a history of science-2-第45章

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me　mercury；　he　found　that　by　allowing　air　to　rush　through　the　mercury　the　metal　became　a　jet　thrown　in　all　directions　against　the　sides　of　the　vessel；　making　a　great；　flaming　shower；　〃like　flashes　of　lightning；〃　as　he　said。　But　it　seemed　to　him　that　there　was　a　difference　between　this　light　and　the　glow　noted　in　the　barometer。　This　was　a　bright　light；　whereas　the　barometer　light　was　only　a　glow。　Pondering　over　this；　Hauksbee　tried　various　experiments；　revolving　pieces　of　amber；　flint；　steel；　and　other　substances　in　his　exhausted　air…pump　receiver；　with　negative；　or　unsatisfactory；　results。　Finally；　it　occurred　to　him　to　revolve　an　exhausted　glass　tube　itself。　Mounting　such　a　globe　of　glass　on　an　axis　so　that　it　could　be　revolved　rapidly　by　a　belt　running　on　a　large　wheel；　he　found　that　by　holding　his　fingers　against　the　whirling　globe　a　purplish　glow　appeared；　giving　sufficient　light　so　that　coarse　print　could　be　read；　and　the　walls　of　a　dark　room　sensibly　lightened　several　feet　away。　As　air　was　admitted　to　the　globe　the　light　gradually　diminished；　and　it　seemed　to　him　that　this　diminished　glow　was　very　similar　in　appearance　to　the　pale　light　seen　in　the　mercurial　barometer。　Could　it　be　that　it　was　the　glass；　and　not　the　mercury；　that　caused　it？　Going　to　a　barometer　he　proceeded　to　rub　the　glass　above　the　column　of　mercury　over　the　vacuum；　without　disturbing　the　mercury；　when；　to　his　astonishment；　the　same　faint　light；　to　all　appearances　identical　with　the　glow　seen　in　the　whirling　globe；　was　produced。　Turning　these　demonstrations　over　in　his　mind；　he　recalled　the　well…known　fact　that　rubbed　glass　attracted　bits　of　paper；　leaf…brass；　and　other　light　substances；　and　that　this　phenomenon　was　supposed　to　be　electrical。　This　led　him　finally　to　determine　the　hitherto　unsuspected　fact；　that　the　glow　in　the　barometer　was　electrical　as　was　also　the　glow　seen　in　his　whirling　globe。　Continuing　his　investigations；　he　soon　discovered　that　solid　glass　rods　when　rubbed　produced　the　same　effects　as　the　tube。　By　mere　chance；　happening　to　hold　a　rubbed　tube　to　his　cheek；　he　felt　the　effect　of　electricity　upon　the　skin　like　〃a　number　of　fine；　limber　hairs；〃　and　this　suggested　to　him　that；　since　the　mysterious　manifestation　was　so　plain；　it　could　be　made　to　show　its　effects　upon　various　substances。　Suspending　some　woollen　threads　over　the　whirling　glass　cylinder；　he　found　that　as　soon　as　he　touched　the　glass　with　his　hands　the　threads；　which　were　waved　about　by　the　wind　of　the　revolution；　suddenly　straightened　themselves　in　a　peculiar　manner；　and　stood　in　a　radical　position；　pointing　to　the　axis　of　the　cylinder。　Encouraged　by　these　successes；　he　continued　his　experiments　with　breathless　expectancy；　and　soon　made　another　important　discovery；　that　of　〃induction；〃　although　the　real　significance　of　this　discovery　was　not　appreciated　by　him　or；　for　that　matter；　by　any　one　else　for　several　generations　following。　This　discovery　was　made　by　placing　two　revolving　cylinders　within　an　inch　of　each　other；　one　with　the　air　exhausted　and　the　other　unexhausted。　Placing　his　hand　on　the　unexhausted　tube　caused　the　light　to　appear　not　only　upon　it；　but　on　the　other　tube　as　well。　A　little　later　he　discovered　that　it　is　not　necessary　to　whirl　the　exhausted　tube　to　produce　this　effect；　but　simply　to　place　it　in　close　proximity　to　the　other　whirling　cylinder。　These　demonstrations　of　Hauksbee　attracted　wide　attention　and　gave　an　impetus　to　investigators　in　the　field　of　electricity；　but　still　no　great　advance　was　made　for　something　like　a　quarter　of　a　century。　Possibly　the　energies　of　the　scientists　were　exhausted　for　the　moment　in　exploring　the　new　fields　thrown　open　to　investigation　by　the　colossal　work　of　Newton。

THE　EXPERIMENTS　OF　STEPHEN　GRAY　In　1729　Stephen　Gray　（died　in　1736）；　an　eccentric　and　irascible　old　pensioner　of　the　Charter　House　in　London；　undertook　some　investigations　along　lines　similar　to　those　of　Hauksbee。　While　experimenting　with　a　glass　tube　for　producing　electricity；　as　Hauksbee　had　done；　he　noticed　that　the　corks　with　which　he　had　stopped　the　ends　of　the　tube　to　exclude　the　dust；　seemed　to　attract　bits　of　paper　and　leaf…brass　as　well　as　the　glass　itself。　He　surmised　at　once　that　this　mysterious　electricity；　or　〃virtue；〃　as　it　was　called；　might　be　transmitted　through　other　substances　as　it　seemed　to　be　through　glass。　〃Having　by　me　an　ivory　ball　of　about　one　and　three…tenths　of　an　inch　in　diameter；〃　he　writes；　〃with　a　hole　through　it；　this　I　fixed　upon　a　fir…stick　about　four　inches　long；　thrusting　the　other　end　into　the　cork；　and　upon　rubbing　the　tube　found　that　the　ball　attracted　and　repelled　the　feather　with　more　vigor　than　the　cork　had　done；　repeating　its　attractions　and　repulsions　for　many　times　together。　I　then　fixed　the　ball　on　longer　sticks；　first　upon　one　of　eight　inches；　and　afterwards　upon　one　of　twenty…four　inches　long；　and　found　the　effect　the　same。　Then　I　made　use　of　iron；　and　then　brass　wire；　to　fix　the　ball　on；　inserting　the　other　end　of　the　wire　in　the　cork；　as　before；　and　found　that　the　attraction　was　the　same　as　when　the　fir…sticks　were　made　use　of；　and　that　when　the　feather　was　held　over　against　any　part　of　the　wire　it　was　attracted　by　it；　but　though　it　was　then　nearer　the　tube；　yet　its　attraction　was　not　so　strong　as　that　of　the　ball。　When　the　wire　of　two　or　three　feet　long　was　used；　its　vibrations；　caused　by　the　rubbing　of　the　tube；　made　it　somewhat　troublesome　to　be　managed。　This　put　me　to　thinking　whether；　if　the　ball　was　hung　by　a　pack…thread　and　suspended　by　a　loop　on　the　tube；　the　electricity　would　not　be　carried　down　the　line　to　the　ball；　I　found　it　to　succeed　accordingly；　for　upon　suspending　the　ball　on　the　tube　by　a　pack…thread　about　three　feet　long；　when　the　tube　had　been　excited　by　rubbing；　the　ivory　ball　attracted　and　repelled　the　leaf…brass　over　which　it　was　held　as　freely　as　it　had　done　when　it　was　suspended　on　sticks　or　wire；　as　did　also　a　ball　of　cork；　and　another　of　lead　that　weighed　one　pound　and　a　quarter。〃　Gray　next　attempted　to　determine　what　other　bodies　would　attract　the　bits　of　paper；　and　for　this　purpose　he　tried　coins；　pieces　of　metal；　and　even　a　tea…kettle；　〃both　empty　and　filled　with　hot　or　cold　water〃；　but　he　found　that　the　attractive　power　appeared　to　be　the　same　regardless　of　the　substance　used。　〃I　next　proceeded；〃　he　continues；　〃to　try　at　what　greater　distances　the　electric　virtues　might　be　carried；　and　having　by　me　a　hollow　walking…cane；　which　I　suppose　was　part　of　a　fishing…rod；　two　feet　seven　inches　long；　I　cut　the　great　end　of　it　to　fit　into　the　bore　of　the　tube；　into　which　it　went　about　five　inches；　then　when　the　cane　was　put　into　the　end　of　the　tube；　and　this　excited；　the　cane　drew　the　leaf…brass　to　the　height　of　more　than　two　inches；　as　did　also　the　ivory　ball；　when　by　a　cork　and　stick　it　had　been　fixed　to　the　end　of　the　cane。。。。　With　several　pieces　of　Spanish　cane　and　fir…sticks　I　afterwards　made　a　rod；　which；　together　with　the　tube；　was　somewhat　more　than　eighteen　feet　long；　which　was　the　greatest　length　I　could　conveniently　use　in　my　chamber；　and　found　the　attraction　very　nearly；　if　not　altogether；　as　strong　as　when　the　ball　was　placed　on　the　shorter　rods。〃　This　experiment　exhausted　the　capacity　of　his　small　room；　but　on　going　to　the　country　a　little　later　he　was　able　to　continue　his　experiments。　〃To　a　pole　of　eighteen　feet　there　was　tied　a　line　of　thirty…four　feet　in　length；　so　that　the　pole　and　line　together　were　fifty…two　feet。　With　the　pole　and　tube　I　stood　in　the　balcony；　the　assistant　below　in　the　court；　where　he　held　the　board　with　the　leaf…brass　on　it。　Then　the　tube　being　excited；　as　usual；　the　electric　virtue　passed　from　the　tube　up　the　pole　and　down　the　line　to　the　ivory　ball；　which　attracted　the　leaf…brass；　and　as　the　ball　passed　over　it　in　its　vibrations　the　leaf…brass　would　follow　it　till　it　was　carried　off　the　board。〃　Gray　next　attempted　to　send　the　electricity　over　a　line　suspended　horizontally。　To　do　this　he　suspended　the　pack…thread　by　pieces　of　string　looped　over　nails　driven　into　beams　for　that　purpose。　But　when　thus　suspended　he　found　that　the　ivory　ball　no　longer　excited　the　leaf…brass；　and　he　guessed　correctly　that　the　explanation　of　this　lay　in　the　fact　that　〃when　the　electric　virtue　came　to　the　loop　that　was　suspended　on　the　beam　it　went　up　the　same　to　the　beam；〃　none　of　it　reaching　the　ball。　As　we　shall　see　from　what　follows；　however；　Gray　had　not　as　yet　determined　that　certain　substances　will　conduct　electricity　while　others　will　not。　But　by　a　lucky　accident　he　made　the　discovery　that　silk；　for　example；　was　a　poor　conductor；　and　could　be　turned　to　account　in　insulating　the　conducting…cord。　A　certain　Mr。　Wheler　had　become　much　interested　in　the　old　pensioner　and　his　work；　and；　as　a　guest　at　the　Wheler　house；　Gray　had　been　repeating　some　of　his　former　experiments　with　the　fishing…rod；　line；　and　ivory　ball。　He　had　finally　exhausted　the　heights　from　which　these　experiments　could　be　made　by　climbing　to　the　clock…tower　and　exciting　bits　of　leaf…brass　on　the　ground　below。　〃As　we　had　no　greater　heights　here；〃　he　says；　〃Mr。　Wheler　was　desirous　to　try　whether　we　could　not　carry　the　electric　virtue　horizontally。　I　then　told　him　of　the　attempt　I　had　made　with　that　design；　but　witho