a history of science-4-第3节

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HENRY　CAVENDISH

This　work　of　Black's　was　followed　by　the　equally　important　work　of　his　former　pupil；　Henry　Cavendish　（1731…1810）；　whose　discovery　of　the　composition　of　many　substances；　notably　of　nitric　acid　and　of　water；　was　of　great　importance；　adding　another　link　to　the　important　chain　of　evidence　against　the　phlogiston　theory。　Cavendish　is　one　of　the　most　eccentric　figures　in　the　history　of　science；　being　widely　known　in　his　own　time　for　his　immense　wealth　and　brilliant　intellect；　and　also　for　his　peculiarities　and　his　morbid　sensibility；　which　made　him　dread　society；　and　probably　did　much　in　determining　his　career。　Fortunately　for　him；　and　incidentally　for　the　cause　of　science；　he　was　able　to　pursue　laboratory　investigations　without　being　obliged　to　mingle　with　his　dreaded　fellow…mortals；　his　every　want　being　provided　for　by　the　immense　fortune　inherited　from　his　father　and　an　uncle。

When　a　young　man；　as　a　pupil　of　Dr。　Black；　he　had　become　imbued　with　the　enthusiasm　of　his　teacher；　continuing　Black's　investigations　as　to　the　properties　of　carbonic…acid　gas　when　free　and　in　combination。　One　of　his　first　investigations　was　reported　in　1766；　when　he　communicated　to　the　Royal　Society　his　experiments　for　ascertaining　the　properties　of　carbonic…acid　and　hydrogen　gas；　in　which　he　first　showed　the　possibility　of　weighing　permanently　elastic　fluids；　although　Torricelli　had　before　this　shown　the　relative　weights　of　a　column　of　air　and　a　column　of　mercury。　Other　important　experiments　were　continued　by　Cavendish；　and　in　1784　he　announced　his　discovery　of　the　composition　of　water；　thus　robbing　it　of　its　time…honored　position　as　an　〃element。〃　But　his　claim　to　priority　in　this　discovery　was　at　once　disputed　by　his　fellow…countryman　James　Watt　and　by　the　Frenchman　Lavoisier。　Lavoisier's　claim　was　soon　disallowed　even　by　his　own　countrymen；　but　for　many　years　a　bitter　controversy　was　carried　on　by　the　partisans　of　Watt　and　Cavendish。　　The　two　principals；　however；　seem。　never　to　have　entered　into　this　controversy　with　anything　like　the　same　ardor　as　some　of　their　successors；　as　they　remained　on　the　best　of　terms。＇1＇　It　is　certain；　at　any　rate；　that　Cavendish　announced　his　discovery　officially　before　Watt　claimed　that　the　announcement　had　been　previously　made　by　him；　〃and；　whether　right　or　wrong；　the　honor　of　scientific　discoveries　seems　to　be　accorded　naturally　to　the　man　who　first　publishes　a　demonstration　of　his　discovery。〃　Englishmen　very　generally　admit　the　justness　of　Cavendish's　claim；　although　the　French　scientist　Arago；　after　reviewing　the　evidence　carefully　in　1833；　decided　in　favor　of　Watt。

It　appears　that　something　like　a　year　before　Cavendish　made　known　his　complete　demonstration　of　the　composition　of　water；　Watt　communicated　to　the　Royal　Society　a　suggestion　that　water　was　composed　of　〃dephlogisticated　air　（oxygen）　and　phlogiston　（hydrogen）　deprived　of　part　of　its　latent　heat。〃　Cavendish　knew　of　the　suggestion；　but　in　his　experiments　refuted　the　idea　that　the　hydrogen　lost　any　of　its　latent　heat。　Furthermore；　Watt　merely　suggested　the　possible　composition　without　proving　it；　although　his　idea　was　practically　correct；　if　we　can　rightly　interpret　the　vagaries　of　the　nomenclature　then　in　use。　But　had　Watt　taken　the　steps　to　demonstrate　his　theory；　the　great　〃Water　Controversy〃　would　have　been　avoided。　Cavendish's　report　of　his　discovery　to　the　Royal　Society　covers　something　like　forty　pages　of　printed　matter。　In　this　he　shows　how；　by　passing　an　electric　spark　through　a　closed　jar　containing　a　mixture　of　hydrogen　gas　and　oxygen；　water　is　invariably　formed；　apparently　by　the　union　of　the　two　gases。　The　experiment　was　first　tried　with　hydrogen　and　common　air；　the　oxygen　of　the　air　uniting　with　the　hydrogen　to　form　water；　leaving　the　nitrogen　of　the　air　still　to　be　accounted　for。　With　pure　oxygen　and　hydrogen；　however；　Cavendish　found　that　pure　water　was　formed；　leaving　slight　traces　of　any　other；　substance　which　might　not　be　interpreted　as　being　Chemical　impurities。　There　was　only　one　possible　explanation　of　this　phenomenonthat　hydrogen　and　oxygen；　when　combined；　form　water。

〃By　experiments　with　the　globe　it　appeared；〃　wrote　Cavendish；　〃that　when　inflammable　and　common　air　are　exploded　in　a　proper　proportion；　almost　all　the　inflammable　air；　and　near　one…fifth　the　common　air；　lose　their　elasticity　and　are　condensed　into　dew。　And　by　this　experiment　it　appears　that　this　dew　is　plain　water；　and　consequently　that　almost　all　the　inflammable　air　is　turned　into　pure　water。

〃In　order　to　examine　the　nature　of　the　matter　condensed　on　firing　a　mixture　of　dephlogisticated　and　inflammable　air；　I　took　a　glass　globe；　holding　8800　grain　measures；　furnished　with　a　brass　cock　and　an　apparatus　for　firing　by　electricity。　　This　globe　was　well　exhausted　by　an　air…pump；　and　then　filled　with　a　mixture　of　inflammable　and　dephlogisticated　air　by　shutting　the　cock；　fastening　the　bent　glass　tube　into　its　mouth；　and　letting　up　the　end　of　it　into　a　glass　jar　inverted　into　water　and　containing　a　mixture　of　19；500　grain　measures　of　dephlogisticated　air；　and　37；000　of　inflammable　air；　so　that；　upon　opening　the　cock；　some　of　this　mixed　air　rushed　through　the　bent　tube　and　filled　the　globe。　The　cock　was　then　shut　and　the　included　air　fired　by　electricity；　by　means　of　which　almost　all　of　it　lost　its　elasticity　（was　condensed　into　water　vapors）。　The　cock　was　then　again　opened　so　as　to　let　in　more　of　the　same　air　to　supply　the　place　of　that　destroyed　by　the　explosion；　which　was　again　fired；　and　the　operation　continued　till　almost　the　whole　of　the　mixture　was　let　into　the　globe　and　exploded。　　By　this　means；　though　the　globe　held　not　more　than　a　sixth　part　of　the　mixture；　almost　the　whole　of　it　was　exploded　therein　without　any　fresh　exhaustion　of　the　globe。〃

At　first　this　condensed　matter　was　〃acid　to　the　taste　and　contained　two　grains　of　nitre；〃　but　Cavendish；　suspecting　that　this　was　due　to　impurities；　tried　another　experiment　that　proved　conclusively　that　his　opinions　were　correct。　〃I　therefore　made　another　experiment；〃　he　says；　〃with　some　more　of　the　same　air　from　plants　in　which　the　proportion　of　inflammable　air　was　greater；　so　that　the　burnt　air　was　almost　completely　phlogisticated；　its　standard　being　one…tenth。　The　condensed　liquor　was　then　not　at　all　acid；　but　seemed　pure　water。〃

From　these　experiments　he　concludes　〃that　when　a　mixture　of　inflammable　and　dephlogisticated　air　is　exploded；　in　such　proportions　that　the　burnt　air　is　not　much　phlogisticated；　the　condensed　liquor　contains　a　little　acid　which　is　always　of　the　nitrous　kind；　whatever　substance　the　dephlogisticated　air　is　procured　from；　but　if　the　proportion　be　such　that　the　burnt　air　is　almost　entirely　phlogisticated；　the　condensed　liquor　is　not　at　all　acid；　but　seems　pure　water；　without　any　addition　whatever。〃＇2＇

These　same　experiments；　which　were　undertaken　to　discover　the　composition　of　water；　led　him　to　discover　also　the　composition　of　nitric　acid。　He　had　observed　that；　in　the　combustion　of　hydrogen　gas　with　common　air；　the　water　was　slightly　tinged　with　acid；　but　that　this　was　not　the　case　when　pure　oxygen　gas　was　used。　　Acting　upon　this　observation；　he　devised　an　experiment　to　determine　the　nature　of　this　acid。　He　constructed　an　apparatus　whereby　an　electric　spark　was　passed　through　a　vessel　containing　common　air。　　After　this　process　had　been　carried　on　for　several　weeks　a　small　amount　of　liquid　was　formed。　This　liquid　combined　with　a　solution　of　potash　to　form　common　nitre；　which　〃detonated　with　charcoal；　sparkled　when　paper　impregnated　with　it　was　burned；　and　gave　out　nitrous　fumes　when　sulphuric　acid　was　poured　on　it。〃　　In　other　words；　the　liquid　was　shown　to　be　nitric　acid。　Now；　since　nothing　but　pure　air　had　been　used　in　the　initial　experiment；　and　since　air　is　composed　of　nitrogen　and　oxygen；　there　seemed　no　room　to　doubt　that　nitric　acid　is　a　combination　of　nitrogen　and　oxygen。

This　discovery　of　the　nature　of　nitric　acid　seems　to　have　been　about　the　last　work　of　importance　that　Cavendish　did　in　the　field　of　chemistry；　although　almost　to　the　hour　of　his　death　he　was　constantly　occupied　with　scientific　observations。　　Even　in　the　last　moments　of　his　life　this　habit　asserted　itself；　according　to　Lord　Brougham。　　〃He　died　on　March　10；　1810；　after　a　short　illness；　probably　the　first；　as　well　as　the　last；　which　he　ever　suffered。　His　habit　of　curious　observation　continued　to　the　end。　He　was　desirous　of　marking　the　progress　of　the　disease　and　the　gradual　extinction　of　the　vital　powers。　　With　these　ends　in　view；　that　he　might　not　be　disturbed；　he　desired　to　be　left　alone。　His　servant；　returning　sooner　than　he　had　wished；　was　ordered　again　to　leave　the　chamber　of　death；　and　when　be　came　back　a　second　time　he　found　his　master　had　expired。＇3＇


JOSEPH　PRIESTLEY

While　the　opulent　but　diffident　Cavendish　was　making　his　important　discoveries；　another　Englishman；　a　poor　country　preacher　named　Joseph　Priestley　（1733…1804）　was　not　only　rivalling　him；　but；　if　anything；　outstripping　him　in　the　pursuit　of　chemical　discoveries。　In　1761　this　young　minister　was　given　a　position　as　tutor　in　a　nonconformist　academy　at　Warrington；　and　here；　for　six　years；　he　was　able　to　pursue　his　studies　in　c