the　blast　furnaces；　in　like　manner　as　carburetted　hydrogen　gas　was
  purified。　　The　ironmaster　supposed　that　it　was　the　presence　of　sulphur
  in　the　air　that　caused　blast…furnaces　to　work　irregularly；　and　to
  make　bad　iron　in　the　summer　months。　　Mr。　Neilson　was　of　opinion　that
  this　was　not　the　true　cause；　and　he　was　rather　disposed　to　think　it
  attributable　to　the　want　of　a　due　proportion　of　oxygen　in　summer；
  when　the　air　was　more　rarefied；　besides　containing　more　aqueous
  vapour　than　in　winter。　　He　therefore　thought　the　true　remedy　was　in
  some　way　or　other　to　throw　in　a　greater　proportion　of　oxygen；　and　he
  suggested　that；　in　order　to　dry　the　air；　it　should　be　passed；　on　its
  way　to　the　furnace；　through　two　long　tunnels　containing　calcined
  lime。　　But　further　inquiry　served　to　correct　his　views；　and　eventually
  led　him　to　the　true　theory　of　blasting。
  Shortly　after；　his　attention　was　directed　by　Mr。　James　Ewing　to　a
  defect　in　one　of　the　Muirkirk　blast…furnaces；　situated　about　half　a
  mile　distant　from　the　blowing…engine；　which　was　found　not　to　work　so
  well　as　others　which　were　situated　close　to　it。　　The　circumstances　of
  the　case　led　Mr。　Neilson　to　form　the　opinion　that；　as　air　increases
  in　volume　according　to　temperature；　if　he　were　to　heat　it　by　passing
  it　through　a　red…hot　vessel；　its　volume　would　be　increased；　according
  to　the　well…known　law；　and　the　blast　might　thus　be　enabled　to　do　more
  duty　in　the　distant　furnace。　　He　proceeded　to　make　a　series　of
  experiments　at　the　Gas…works；　trying　the　effect　of　heated　air　on　the
  illuminating　power　of　gas；　by　bringing　up　a　stream　of　it　in　a　tube　so
  as　to　surround　the　gas…burner。　　He　found　that　by　this　means　the
  combustion　of　the　gas　was　rendered　more　intense；　and　its　illuminating
  power　greatly　increased。　　He　proceeded　to　try　a　similar　experiment　on
  a　common　smith's　fire；　by　blowing　the　fire　with　heated　air；　and　the
  effect　was　the　same；　the　fire　was　much　more　brilliant；　and
  accompanied　by　an　unusually　intense　degree　of　heat。
  Having　obtained　such　marked　results　by　these　small　experiments；　it
  naturally　occurred　to　him　that　a　similar　increase　in　intensity　of
  combustion　and　temperature　would　attend　the　application　of　the
  process　to　the　blast…furnace　on　a　large　scale；　but　being　only　a
  gas…maker；　he　had　the　greatest　difficulty　in　persuading　any
  ironmaster　to　permit　him　to　make　the　necessary　experiment's　with
  blast…furnaces　actually　at　work。　　Besides；　his　theory　was　altogether
  at　variance　with　the　established　practice；　which　was　to　supply　air　as
  cold　as　possible；　the　prevailing　idea　being　that　the　coldness　of　the
  air　in　winter　was　the　cause　of　the　best　iron　being　then　produced。
  Acting　on　these　views；　the　efforts　of　the　ironmasters　had　always　been
  directed　to　the　cooling　of　the　blast；　and　various　expedients　were
  devised　for　the　purpose。　　Thus　the　regulator　was　painted　white；　as
  being　the　coolest　colour；　the　air　was　passed　over　cold　water；　and　in
  some　cases　the　air　pipes　were　even　surrounded　by　ice；　all　with　the
  object　of　keeping　the　blast　cold。　　When；　therefore；　Mr。　Neilson
  proposed　entirely　to　reverse　the　process；　and　to　employ　hot　instead
  of　cold　blast；　the　incredulity　of　the　ironmasters　may　well　be
  imagined。　　What！　　Neilson；　a　mere　maker　of　gas；　undertake　to　instruct
  practical　men　in　the　manufacture　of　iron！　　And　to　suppose　that　heated
  air　can　be　used　for　the　purpose！　　It　was　presumption　in　the　extreme；
  or　at　best　the　mere　visionary　idea　of　a　person　altogether
  unacquainted　with　the　subject！
  At　length；　however；　Mr。　Neilson　succeeded　in　inducing　Mr。　Charles
  Macintosh　of　Crossbasket；　and　Mr。　Colin　Dunlop　of　the　Clyde　Iron
  Works；　to　allow　him　to　make　a　trial　of　the　hot　air　process。　　In　the
  first　imperfect　attempts　the　air　was　heated　to　little　more　than　80
  degrees　Fahrenheit；　yet　the　results　were　satisfactory；　and　the
  scoriae　from　the　furnace　evidently　contained　less　iron。　　He　was
  therefore　desirous　of　trying　his　plan　upon　a　more　extensive　scale；
  with　the　object；　if　possible；　of　thoroughly　establishing　the
  soundness　of　his　principle。　　In　this　he　was　a　good　deal　hampered　even
  by　those　ironmasters　who　were　his　friends；　and　had　promised　him　the
  requisite　opportunities　for　making　a　fair　trial　of　the　new　process。
  They　strongly　objected　to　his　making　the　necessary　alterations　in　the
  furnaces；　and　he　seemed　to　be　as　far　from　a　satisfactory　experiment
  as　ever。　　In　one　instance；　where　he　had　so　far　succeeded　as　to　be
  allowed　to　heat　the　blast…main；　he　asked　permission　to　introduce
  deflecting　plates　in　the　main　or　to　put　a　bend　in　the　pipe；　so　as　to
  bring　the　blast　more　closely　against　the　heated　sides　of　the　pipe；
  and　also　increase　the　area　of　heating　surface；　in　order　to　raise　the
  temperature　to　a　higher　point；　but　this　was　refused；　and　it　was　said
  that　if　even　a　bend　were　put　in　the　pipe　the　furnace　would　stop
  working。　　These　prejudices　proved　a　serious　difficulty　in　the　way　of
  our　inventor；　and　several　more　years　passed　before　he　was　allowed　to
  put　a　bend　in　the　blast…main。　　After　many　years　of　perseverance；　he
  was；　however；　at　length　enabled　to　work　out　his　plan　into　a　definite
  shape　at　the　Clyde　Iron　Works；　and　its　practical　value　was　at　once
  admitted。　　At　the　meeting　of　the　Mechanical　Engineers'　Society　held　in
  May；　1859；　Mr。　Neilson　explained　that　his　invention　consisted　solely
  in　the　principle　of　heating　the　blast　between　the　engine　and　the
  furnace；　and　was　not　associated　with　any　particular　construction　of
  the　intermediate　heating　apparatus。　　This；　he　said；　was　the　cause　of
  its　success；　and　in　some　respects　it　resembled　the　invention　of　his
  countryman；　James　Watt；　who；　in　connection　with　the　steam…engine；
  invented　the　plan　of　condensing　the　steam　in　a　separate　vessel；　and
  was　successful　in　maintaining　his　invention　by　not　limiting　it　to　any
  particular　construction　of　the　condenser。　　On　the　same　occasion　he
  took　the　opportunity　of　acknowledging　the　firmness　with　which　the
  English　ironmasters　had　stood　by　him　when　attempts　were　made　to
  deprive　him　of　the　benefits　of　his　invention；　and　to　them　he
  acknowledged　he　was　mainly　indebted　for　the　successful　issue　of　the
  severe　contests　he　had　to　undergo。　　For　there　were；　of　course；　certain
  of　the　ironmasters；　both　English　and　Scotch；　supporters　of　the　cause
  of　free　trade　in　others'　inventions；　who　sought　to　resist　the　patent；
  after　it　had　come　into　general　use；　and　had　been　recognised　as　one　of
  the　most　valuable　improvements　of　modem　times。*
  ＇footnote。。。
  Mr。　Mushet　described　it　as　〃a　wonderful　discovery；〃　and　one　of　the
  〃most　novel　and　beautiful　improvements　in　his　time。〃　　Professor
  Gregory　of　Aberdeen　characterized　it　as　〃the　greatest　improvement
  with　which　he　was　acquainted。〃　　Mr。　Jessop；　an　extensive　English　iron
  manufacturer；　declared　it　to　be　〃of　as　great　advantage　in　the　iron
  trade　as　Arkwright's　machinery　was　in　the　cotton…spinning　trade；　and
  Mr。　Fairbairn；　in　his　contribution　on　〃Iron〃　in　the　Encyclopaedia
  Britannica；　says　that　it　〃has　effected　an　entire　revolution　in　the
  iron　industry　of　Great　Britain；　and　forms　the　last　era　in　the　history
  of　this　material。〃
  。。。＇
  The　patent　was　secured　in　1828　for　a　term　of　fourteen　years；　but；　as
  Mr。　Neilson　did　not　himself　possess　the　requisite　capital　to　enable
  him　to　perfect　the　invention；　or　to　defend　it　if　attacked；　he　found
  it　necessary　to　invite　other　gentlemen；　able　to　support　him　in　these
  respects；　to　share　its　profits；　retaining　for　himself　only
  three…tenths　of　the　whole。　　His　partners　were　Mr。　Charles　Macintosh；
  Mr。　Colin　Dunlop；　and　Mr。John　Wilson　of　Dundyvan。　　The　charge　made　by
  them　was　only　a　shilling　a　ton　for　all　iron　produced　by　the　new
  process；　this　low　rate　being　fixed　in　order　to　ensure　the
  introduction　of　the　patent　into　general　use；　as　well　as　to　reduce　to
  a　minimum　the　temptations　of　the　ironmasters　to　infringe　it。
  The　first　trials　of　the　process　were　made　at　the　blast…furnaces　of
  Clyde　and　Calder；　from　whence　the　use　of　the　hot　blast　gradually
  extended　to　the　other　iron…mining　districts。　　In　the　course　of　a　few
  years　every　furnace　in　Scotland；　with　one　exception　（that　at　Carron）；
  had　adopted　the　improvement；　while　it　was　also　employed　in　half　the
  furnaces　of　England　and　Wales；　and　in　many　of　the　furnaces　on　the
  Continent　and　in　America。　　In　course　of　time；　and　with　increasing
  experience；　various　improvements　were　introduced　in　the　process；　more
  particularly　in　the　shape　of　the　air…heating　vessels；　the　last　form
  adopted　being　that　of　a　congeries　of　tubes；　similar　to　the　tubular
  arrangement　in　the　boiler　of　the　locomotive；　by　which　the　greatest
  extent　of　heating　surface　was　provided　for　the　thorough　heating　of
  the　air。　　By　these　modifications　the　temperature　of　the　air　introduced
  into　the　furnace　has　been　raised　from　240　degrees　to　600　degrees；　or
  the　temperature　of　melting　lead。　　To　protect　the　nozzle　of　the
  air…pipe　as　it　entered　the　furnace　against　the　action　of　the　intense
  heat　to　which