a history of science-4-第2节

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another　in　many　respects；　were　alike　in　combustibility。　From　this　he　argued　that　all　combustible　substances　must　contain　a　common　principle；　and　this　principle　he　named　phlogiston。　This　phlogiston　he　believed　to　be　intimately　associated　in　combination　with　other　substances　in　nature；　and　in　that　condition　not　perceivable　by　the　senses；　but　it　was　supposed　to　escape　as　a　substance　burned；　and　become　apparent　to　the　senses　as　fire　or　flame。　In　other　words；　phlogiston　was　something　imprisoned　in　a　combustible　structure　（itself　forming　part　of　the　structure）；　and　only　liberated　when　this　structure　was　destroyed。　Fire；　or　flame；　was　FREE　phlogiston；　while　the　imprisoned　phlogiston　was　called　COMBINED　PHLOGISTON；　or　combined　fire。　The　peculiar　quality　of　this　strange　substance　was　that　it　disliked　freedom　and　was　always　striving　to　conceal　itself　in　some　combustible　substance。　　Boyle's　tentative　suggestion　that　heat　was　simply　motion　was　apparently　not　accepted　by　Stahl；　or　perhaps　it　was　unknown　to　him。

According　to　the　phlogistic　theory；　the　part　remaining　after　a　substance　was　burned　was　simply　the　original　substance　deprived　of　phlogiston。　To　restore　the　original　combustible　substance；　it　was　necessary　to　heat　the　residue　of　the　combustion　with　something　that　burned　easily；　so　that　the　freed　phlogiston　might　again　combine　with　the　ashes。　This　was　explained　by　the　supposition　that　the　more　combustible　a　substance　was　the　more　phlogiston　it　contained；　and　since　free　phlogiston　sought　always　to　combine　with　some　suitable　substance；　it　was　only　necessary　to　mix　the　phlogisticating　agents；　such　as　charcoal；　phosphorus；　oils；　fats；　etc。；　with　the　ashes　of　the　original　substance；　and　heat　the　mixture；　the　phlogiston　thus　freed　uniting　at　once　with　the　ashes。　　This　theory　fitted　very　nicely　as　applied　to　the　calcined　lead　revivified　by　the　grains　of　wheat；　although　with　some　other　products　of　calcination　it　did　not　seem　to　apply　at　all。

It　will　be　seen　from　this　that　the　phlogistic　theory　was　a　step　towards　chemistry　and　away　from　alchemy。　　It　led　away　from　the　idea　of　a　〃spirit〃　in　metals　that　could　not　be　seen；　felt；　or　appreciated　by　any　of　the　senses；　and　substituted　for　it　a　principle　which；　although　a　falsely　conceived　one；　was　still　much　more　tangible　than　the　〃spirit；〃　since　it　could　be　seen　and　felt　as　free　phlogiston　and　weighed　and　measured　as　combined　phlogiston。　The　definiteness　of　the　statement　that　a　metal；　for　example；　was　composed　of　phlogiston　and　an　element　was　much　less　enigmatic；　even　if　wrong；　than　the　statement　of　the　alchemist　that　〃metals　are　produced　by　the　spiritual　action　of　the　three　principles；　salt；　mercury；　sulphur〃particularly　when　it　is　explained　that　salt；　mercury；　and　sulphur　were　really　not　what　their　names　implied；　and　that　there　was　no　universally　accepted　belief　as　to　what　they　really　were。

The　metals；　which　are　now　regarded　as　elementary　bodies；　were　considered　compounds　by　the　phlogistians；　and　they　believed　that　the　calcining　of　a　metal　was　a　process　of　simplification。　They　noted；　however；　that　the　remains　of　calcination　weighed　more　than　the　original　product；　and　the　natural　inference　from　this　would　be　that　the　metal　must　have　taken　in　some　substance　rather　than　have　given　off　anything。　　But　the　phlogistians　had　not　learned　the　all…important　significance　of　weights；　and　their　explanation　of　variation　in　weight　was　either　that　such　gain　or　loss　was　an　unimportant　〃accident〃　at　best；　or　that　phlogiston；　being　light；　tended　to　lighten　any　substance　containing　it；　so　that　driving　it　out　of　the　metal　by　calcination　naturally　left　the　residue　heavier。

At　first　the　phlogiston　theory　seemed　to　explain　in　an　indisputable　way　all　the　known　chemical　phenomena。　　Gradually；　however；　as　experiments　multiplied；　it　became　evident　that　the　plain　theory　as　stated　by　Stahl　and　his　followers　failed　to　explain　satisfactorily　certain　laboratory　reactions。　　To　meet　these　new　conditions；　certain　modifications　were　introduced　from　time　to　time；　giving　the　theory　a　flexibility　that　would　allow　it　to　cover　all　cases。　But　as　the　number　of　inexplicable　experiments　continued　to　increase；　and　new　modifications　to　the　theory　became　necessary；　it　was　found　that　some　of　these　modifications　were　directly　contradictory　to　others；　and　thus　the　simple　theory　became　too　cumbersome　from　the　number　of　its　modifications。　Its　supporters　disagreed　among　themselves；　first　as　to　the　explanation　of　certain　phenomena　that　did　not　seem　to　accord　with　the　phlogistic　theory；　and　a　little　later　as　to　the　theory　itself。　　But　as　yet　there　was　no　satisfactory　substitute　for　this　theory；　which；　even　if　unsatisfactory；　seemed　better　than　anything　that　had　gone　before　or　could　be　suggested。

But　the　good　effects　of　the　era　of　experimental　research；　to　which　the　theory　of　Stahl　had　given　such　an　impetus；　were　showing　in　the　attitude　of　the　experimenters。　The　works　of　some　of　the　older　writers；　such　as　Boyle　and　Hooke；　were　again　sought　out　in　their　dusty　corners　and　consulted；　and　their　surmises　as　to　the　possible　mixture　of　various　gases　in　the　air　were　more　carefully　considered。　　Still　the　phlogiston　theory　was　firmly　grounded　in　the　minds　of　the　philosophers；　who　can　hardly　be　censured　for　adhering　to　it；　at　least　until　some　satisfactory　substitute　was　offered。　　The　foundation　for　such　a　theory　was　finally　laid；　as　we　shall　see　presently；　by　the　work　of　Black；　Priestley；　Cavendish；　and　Lavoisier；　in　the　eighteenth　century；　but　the　phlogiston　theory　cannot　be　said　to　have　finally　succumbed　until　the　opening　years　of　the　nineteenth　century。



II。　THE　BEGINNINGS　OF　MODERN　CHEMISTRY

THE　〃PNEUMATIC〃　CHEMISTS

Modern　chemistry　may　be　said　to　have　its　beginning　with　the　work　of　Stephen　Hales　（1677…1761）；　who　early　in　the　eighteenth　century　began　his　important　study　of　the　elasticity　of　air。　Departing　from　the　point　of　view　of　most　of　the　scientists　of　the　time；　be　considered　air　to　be　〃a　fine　elastic　fluid；　with　particles　of　very　different　nature　floating　in　it〃　；　and　he　showed　that　these　〃particles〃　could　be　separated。　He　pointed　out；　also；　that　various　gases；　or　〃airs；〃　as　he　called　them；　were　contained　in　many　solid　substances。　The　importance　of　his　work；　however；　lies　in　the　fact　that　his　general　studies　were　along　lines　leading　away　from　the　accepted　doctrines　of　the　time；　and　that　they　gave　the　impetus　to　the　investigation　of　the　properties　of　gases　by　such　chemists　as　Black；　Priestley；　Cavendish；　and　Lavoisier；　whose　specific　discoveries　are　the　foundation…stones　of　modern　chemistry。


JOSEPH　BLACK

The　careful　studies　of　Hales　were　continued　by　his　younger　confrere；　Dr。　Joseph　Black　（1728…1799）；　whose　experiments　in　the　weights　of　gases　and　other　chemicals　were　first　steps　in　quantitative　chemistry。　But　even　more　important　than　his　discoveries　of　chemical　properties　in　general　was　his　discovery　of　the　properties　of　carbonic…acid　gas。

Black　had　been　educated　for　the　medical　profession　in　the　University　of　Glasgow；　being　a　friend　and　pupil　of　the　famous　Dr。　William　Cullen。　　But　his　liking　was　for　the　chemical　laboratory　rather　than　for　the　practice　of　medicine。　　Within　three　years　after　completing　his　medical　course；　and　when　only　twenty…three　years　of　age；　he　made　the　discovery　of　the　properties　of　carbonic　acid；　which　he　called　by　the　name　of　〃fixed　air。〃　　After　discovering　this　gas；　Black　made　a　long　series　of　experiments；　by　which　he　was　able　to　show　how　widely　it　was　distributed　throughout　nature。　　Thus；　in　1757；　be　discovered　that　the　bubbles　given　off　in　the　process　of　brewing；　where　there　was　vegetable　fermentation；　were　composed　of　it。　To　prove　this；　he　collected　the　contents　of　these　bubbles　in　a　bottle　containing　lime…water。　When　this　bottle　was　shaken　violently；　so　that　the　lime…water　and　the　carbonic　acid　became　thoroughly　mixed；　an　insoluble　white　powder　was　precipitated　from　the　solution；　the　carbonic　acid　having　combined　chemically　with　the　lime　to　form　the　insoluble　calcium　carbonate；　or　chalk。　　This　experiment　suggested　another。　Fixing　a　piece　of　burning　charcoal　in　the　end　of　a　bellows；　he　arranged　a　tube　so　that　the　gas　coming　from　the　charcoal　would　pass　through　the　lime…water；　and；　as　in　the　case　of　the　bubbles　from　the　brewer's　vat；　he　found　that　the　white　precipitate　was　thrown　down；　in　short；　that　carbonic　acid　was　given　off　in　combustion。　Shortly　after；　Black　discovered　that　by　blowing　through　a　glass　tube　inserted　into　lime…water；　chalk　was　precipitated；　thus　proving　that　carbonic　acid　was　being　constantly　thrown　off　in　respiration。

The　effect　of　Black's　discoveries　was　revolutionary；　and　the　attitude　of　mind　of　the　chemists　towards　gases；　or　〃airs；〃　was　changed　from　that　time　forward。　Most　of　the　chemists；　however；　attempted　to　harmonize　the　new　facts　with　the　older　theoriesto　explain　all　the　phenomena　on　the　basis　of　the　phlogiston　theory；　which　was　still　dominant。　But　while　many　of　Black's　discoveries　could　not　be　made　to　harmonize　with　that　theory；　they　did　not　directly　overthrow　it。　It　required　the　additional　discoveries　of　some　of　Black's　fellow…scientists　to　complete　its　downfall；　as　we　shall　see。


HENRY　CAVENDISH

This　work　of　Black's　was　followed　by　the　equally　important　work　of　his　f