a history of science-2-第26章

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magnet；　it　should　be　remembered　that　numberless　theories　to　explain　the　action　of　the　electric　needle　had　been　advanced。　Columbus　and　Paracelsus；　for　example；　believed　that　the　magnet　was　attracted　by　some　point　in　the　heavens；　such　as　a　magnetic　star。　Gilbert　himself　tells　of　some　of　the　beliefs　that　had　been　held　by　his　predecessors；　many　of　whom　he　declares　〃wilfully　falsify。〃　One　of　his　first　steps　was　to　refute　by　experiment　such　assertions　as　that　of　Cardan；　that　〃a　wound　by　a　magnetized　needle　was　painless〃；　and　also　the　assertion　of　Fracastoni　that　loadstone　attracts　silver；　or　that　of　Scalinger；　that　the　diamond　will　attract　iron；　and　the　statement　of　Matthiolus　that　〃iron　rubbed　with　garlic　is　no　longer　attracted　to　the　loadstone。〃　Gilbert　made　extensive　experiments　to　explain　the　dipping　of　the　needle；　which　had　been　first　noticed　by　William　Norman。　His　deduction　as　to　this　phenomenon　led　him　to　believe　that　this　was　also　explained　by　the　magnetic　attraction　of　the　earth；　and　to　predict　where　the　vertical　dip　would　be　found。　These　deductions　seem　the　more　wonderful　because　at　the　time　he　made　them　the　dip　had　just　been　discovered；　and　had　not　been　studied　except　at　London。　His　theory　of　the　dip　was；　therefore；　a　scientific　prediction；　based　on　a　preconceived　hypothesis。　Gilbert　found　the　dip　to　be　72　degrees　at　London；　eight　years　later　Hudson　found　the　dip　at　75　degrees　22'　north　latitude　to　be　89　degrees　30'；　but　it　was　not　until　over　two　hundred　years　later；　in　1831；　that　the　vertical　dip　was　first　observed　by　Sir　James　Ross　at　about　70　degrees　5'　north　latitude；　and　96　degrees　43'　west　longitude。　This　was　not　the　exact　point　assumed　by　Gilbert；　and　his　scientific　predictions；　therefore；　were　not　quite　correct；　but　such　comparatively　slight　and　excusable　errors　mar　but　little　the　excellence　of　his　work　as　a　whole。　A　brief　epitome　of　some　of　his　other　important　discoveries　suffices　to　show　that　the　exalted　position　in　science　accorded　him　by　contemporaries；　as　well　as　succeeding　generations　of　scientists；　was　well　merited。　He　was　first　to　distinguish　between　magnetism　and　electricity；　giving　the　latter　its　name。　He　discovered　also　the　〃electrical　charge；〃　and　pointed　the　way　to　the　discovery　of　insulation　by　showing　that　the　charge　could　be　retained　some　time　in　the　excited　body　by　covering　it　with　some　non…conducting　substance；　such　as　silk；　although；　of　course；　electrical　conduction　can　hardly　be　said　to　have　been　more　than　vaguely　surmised；　if　understood　at　all　by　him。　The　first　electrical　instrument　ever　made；　and　known　as　such；　was　invented　by　him；　as　was　also　the　first　magnetometer；　and　the　first　electrical　indicating　device。　Although　three　centuries　have　elapsed　since　his　death；　the　method　of　magnetizing　iron　first　introduced　by　him　is　in　common　use　to…day。　He　made　exhaustive　experiments　with　a　needle　balanced　on　a　pivot　to　see　how　many　substances　he　could　find　which；　like　amber；　on　being　rubbed　affected　the　needle。　In　this　way　he　discovered　that　light　substances　were　attracted　by　alum；　mica；　arsenic；　sealing…wax；　lac　sulphur；　slags；　beryl；　amethyst；　rock…crystal；　sapphire；　jet；　carbuncle；　diamond；　opal；　Bristol　stone；　glass；　glass　of　antimony；　gum…mastic；　hard　resin；　rock…salt；　and；　of　course；　amber。　He　discovered　also　that　atmospheric　conditions　affected　the　production　of　electricity；　dryness　being　unfavorable　and　moisture　favorable。　Galileo's　estimate　of　this　first　electrician　is　the　verdict　of　succeeding　generations。　〃I　extremely　admire　and　envy　this　author；〃　he　said。　〃I　think　him　worthy　of　the　greatest　praise　for　the　many　new　and　true　observations　which　he　has　made；　to　the　disgrace　of　so　many　vain　and　fabling　authors。〃

STUDIES　OF　LIGHT；　HEAT；　AND　ATMOSPHERIC　PRESSURE　We　have　seen　that　Gilbert　was　by　no　means　lacking　in　versatility；　yet　the　investigations　upon　which　his　fame　is　founded　were　all　pursued　along　one　line；　so　that　the　father　of　magnetism　may　be　considered　one　of　the　earliest　of　specialists　in　physical　science。　Most　workers　of　the　time；　on　the　other　band；　extended　their　investigations　in　many　directions。　The　sum　total　of　scientific　knowledge　of　that　day　had　not　bulked　so　large　as　to　exclude　the　possibility　that　one　man　might　master　it　all。　So　we　find　a　Galileo；　for　example；　making　revolutionary　discoveries　in　astronomy；　and　performing　fundamental　experiments　in　various　fields　of　physics。　Galileo's　great　contemporary；　Kepler；　was　almost　equally　versatile；　though　his　astronomical　studies　were　of　such　pre…eminent　importance　that　his　other　investigations　sink　into　relative　insignificance。　Yet　he　performed　some　notable　experiments　in　at　least　one　department　of　physics。　These　experiments　had　to　do　with　the　refraction　of　light；　a　subject　which　Kepler　was　led　to　investigate；　in　part　at　least；　through　his　interest　in　the　telescope。　We　have　seen　that　Ptolemy　in　the　Alexandrian　time；　and　Alhazen；　the　Arab；　made　studies　of　refraction。　Kepler　repeated　their　experiments；　and；　striving　as　always　to　generalize　his　observations；　he　attempted　to　find　the　law　that　governed　the　observed　change　of　direction　which　a　ray　of　light　assumes　in　passing　from　one　medium　to　another。　Kepler　measured　the　angle　of　refraction　by　means　of　a　simple　yet　ingenious　trough…like　apparatus　which　enabled　him　to　compare　readily　the　direct　and　refracted　rays。　He　discovered　that　when　a　ray　of　light　passes　through　a　glass　plate；　if　it　strikes　the　farther　surface　of　the　glass　at　an　angle　greater　than　45　degrees　it　will　be　totally　refracted　instead　of　passing　through　into　the　air。　He　could　not　well　fail　to　know　that　different　mediums　refract　light　differently；　and　that　for　the　same　medium　the　amount　of　light　valies　with　the　change　in　the　angle　of　incidence。　He　was　not　able；　however；　to　generalize　his　observations　as　he　desired；　and　to　the　last　the　law　that　governs　refraction　escaped　him。　It　remained　for　Willebrord　Snell；　a　Dutchman；　about　the　year　1621；　to　discover　the　law　in　question；　and　for　Descartes；　a　little　later；　to　formulate　it。　Descartes；　indeed；　has　sometimes　been　supposed　to　be　the　discoverer　of　the　law。　There　is　reason　to　believe　that　he　based　his　generalizations　on　the　experiment　of　Snell；　though　he　did　not　openly　acknowledge　his　indebtedness。　The　law；　as　Descartes　expressed　it；　states　that　the　sine　of　the　angle　of　incidence　bears　a　fixed　ratio　to　the　sine　of　the　angle　of　refraction　for　any　given　medium。　Here；　then；　was　another　illustration　of　the　fact　that　almost　infinitely　varied　phenomena　may　be　brought　within　the　scope　of　a　simple　law。　Once　the　law　had　been　expressed；　it　could　be　tested　and　verified　with　the　greatest　ease；　and；　as　usual；　the　discovery　being　made；　it　seems　surprising　that　earlier　investigatorsin　particular　so　sagacious　a　guesser　as　Keplershould　have　missed　it。　Galileo　himself　must　have　been　to　some　extent　a　student　of　light；　since；　as　we　have　seen；　he　made　such　notable　contributions　to　practical　optics　through　perfecting　the　telescope；　but　he　seems　not　to　have　added　anything　to　the　theory　of　light。　The　subject　of　heat；　however；　attracted　his　attention　in　a　somewhat　different　way；　and　he　was　led　to　the　invention　of　the　first　contrivance　for　measuring　temperatures。　His　thermometer　was　based　on　the　afterwards　fa