a history of science-4-第14章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


notes　soon　came　under　the　observation　of　other　chemists；　notably　of　Professors　Gustav　Hinrichs　in　America；　Dmitri　Mendeleeff　in　Russia；　and　Lothar　Meyer　in　Germany。　　Mendeleeff　gave　the　discovery　fullest　expression；　explicating　it　in　1869；　under　the　title　of　〃the　periodic　law。〃

Though　this　early　exposition　of　what　has　since　been　admitted　to　be　a　most　important　discovery　was　very　fully　outlined；　the　generality　of　chemists　gave　it　little　heed　till　a　decade　or　so　later；　when　three　new　elements；　gallium；　scandium；　and　germanium；　were　discovered；　which；　on　being　analyzed；　were　quite　unexpectedly　found　to　fit　into　three　gaps　which　Mendeleeff　had　left　in　his　periodic　scale。　In　effect　the　periodic　law　had　enabled　Mendeleeff　to　predicate　the　existence　of　the　new　elements　years　before　they　were　discovered。　Surely　a　system　that　leads　to　such　results　is　no　mere　vagary。　So　very　soon　the　periodic　law　took　its　place　as　one　of　the　most　important　generalizations　of　chemical　science。

This　law　of　periodicity　was　put　forward　as　an　expression　of　observed　relations　independent　of　hypothesis；　but　of　course　the　theoretical　bearings　of　these　facts　could　not　be　overlooked。　As　Professor　J。　H。　Gladstone　has　said；　it　forces　upon　us　〃the　conviction　that　the　elements　are　not　separate　bodies　created　without　reference　to　one　another；　but　that　they　have　been　originally　fashioned；　or　have　been　built　up；　from　one　another；　according　to　some　general　plan。〃　　It　is　but　a　short　step　from　that　proposition　to　the　Proutian　hypothesis。


NEW　WEAPONSSPECTROSCOPE　AND　CAMERA

But　the　atomic　weights　are　not　alone　in　suggesting　the　compound　nature　of　the　alleged　elements。　　Evidence　of　a　totally　different　kind　has　contributed　to　the　same　end；　from　a　source　that　could　hardly　have　been　imagined　when　the　Proutian　hypothesis；　was　formulated；　through　the　tradition　of　a　novel　weapon　to　the　armamentarium　of　the　chemistthe　spectroscope。　　The　perfection　of　this　instrument；　in　the　hands　of　two　German　scientists；　Gustav　Robert　Kirchhoff　and　Robert　Wilhelm　Bunsen；　came　about　through　the　investigation；　towards　the　middle　of　the　century；　of　the　meaning　of　the　dark　lines　which　had　been　observed　in　the　solar　spectrum　by　Fraunhofer　as　early　as　1815；　and　by　Wollaston　a　decade　earlier。　It　was　suspected　by　Stokes　and　by　Fox　Talbot　in　England；　but　first　brought　to　demonstration　by　Kirchhoff　and　Bunsen；　that　these　lines；　which　were　known　to　occupy　definite　positions　in　the　spectrum；　are　really　indicative　of　particular　elementary　substances。　By　means　of　the　spectroscope；　which　is　essentially　a　magnifying　lens　attached　to　a　prism　of　glass；　it　is　possible　to　locate　the　lines　with　great　accuracy；　and　it　was　soon　shown　that　here　was　a　new　means　of　chemical　analysis　of　the　most　exquisite　delicacy。　It　was　found；　for　example；　that　the　spectroscope　could　detect　the　presence　of　a　quantity　of　sodium　so　infinitesimal　as　the　one　two…hundred…thousandth　of　a　grain。　　But　what　was　even　more　important；　the　spectroscope　put　no　limit　upon　the　distance　of　location　of　the　substance　it　tested；　provided　only　that　sufficient　light　came　from　it。　The　experiments　it　recorded　might　be　performed　in　the　sun；　or　in　the　most　distant　stars　or　nebulae；　indeed；　one　of　the　earliest　feats　of　the　instrument　was　to　wrench　from　the　sun　the　secret　of　his　chemical　constitution。

To　render　the　utility　of　the　spectroscope　complete；　however；　it　was　necessary　to　link　with　it　another　new　chemical　agencynamely；　photography。　　This　now　familiar　process　is　based　on　the　property　of　light　to　decompose　certain　unstable　compounds　of　silver；　and　thus　alter　their　chemical　composition。　Davy　and　Wedgwood　barely　escaped　the　discovery　of　the　value　of　the　photographic　method　early　in　the　nineteenth　century。　Their　successors　quite　overlooked　it　until　about　1826；　when　Louis　J。　M。　Daguerre；　the　French　chemist；　took　the　matter　in　hand；　and　after　many　years　of　experimentation　brought　it　to　relative　perfection　in　1839；　in　which　year　the　famous　daguerreotype　first　brought　the　matter　to　popular　attention。　In　the　same　year　Mr。　Fox　Talbot　read　a　paper　on　the　subject　before　the　Royal　Society；　and　soon　afterwards　the　efforts　of　Herschel　and　numerous　other　natural　philosophers　contributed　to　the　advancement　of　the　new　method。

In　1843　Dr。　John　W。　Draper；　the　famous　English…American　chemist　and　physiologist；　showed　that　by　photography　the　Fraunhofer　lines　in　the　solar　spectrum　might　be　mapped　with　absolute　accuracy；　also　proving　that　the　silvered　film　revealed　many　lines　invisible　to　the　unaided　eye。　The　value　of　this　method　of　observation　was　recognized　at　once；　and；　as　soon　as　the　spectroscope　was　perfected；　the　photographic　method；　in　conjunction　with　its　use；　became　invaluable　to　the　chemist。　By　this　means　comparisons　of　spectra　may　be　made　with　a　degree　of　accuracy　not　otherwise　obtainable；　and；　in　case　of　the　stars；　whole　clusters　of　spectra　may　be　placed　on　record　at　a　single　observation。

As　the　examination　of　the　sun　and　stars　proceeded；　chemists　were　amazed　or　delighted；　according　to　their　various　preconceptions；　to　witness　the　proof　that　many　familiar　terrestrial　elements　are　to　be　found　in　the　celestial　bodies。　　But　what　perhaps　surprised　them　most　was　to　observe　the　enormous　preponderance　in　the　sidereal　bodies　of　the　element　hydrogen。　Not　only　are　there　vast　quantities　of　this　element　in　the　sun's　atmosphere；　but　some　other　suns　appeared　to　show　hydrogen　lines　almost　exclusively　in　their　spectra。　　Presently　it　appeared　that　the　stars　of　which　this　is　true　are　those　white　stars；　such　as　Sirius；　which　had　been　conjectured　to　be　the　hottest；　whereas　stars　that　are　only　red…hot；　like　our　sun；　show　also　the　vapors　of　many　other　elements；　including　iron　and　other　metals。

In　1878　Professor　J。　Norman　Lockyer；　in　a　paper　before　the　Royal　Society；　called　attention　to　the　possible　significance　of　this　series　of　observations。　He　urged　that　the　fact　of　the　sun　showing　fewer　elements　than　are　observed　here　on　the　cool　earth；　while　stars　much　hotter　than　the　sun　show　chiefly　one　element；　and　that　one　hydrogen；　the　lightest　of　known　elements；　seemed　to　give　color　to　the　possibility　that　our　alleged　elements　are　really　compounds；　which　at　the　temperature　of　the　hottest　stars　may　be　decomposed　into　hydrogen；　the　latter　〃element〃　itself　being　also　doubtless　a　compound；　which　might　be　resolved　under　yet　more　trying　conditions。

Here；　then；　was　what　might　be　termed　direct　experimental　evidence　for　the　hypothesis　of　Prout。　　Unfortunately；　however；　it　is　evidence　of　a　kind　which　only　a　few　experts　are　competent　to　discussso　very　delicate　a　matter　is　the　spectral　analysis　of　the　stars。　What　is　still　more　unfortunate；　the　experts　do　not　agree　among　themselves　as　to　the　validity　of　Professor　Lockyer's　conclusions。　Some；　like　Professor　Crookes；　have　accepted　them　with　acclaim；　hailing　Lockyer　as　〃the　Darwin　of　the　inorganic　world；〃　while　others　have　sought　a　different　explanation　of　the　facts　he　brings　forward。　As　yet　it　cannot　be　said　that　the　controversy　has　been　brought　to　final　settlement。　　Still；　it　is　hardly　to　be　doubted　that　now；　since　the　periodic　law　has　seemed　to　join　hands　with　the　spectroscope；　a　belief　in　the　compound　nature　of　the　so…called　elements　is　rapidly　gaining　ground　among　chemists。　　More　and　more　general　becomes　the　belief　that　the　Daltonian　atom　is　really　a　compound　radical；　and　that　back　of　the　seeming　diversity　of　the　alleged　elements　is　a　single　form　of　primordial　matter。　　Indeed；　in　very　recent　months；　direct　experimental　evidence　for　this　view　has　at　last　come　to　hand；　through　the　study　of　radio…active　substances。　　In　a　later　chapter　we　shall　have　occasion　to　inquire　how　this　came　about。



IV。　ANATOMY　AND　PHYSIOLOGY　IN　THE　EIGHTEENTH　CENTURY

ALBRECHT　VON　HALLER

An　epoch　in　physiology　was　made　in　the　eighteenth　century　by　the　genius　and　efforts　of　Albrecht　von　Haller　（1708…1777）；　of　Berne；　who　is　perhaps　as　worthy　of　the　title　〃The　Great〃　as　any　philosopher　who　has　been　so　christened　by　his　contemporaries　since　the　time　of　Hippocrates。　　Celebrated　as　a　physician；　he　was　proficient　in　various　fields；　being　equally　famed　in　his　own　time　as　poet；　botanist；　and　statesman；　and　dividing　his　attention　between　art　and　science。

As　a　child