infusion；　that　then；　although　you　might　have　plenty　of　animalcules　in
  the　first　flask；　you　would　certainly　obtain　none　from　the　second。
  These　experiments；　you　see；　all　tended　towards　one　conclusionthat　the
  infusoria　were　developed　from　little　minute　spores　or　eggs　which　were
  constantly　floating　in　the　atmosphere；　which　lose　their　power　of
  germination　if　subjected　to　heat。　　But　one　observer　now　made　another
  experiment　which　seemed　to　go　entirely　the　other　way；　and　puzzled　him
  altogether。　He　took　some　of　this　boiled　infusion　that　I　have　been
  speaking　of；　and　by　the　use　of　a　mercurial　batha　kind　of　trough　used
  in　laboratorieshe　deftly　inverted　a　vessel　containing　the　infusion
  into　the　mercury；　so　that　the　latter　reached　a　little　beyond　the　level
  of　the　mouth　of　the　'inverted'　vessel。　　You　see　that　he　thus　had　a
  quantity　of　the　infusion　shut　off　from　any　possible　communication　with
  the　outer　air　by　being　inverted　upon　a　bed　of　mercury。
  He　then　prepared　some　pure　oxygen　and　nitrogen　gases；　and　passed　them　by
  means　of　a　tube　going　from　the　outside　of　the　vessel；　up　through　the
  mercury　into　the　infusion；　so　that　he　thus　had　it　exposed　to　a
  perfectly　pure　atmosphere　of　the　same　constituents　as　the　external　air。
  Of　course；　he　expected　he　would　get　no　infusorial　animalcules　at　all　in
  that　infusion；　but；　to　his　great　dismay　and　discomfiture；　he　found　he
  almost　always　did　get　them。
  Furthermore；　it　has　been　found　that　experiments　made　in　the　manner
  described　above　answer　well　with　most　infusions；　but　that　if　you　fill
  the　vessel　with　boiled　milk；　and　then　stop　the　neck　with　cotton…wool；
  you　'will'　have　infusoria。　　So　that　you　see　there　were　two　experiments
  that　brought　you　to　one　kind　of　conclusion；　and　three　to　another；　which
  was　a　most　unsatisfactory　state　of　things　to　arrive　at　in　a　scientific
  inquiry。
  Some　few　years　after　this；　the　question　began　to　be　very　hotly　discussed
  in　France。　　There　was　M。　Pouchet；　a　professor　at　Rouen；　a　very　learned
  man；　but　certainly　not　a　very　rigid　experimentalist。　　He　published　a
  number　of　experiments　of　his　own；　some　of　which　were　very　ingenious；　to
  show　that　if　you　went　to　work　in　a　proper　way；　there　was　a　truth　in　the
  doctrine　of　spontaneous　generation。　　Well；　it　was　one　of　the　most
  fortunate　things　in　the　world　that　M。　　Pouchet　took　up　this　question；
  because　it　induced　a　distinguished　French　chemist；　M。　Pasteur；　to　take
  up　the　question　on　the　other　side；　and　he　has　certainly　worked　it　out　in
  the　most　perfect　manner。　　I　am　glad　to　say；　too；　that　he　has　published
  his　researches　in　time　to　enable　me　to　give　you　an　account　of　them。　　He
  verified　all　the　experiments　which　I　have　just　mentioned　to　youand
  then　finding　those　extraordinary　anomalies；　as　in　the　case　of　the
  mercury　bath　and　the　milk；　he　set　himself　to　work　to　discover　their
  nature。　　In　the　case　of　milk　he　found　it　to　be　a　question　of
  temperature。　　Milk　in　a　fresh　state　is　slightly　alkaline；　and　it　is　a
  very　curious　circumstance；　but　this　very　slight　degree　of　alkalinity
  seems　to　have　the　effect　of　preserving　the　organisms　which　fall　into　it
  from　the　air　from　being　destroyed　at　a　temperature　of　212　degrees；
  which　is　the　boiling　point。　　But　if　you　raise　the　temperature　10　degrees
  when　you　boil　it；　the　milk　behaves　like　everything　else；　and　if　the　air
  with　which　it　comes　in　contact；　after　being　boiled　at　this　temperature；
  is　passed　through　a　red…hot　tube；　you　will　not　get　a　trace　of
  organisms。
  He　then　turned　his　attention　to　the　mercury　bath；　and　found　on
  examination　that　the　surface　of　the　mercury　was　almost　always　covered
  with　a　very　fine　dust。　　He　found　that　even　the　mercury　itself　was
  positively　full　of　organic　matters；　that　from　being　constantly　exposed
  to　the　air；　it　had　collected　an　immense　number　of　these　infusorial
  organisms　from　the　air。　　Well；　under　these　circumstances　he　felt　that
  the　case　was　quite　clear；　and　that　the　mercury　was　not　what　it　had
  appeared　to　M。　Schwann　to　be；a　bar　to　the　admission　of　these
  organisms；　but　that；　in　reality；　it　acted　as　a　reservoir　from　which　the
  infusion　was　immediately　supplied　with　the　large　quantity　that　had　so
  puzzled　him。
  But　not　content　with　explaining　the　experiments　of　others；　M。　Pasteur
  went　to　work　to　satisfy　himself　completely。　　He　said　to　himself：　〃If　my
  view　is　right；　and　if；　in　point　of　fact；　all　these　appearances　of
  spontaneous　generation　are　altogether　due　to　the　falling　of　minute
  germs　suspended　in　the　atmosphere；why；　I　ought　not　only　to　be　able　to
  show　the　germs；　but　I　ought　to　be　able　to　catch　and　sow　them；　and
  produce　the　resulting　organisms。〃　　He；　accordingly；　constructed　a　very
  ingenious　apparatus　to　enable　him　to　accomplish　this　trapping　of　this
  〃germ　dust〃　in　the　air。　　He　fixed　in　the　window　of　his　room　a　glass
  tube；　in　the　centre　of　which　he　had　placed　a　ball　of　gun…cotton；　which；
  as　you　all　know；　is　ordinary　cotton…wool；　which；　from　having　been
  steeped　in　strong　acid；　is　converted　into　a　substance　of　great　explosive
  power。　　It　is　also　soluble　in　alcohol　and　ether。　　One　end　of　the　glass
  tube　was；　of　course；　open　to　the　external　air；　and　at　the　other　end　of
  it　he　placed　an　aspirator；　a　contrivance　for　causing　a　current　of　the
  external　air　to　pass　through　the　tube。　　He　kept　this　apparatus　going
  for　four…and…twenty　hours；　and　then　removed　the　'dusted'　gun…cotton；
  and　dissolved　it　in　alcohol　and　ether。　　He　then　allowed　this　to　stand
  for　a　few　hours；　and　the　result　was；　that　a　very　fine　dust　was
  gradually　deposited　at　the　bottom　of　it。　　That　dust；　on　being
  transferred　to　the　stage　of　a　microscope；　was　found　to　contain　an
  enormous　number　of　starch　grains。　　You　know　that　the　materials　of　our
  food　and　the　greater　portion　of　plants　are　composed　of　starch；　and　we
  are　constantly　making　use　of　it　in　a　variety　of　ways；　so　that　there　is
  always　a　quantity　of　it　suspended　in　the　air。　　It　is　these　starch
  grains　which　form　many　of　those　bright　specks　that　we　see　dancing　in　a
  ray　of　light　sometimes。　　But　besides　these；　M。　Pasteur　found　also　an
  immense　number　of　other　organic　substances　such　as　spores　of　fungi；
  which　had　been　floating　about　in　the　air　and　had　got　caged　in　this　way。
  He　went　farther；　and　said　to　himself；　〃If　these　really　are　the　things
  that　give　rise　to　the　appearance　of　spontaneous　generation；　I　ought　to
  be　able　to　take　a　ball　of　this　'dusted'　gun…cotton　and　put　it　into　one
  of　my　vessels；　containing　that　boiled　infusion　which　has　been　kept　away
  from　the　air；　and　in　which　no　infusoria　are　at　present　developed；　and
  then；　if　I　am　right；　the　introduction　of　this　gun…cotton　will　give　rise
  to　organisms。〃
  Accordingly；　he　took　one　of　these　vessels　of　infusion；　which　had　been
  kept　eighteen　months；　without　the　least　appearance　of　life；　and　by　a
  most　ingenious　contrivance；　he　managed　to　break　it　open　and　introduce
  such　a　ball　of　gun…cotton；　without　allowing　the　infusion　or　the　cotton
  ball　to　come　into　contact　with　any　air　but　that　which　had　been　subjected
  to　a　red　heat；　and　in　twenty…four　hours　he　had　the　satisfaction　of
  finding　all　the　indications　of　what　had　been　hitherto　called
  spontaneous　generation。　　He　had　succeeded　in　catching　the　germs　and
  developing　organisms　in　the　way　he　had　anticipated。
  It　now　struck　him　that　the　truth　of　his　conclusions　might　be
  demonstrated　without　all　the　apparatus　he　had　employed。　　To　do　this；　he
  took　some　decaying　animal　or　vegetable　substance；　such　as　urine；　which
  is　an　extremely　decomposable　substance；　or　the　juice　of　yeast；　or
  perhaps　some　other　artificial　preparation；　and　filled　a　vessel　having　a
  long　tubular　neck　with　it。　　He　then　boiled　the　liquid　and　bent　that
  long　neck　into　an　S　shape　or　zig…zag；　leaving　it　open　at　the　end。　　The
  infusion　then　gave　no　trace　of　any　appearance　of　spontaneous
  generation；　however　long　it　might　be　left；　as　all　the　germs　in　the　air
  were　deposited　in　the　beginning　of　the　bent　neck。　　He　then　cut　the　tube
  close　to　the　vessel；　and　allowed　the　ordinary　air　to　have　free　and
  direct　access；　and　the　result　of　that　was　the　appearance　of　organisms　in
  it；　as　soon　as　the　infusion　had　been　allowed　to　stand　long　enough　to
  allow　of　the　growth　of　those　it　received　from　the　air；　which　was　about
  forty…eight　hours。　　The　result　of　M。　Pasteur's　experiments　proved；
  therefore；　in　the　most　conclusive　manner；　that　all　the　appearances　of
  spontaneous　generation　arose　from　nothing　more　than　the　deposition　of
  the　germs　of　organisms　which　were　constantly　floating　in　the　air。
  To　this　conclusion；　however；　the　objection　was　made；　that　if　that　were
  the　cause；　then　the　air　would　contain　such　an　enormous　number　of　these
  germs；　that　it　would　be　a　continual　fog。　　But　M。　Pasteur　replied　that
  they　are　not　there　in　anything　like　the　number　we　might　suppose；　and
  that　an　exaggerated　view　has　been　held　on　that　subject；　he　showed　that
  the　chances　of　animal　or　vegetable　life　appearing　in　infusions；　depend
  entirely　on　the　conditions　under　which　they　are　exposed。　　If　they　are
  exposed　to　the　ordinary　atmosphere　around　us；　why；　of　course；　you　may
  have　organisms　appearing　early。　　But；　on　the　other　hand；　if　they　are
  exposed　to　air　from　a　great　height；　or　from　some　very　quiet　cellar；　you
  will　often　not　find　a　single　trace　of　life。
  So　that