concluded　that　electrified　bodies　first　attract　bodies　not　electrified；　〃charge〃　them　with　electricity；　and　then　repel　them；　the　body　so　charged　not　being　attracted　again　until　it　has　discharged　its　electricity　by　touching　something。　〃On　making　the　experiment　related　by　Otto　von　Guericke；〃　he　says；　〃which　consists　in　making　a　ball　of　sulphur　rendered　electrical　to　repel　a　down　feather；　I　perceived　that　the　same　effects　were　produced　not　only　by　the　tube；　but　by　all　electric　bodies　whatsoever；　and　I　discovered　that　which　accounts　for　a　great　part　of　the　irregularities　and；　if　I　may　use　the　term；　of　the　caprices　that　seem　to　accompany　most　of　the　experiments　on　electricity。　This　principle　is　that　electric　bodies　attract　all　that　are　not　so；　and　repel　them　as　soon　as　they　are　become　electric　by　the　vicinity　or　contact　of　the　electric　body。　Thus　gold…leaf　is　first　attracted　by　the　tube；　and　acquires　an　electricity　by　approaching　it；　and　of　consequence　is　immediately　repelled　by　it。　Nor　is　it　reattracted　while　it　retains　its　electric　quality。　But　if　while　it　is　thus　sustained　in　the　air　it　chance　to　light　on　some　other　body；　it　straightway　loses　its　electricity；　and　in　consequence　is　reattracted　by　the　tube；　which；　after　having　given　it　a　new　electricity；　repels　it　a　second　time；　which　continues　as　long　as　the　tube　keeps　its　electricity。　Upon　applying　this　principle　to　the　various　experiments　of　electricity；　one　will　be　surprised　at　the　number　of　obscure　and　puzzling　facts　that　it　clears　up。　For　Mr。　Hauksbee's　famous　experiment　of　the　glass　globe；　in　which　silk　threads　are　put；　is　a　necessary　consequence　of　it。　When　these　threads　are　arranged　in　the　form　of　rays　by　the　electricity　of　the　sides　of　the　globe；　if　the　finger　be　put　near　the　outside　of　the　globe　the　silk　threads　within　fly　from　it；　as　is　well　known；　which　happens　only　because　the　finger　or　any　other　body　applied　near　the　glass　globe　is　thereby　rendered　electrical；　and　consequently　repels　the　silk　threads　which　are　endowed　with　the　same　quality。　With　a　little　reflection　we　may　in　the　same　manner　account　for　most　of　the　other　phenomena；　and　which　seem　inexplicable　without　attending　to　this　principle。　〃Chance　has　thrown　in　my　way　another　principle；　more　universal　and　remarkable　than　the　preceding　one；　and　which　throws　a　new　light　on　the　subject　of　electricity。　This　principle　is　that　there　are　two　distinct　electricities；　very　different　from　each　other；　one　of　which　I　call　vitreous　electricity　and　the　other　resinous　electricity。　The　first　is　that　of　glass；　rock…crystal；　precious　stones；　hair　of　animals；　wool；　and　many　other　bodies。　The　second　is　that　of　amber；　copal；　gumsack；　silk　thread；　paper；　and　a　number　of　other　substances。　The　characteristic　of　these　two　electricities　is　that　a　body　of　the　vitreous　electricity；　for　example；　repels　all　such　as　are　of　the　same　electricity；　and　on　the　contrary　attracts　all　those　of　the　resinous　electricity；　so　that　the　tube；　made　electrical；　will　repel　glass；　crystal；　hair　of　animals；　etc。；　when　rendered　electric；　and　will　attract　silk　thread；　paper；　etc。；　though　rendered　electrical　likewise。　Amber；　on　the　contrary；　will　attract　electric　glass　and　other　substances　of　the　same　class；　and　will　repel　gum…sack；　copal；　silk　thread；　etc。　Two　silk　ribbons　rendered　electrical　will　repel　each　other；　two　woollen　threads　will　do　the　like；　but　a　woollen　thread　and　a　silken　thread　will　mutually　attract　each　other。　This　principle　very　naturally　explains　why　the　ends　of　threads　of　silk　or　wool　recede　from　each　other；　in　the　form　of　pencil　or　broom；　when　they　have　acquired　an　electric　quality。　From　this　principle　one　may　with　the　same　ease　deduce　the　explanation　of　a　great　number　of　other　phenomena；　and　it　is　probable　that　this　truth　will　lead　us　to　the　further　discovery　of　many　other　things。　〃In　order　to　know　immediately　to　which　of　the　two　classes　of　electrics　belongs　any　body　whatsoever；　one　need　only　render　electric　a　silk　thread；　which　is　known　to　be　of　the　resinuous　electricity；　and　see　whether　that　body；　rendered　electrical；　attracts　or　repels　it。　If　it　attracts　it；　it　is　certainly　of　the　kind　of　electricity　which　I　call　VITREOUS；　if；　on　the　contrary；　it　repels　it；　it　is　of　the　same　kind　of　electricity　with　the　silkthat　is；　of　the　RESINOUS。　I　have　likewise　observed　that　communicated　electricity　retains　the　same　prope