a history of science-2-第4部分

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　this　layer　can　reach　the　spectator　at　any　given　point；　he　hit　upon　a　means　of　measurement　that　seemed　to　solve　the　hitherto　inscrutable　problem　as　to　the　atmospheric　depth。　Like　the　measurements　of　Aristarchus　and　Eratosthenes；　this　calculation　of　Alhazen　is　simple　enough　in　theory。　Its　defect　consists　largely　in　the　difficulty　of　fixing　its　terms　with　precision；　combined　with　the　further　fact　that　the　rays　of　the　sun；　in　taking　the　slanting　course　through　the　earth's　atmosphere；　are　really　deflected　from　a　straight　line　in　virtue　of　the　constantly　increasing　density　of　the　air　near　the　earth's　surface。　Alhazen　must　have　been　aware　of　this　latter　fact；　since　it　was　known　to　the　later　Alexandrian　astronomers；　but　he　takes　no　account　of　it　in　the　present　measurement。　The　diagram　will　make　the　method　of　Alhazen　clear。　His　important　premises　are　two：　first；　the　well…recognized　fact　that；　when　light　is　reflected　from　any　surface；　the　angle　of　incidence　is　equal　to　the　angle　of　reflection；　and；　second；　the　much　more　doubtful　observation　that　twilight　continues　until　such　time　as　the　sun；　according　to　a　simple　calculation；　is　nineteen　degrees　below　the　horizon。　Referring　to　the　diagram；　let　the　inner　circle　represent　the　earth's　surface；　the　outer　circle　the　limits　of　the　atmosphere；　C　being　the　earth's　centre；　and　RR　radii　of　the　earth。　Then　the　observer　at　the　point　A　will　continue　to　receive　the　reflected　rays　of　the　sun　until　that　body　reaches　the　point　S；　which　is；　according　to　the　hypothesis；　nineteen　degrees　below　the　horizon　line　of　the　observer　at　A。　This　horizon　line；　being　represented　by　AH；　and　the　sun's　ray　by　SM；　the　angle　HMS　is　an　angle　of　nineteen　degrees。　The　complementary　angle　SMA　is；　obviously；　an　angle　of　（180…19）　one　hundred　and　sixty…one　degrees。　But　since　M　is　the　reflecting　surface　and　the　angle　of　incidence　equals　the　angle　of　reflection；　the　angle　AMC　is　an　angle　of　one…half　of　one　hundred　and　sixty…one　degrees；　or　eighty　degrees　and　thirty　minutes。　Now　this　angle　AMC；　being　known；　the　right…angled　triangle　MAC　is　easily　resolved；　since　the　side　AC　of　that　triangle；　being　the　radius　of　the　earth；　is　a　known　dimension。　Resolution　of　this　triangle　gives　us　the　length　of　the　hypotenuse　MC；　and　the　difference　between　this　and　the　radius　（AC）；　or　CD；　is　obviously　the　height　of　the　atmosphere　（h）；　which　was　the　measurement　desired。　According　to　the　calculation　of　Alhazen；　this　h；　or　the　height　of　the　atmosphere；　represents　from　twenty　to　thirty　miles。　The　modern　computation　extends　this　to　about　fifty　miles。　But；　considering　the　various　ambiguities　that　necessarily　attended　the　experiment；　the　result　was　a　remarkably　close　approximation　to　the　truth。　Turning　from　physics　to　chemistry；　we　find　as　perhaps　the　greatest　Arabian　name　that　of　Geber；　who　taught　in　the　College　of　Seville　in　the　first　half　of　the　eighth　century。　The　most　important　researches　of　this　really　remarkable　experimenter　had　to　do　with　the　acids。　The　ancient　world　had　had　no　knowledge　of　any　acid　more　powerful　than　acetic。　Geber；　however；　vastly　increased　the　possibilities　of　chemical　experiment　by　the　discovery　of　sulphuric；　nitric；　and　nitromuriatic　acids。　He　made　use　also　of　the　processes　of　sublimation　and　filtration；　and　his　works　describe　the　water　bath　and　the　chemical　oven。　Among　the　important　chemicals　which　he　first　differentiated　is　oxide　of　mercury；　and　his　studies　of　sulphur　in　its　various　compounds　have　peculiar　interest。　In　particular　is　this　true　of　his　observation　that；　tinder　certain　conditions　of　oxidation；　the　weight　of　a　metal　was　lessened。　From　the　record　of　these　studies　in　the　fields　of　astronomy；　physics；　and　chemistry；　we　turn　to　a　somewhat　extended　survey　of　the　Arabian　advances　in　the　field　of　medicine。

ARABIAN　MEDICINE　The　influence　of　Arabian　physicians　rested　chiefly　upon　their　use　of　drugs　rather　than　upon　anatomical　knowledge。　Like　the　mediaeval　Christians；　they　looked　with　horror　on　dissection　of　the　human　body；　yet　there　were　always　among　them　investigators　who　turned　constantly　to　nature　herself　for　hidden　truths；　and　were　ready　to　uphold　the　superiority　of　actual　observation　to　mere　reading。　Thus　the　physician　Abd　el…Letif；　while　in　Egypt；　made　careful　studies　of　a　mound　of　bones　containing　more　than　twenty　thousand　skeletons。　While　examining　these　bones　he　discovered　that　the　lower　jaw　consists　of　a　single　bone；　not　of　two；　as　had　been　taught　by　Galen。　He　also　discovered　several　other　important　mistakes　in　Galenic　anatomy；　and　was　so　impressed　with　his　discoveries　that　he　contemplated　writing　a　work　on　anatomy　which　should　correct　the　great　classical　authority's　mistakes。　It　was　the　Arabs　who　invented　the　apothecary；　and　their　pharmacopoeia；　issued　from　the　hospital　at　Gondisapor；　and　elaborated　from　time　to　time；　formed　the　basis　for　Western　pharmacopoeias。　Just　how　many　drugs　originated　with　them；　and　how　many　were　borrowed　from　the　Hindoos；　Jews；　Syrians；　and　Persians；　cannot　be　determined。　It　is　certain；　however；　that　through　them　various　new　and　useful　drugs；　such　as　senna；　aconite；　rhubarb；　camphor；　and　mercury；　were　handed　down　through　the　Middle　Ages；　and　that　they　are　responsible　for　the　introduction　of　alcohol　in　the　field　of　therapeutics。　In　mediaeval　Europe；　Arabian　science　came　to　be　regarded　with　superstitious　awe；　and　the　works　of　certain　Arabian　physicians　were　exalted　to　a　position　above　all　the　ancient　writers。　In　modern　times；　however；　there　has　been　a　reaction　and　a　tendency　to　depreciation　of　their　work。　By　some　they　are　held　to　be　mere　copyists　or　translators　of　Greek　books；　and　in　no　sense　original　investigators　in　medicine。　Yet　there　can　be　little　doubt　that　while　the　Arabians　did　copy　and　translate　freely；　they　also　originated　and　added　considerably　to　medical　knowledge。　It　is　certain　that　in　the　time　when　Christian　monarchs　in　western　Europe　were　paying　little　attention　to　science　or　education；　the　caliphs　and　vizirs　were　encouraging　physicians　and　philosophers；　building　schools；　and　erecting　libraries　and　hospitals。　They　made　at　least　a　creditable　effort　to　uphold　and　advance　upon　the　scientific　standards　of　an　earlier　age。　The　first　distinguished　Arabian　physician　was　Harets　ben　Kaladah；　who　received　his　education　in　the　Nestonian　school　at　Gondisapor；　about　the　beginning　of　the　seventh　century。　Notwithstanding　the　fact　that　Harets　was　a　Christian；　he　was　chosen　by　Mohammed　as　his　chief　medical　adviser；　and　recommended　as　such　to　his　successor；　the　Caliph　Abu　Bekr。　Thus；　at　the　very　outset；　the　science　of　medicine　was　divorced　from　religion　among　the　Arabians；　for　if　the　prophet　himself　could　employ　the　services　of　an　unbeliever；　surely　others　might　follow　his　example。　And　that　this　example　was　followed　is　shown　in　the　fact　that　many　Christian　physicians　were　raised　to　honorable　positions　by　succeeding　generations　of　Arabian　monarchs。　This　broad…minded　view　of　medicine　taken　by　the　Arabs　undoubtedly　assisted　as　much　as　any　one　single　factor　in　upbuilding　the　science；　just　as　the　narrow　and　superstitious　view　taken　by　Western　nations　helped　to　destroy　it。　The　education　of　the　Arabians　made　it　natural　for　them　to　associate　medicine　with　the　natural　sciences；　rather　than　with　religion。　An　Arabian　savant　was　supposed　to　be　equally　well　educated　in　philosophy；　jurisprudence；　theology；　mathematics；　and　medicine；　and　to　practise　law；　theology；　and　medicine　with　equal　skill　upon　occasion。　It　is　easy　to　understand；　therefore；　why　these　religious　fanatics　were　willing　to　employ　unbelieving　physicians；　and　their　physicians　themselves　to　turn　to　the　scientific　works　of　Hippocrates　and　Galen　for　medical　instruction；　rather　than　to　religious　works。　Even　Mohammed　himself　professed　some　knowledge　of　medicine；　and　often　relied　upon　this　knowledge　in　treating　ailments　rather　than　upon　prayers　or　incantations。　He　is　said；　for　example；　to　have　recommended　and　applied　the　cautery　in　the　case　of　a　friend　who；　when　suffering　from　angina；　had　sought　his　aid。　The　list　of　eminent　Arabian　physicians　is　too　long　to　be　given　here；　but　some　of　them　are　of　such　importance　in　their　influence　upon　later　medicine　that　they　cannot　be　entirely　ignored。　One　of　the　first　of　these　was　Honain　ben　Isaac　（809…873　A。D。）；　a　Christian　Arab　of　Bagdad。　He　made　translations　of　the　works　of　Hippocrates；　and　practised　the　art　along　the　lines　indicated　by　his　teachings　and　those　of　Galen。　He　is　considered　the　greatest　translator　of　the　ninth　century　and　one　of　the　greatest　philosophers　of　that　period。　Another　great　Arabian　physician；　whose　work　was　just　beginning　as　Honain's　was　drawing　to　a　close；　was　Rhazes　（850…923　A。D。）；　who　during　his　life　was　no　less　noted　as　a　philosopher　and　musician　than　as　a　physician。　He　continued　the　work　of　Honain；　and　advanced　therapeutics　by　introducing　more　extensive　use　of　chemical　remedies；　such　as　mercurial　ointments；　sulphuric　acid；　and　aqua　vitae。　He　is　also　credited　with　being　the　first　physician　to　describe　small…pox　and　measles　accurately。　While　Rhazes　was　still　alive　another　Arabian；　Haly　Abbas　（died　about　994）；　was　writing　his　famous　encyclopaedia　of　medicine；　called　The　Royal　Book。　But　the　names　of　all　these　great　physicians　have　been　considerably　obscured　by　the　reputation　of　Avicenna　（980…1037