a history of science-4-第4章

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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　chemistry　and　electricity。　In　1766；　while　on　a　visit　to　London；　he　met　Benjamin　Franklin；　at　whose　suggestion　he　published　his　History　of　Electricity。　　From　this　time　on　he　made　steady　progress　in　scientific　investigations；　keeping　up　his　ecclesiastical　duties　at　the　same　time。　In　1780　he　removed　to　Birmingham；　having　there　for　associates　such　scientists　as　James　Watt；　Boulton；　and　Erasmus　Darwin。

Eleven　years　later；　on　the　anniversary　of　the　fall　of　the　Bastile　in　Paris；　a　fanatical　mob；　knowing　Priestley's　sympathies　with　the　French　revolutionists；　attacked　his　house　and　chapel；　burning　both　and　destroying　a　great　number　of　valuable　papers　and　scientific　instruments。　Priestley　and　his　family　escaped　violence　by　flight；　but　his　most　cherished　possessions　were　destroyed；　and　three　years　later　he　quitted　England　forever；　removing　to　the　United　States；　whose　struggle　for　liberty　he　had　championed。　The　last　ten　years　of　his　life　were　spent　at　Northumberland；　Pennsylvania；　where　he　continued　his　scientific　researches。

Early　in　his　scientific　career　Priestley　began　investigations　upon　the　〃fixed　air〃　of　Dr。　Black；　and；　oddly　enough；　he　was　stimulated　to　this　by　the　same　thing　that　had　influenced　Blackthat　is；　his　residence　in　the　immediate　neighborhood　of　a　brewery。　It　was　during　the　course　of　a　series　of　experiments　on　this　and　other　gases　that　he　made　his　greatest　discovery；　that　of　oxygen；　or　〃dephlogisticated　air；〃　as　he　called　it。　The　story　of　this　important　discovery　is　probably　best　told　in　Priestley's　own　words：

〃There　are；　I　believe；　very　few　maxims　in　philosophy　that　have　laid　firmer　hold　upon　the　mind　than　that　air；　meaning　atmospheric　air；　is　a　simple　elementary　substance；　indestructible　and　unalterable；　at　least　as　much　so　as　water　is　supposed　to　be。　　In　the　course　of　my　inquiries　I　was；　however；　soon　satisfied　that　atmospheric　air　is　not　an　unalterable　thing；　for　that；　according　to　my　first　hypothesis；　the　phlogiston　with　which　it　becomes　loaded　from　bodies　burning　in　it；　and　the　animals　breathing　it；　and　various　other　chemical　processes；　so　far　alters　and　depraves　it　as　to　render　it　altogether　unfit　for　inflammation；　respiration；　and　other　purposes　to　which　it　is　subservient；　and　I　had　discovered　that　agitation　in　the　water；　the　process　of　vegetation；　and　probably　other　natural　processes；　restore　it　to　its　original　purity。。。。

〃Having　procured　a　lens　of　twelve　inches　diameter　and　twenty　inches　local　distance；　I　proceeded　with　the　greatest　alacrity；　by　the　help　of　it；　to　discover　what　kind　of　air　a　great　variety　of　substances　would　yield；　putting　them　into　the　vessel；　which　I　filled　with　quicksilver；　and　kept　inverted　in　a　basin　of　the　same　。。。。　With　this　apparatus；　after　a　variety　of　experiments　。。。。　on　the　1st　of　August；　1774；　I　endeavored　to　extract　air　from　mercurius　calcinatus　per　se；　and　I　presently　found　that；　by　means　of　this　lens；　air　was　expelled　from　it　very　readily。　Having　got　about　three　or　four　times　as　much　as　the　bulk　of　my　materials；　I　admitted　water　to　it；　and　found　that　it　was　not　imbibed　by　it。　But　what　surprised　me　more　than　I　can　express　was　that　a　candle　burned　in　this　air　with　a　remarkably　vigorous　flame；　very　much　like　that　enlarged　flame　with　which　a　candle　burns　in　nitrous　oxide；　exposed　to　iron　or　liver　of　sulphur；　but　as　I　had　got　nothing　like　this　remarkable　appearance　from　any　kind　of　air　besides　this　particular　modification　of　vitrous　air；　and　I　knew　no　vitrous　acid　was　used　in　the　preparation　of　mercurius　calcinatus；　I　was　utterly　at　a　loss　to　account　for　it。〃＇4＇


The　〃new　air〃　was；　of　course；　oxygen。　　Priestley　at　once　proceeded　to　examine　it　by　a　long　series　of　careful　experiments；　in　which；　as　will　be　seen；　he　discovered　most　of　the　remarkable　qualities　of　this　gas。　Continuing　his　description　of　these　experiments；　he　says：

〃The　flame　of　the　candle；　besides　being　larger；　burned　with　more　splendor　and　heat　than　in　that　species　of　nitrous　air；　and　a　piece　of　red…hot　wood　sparkled　in　it；　exactly　like　paper　dipped　in　a　solution　of　nitre；　and　it　consumed　very　fast；　an　experiment　that　I　had　never　thought　of　trying　with　dephlogisticated　nitrous　air。

〃。　。　。　I　had　so　little　suspicion　of　the　air　from　the　mercurius　calcinatus；　etc。；　being　wholesome；　that　I　had　not　even　thought　of　applying　it　to　the　test　of　nitrous　air；　but　thinking　（as　my　reader　must　imagine　I　frequently　must　have　done）　on　the　candle　burning　in　it　after　long　agitation　in　water；　it　occurred　to　me　at　last　to　make　the　experiment；　and；　putting　one　measure　of　nitrous　air　to　two　measures　of　this　air；　I　found　not　only　that　it　was　diminished；　but　that　it　was　diminished　quite　as　much　as　common　air；　and　that　the　redness　of　the　mixture　was　likewise　equal　to　a　similar　mixture　of　nitrous　and　common　air。。。。　The　next　day　I　was　more　surprised　than　ever　I　had　been　before　with　finding　that；　after　the　above…mentioned　mixture　of　nitrous　air　and　the　air　from　mercurius　calcinatus　had　stood　all　night；　。　。　。　a　candle　burned　in　it；　even　better　than　in　common　air。〃

A　little　later　Priestley　discovered　that　〃dephlogisticated　air　。