yeast-第4章

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so　far　as　yeast　is　concerned；　the　doctrine　of　spontaneous　generation　is
absolutely　out　of　court。　　And　not　only　so；　but　the　yeast　must　be　alive
in　order　to　exert　these　peculiar　properties。　　If　it　be　crushed；　if　it　be
heated　so　far　that　its　life　is　destroyed；　that　peculiar　power　of
fermentation　is　not　excited。　　Thus　we　have　come　to　this　conclusion；　as
the　result　of　our　inquiry；　that　the　fermentation　of　sugar；　the
splitting　of　the　sugar　into　alcohol　and　carbonic　acid；　glycerine；　and
succinic　acid；　is　the　result　of　nothing　but　the　vital　activity　of　this
little　fungus；　the　torula。

And　now　comes　the　further　exceedingly　difficult　inquiryhow　is　it　that
this　plant；　the　torula；　produces　this　singular　operation　of　the
splitting　up　of　the　sugar？　　Fabroni；　to　whom　I　referred　some　time　ago；
imagined　that　the　effervescence　of　fermentation　was　produced　in　just　the
same　way　as　the　effervescence　of　a　sedlitz　powder；　that　the　yeast　was　a
kind　of　acid；　and　that　the　sugar　was　a　combination　of　carbonic　acid　and
some　base　to　form　the　alcohol；　and　that　the　yeast　combined　with　this
substance；　and　set　free　the　carbonic　acid；　just　as　when　you　add
carbonate　of　soda　to　acid　you　turn　out　the　carbonic　acid。　　But　of　course
the　discovery　of　Lavoisier　that　the　carbonic　acid　and　the　alcohol　taken
together　are　very　nearly　equal　in　weight　to　the　sugar；　completely　upset
this　hypothesis。　　Another　view　was　therefore　taken　by　the　French
chemist；　Thenard；　and　it　is　still　held　by　a　very　eminent　chemist；　M。
Pasteur；　and　their　view　is　this；　that　the　yeast；　so　to　speak；　eats　a
little　of　the　sugar；　turns　a　little　of　it　to　its　own　purposes；　and　by
so　doing　gives　such　a　shape　to　the　sugar　that　the　rest　of　it　breaks　up
into　carbonic　acid　and　alcohol。

Well；　then；　there　is　a　third　hypothesis；　which　is　maintained　by　another
very　distinguished　chemist；　Liebig；　which　denies　either　of　the　other
two；　and　which　declares　that　the　particles　of　the　sugar　are；　as　it
were；　shaken　asunder　by　the　forces　at　work　in　the　yeast　plant。　　Now　I
am　not　going　to　take　you　into　these　refinements　of　chemical　theory；　I
cannot　for　a　moment　pretend　to　do　so；　but　I　may　put　the　case　before　you
by　an　analogy。　　Suppose　you　compare　the　sugar　to　a　card　house；　and
suppose　you　compare　the　yeast　to　a　child　coming　near　the　card　house；
then　Fabroni's　hypothesis　was　that　the　child　took　half　the　cards　away；
Thenard's　and　Pasteur's　hypothesis　is　that　the　child　pulls　out　the
bottom　card　and　thus　makes　it　tumble　to　pieces；　and　Liebig's　hypothesis
is　that　the　child　comes　by　and　shakes　the　table　and　tumbles　the　house
down。　　I　appeal　to　my　friend　here　（Professor　Roscoe）　whether　that　is　not
a　fair　statement　of　the　case。

Having　thus；　as　far　as　I　can；　discussed　the　general　state　of　the
question；　it　remains　only　that　I　should　speak　of　some　of　those
collateral　results　which　have　come　in　a　very　remarkable　way　out　of　the
investigation　of　yeast。　　I　told　you　that　it　was　very　early　observed　that
the　yeast　plant　consisted　of　a　bag　made　up　of　the　same　material　as　that
which　composes　wood；　and　of　an　interior　semifluid　mass　which　contains　a
substance；　identical　in　its　composition；　in　a　broad　sense；　with　that
which　constitutes　the　flesh　of　animals。　　Subsequently；　after　the
structure　of　the　yeast　plant　had　been　carefully　observed；　it　was
discovered　that　all　plants；　high　and　low；　are　made　up　of　separate　bags
or　〃cells；〃　as　they　are　called；　these　bags　or　cells　having　the
composition　of　the　pure　matter　of　wood；　having　the　same　composition；
broadly　speaking；　as　the　sac　of　the　yeast　plant；　and　having　in　their
interior　a　more　or　less　fluid　substance　containing　a　matter　of　the　same
nature　as　the　protein　substance　of　the　yeast　plant。　　And　therefore　this
remarkable　result　came　outthat　however　much　a　plant　may　differ　from　an
animal；　yet　that　the　essential　constituent　of　the　contents　of　these
various　cells　or　sacs　of　which　the　plant　is　made　up；　the　nitrogenous
protein　matter；　is　the　same　in　the　animal　as　in　the　plant。　　And　not　only
was　this　gradually　discovered；　but　it　was　found　that　these　semifluid
contents　of　the　plant　cell　had；　in　many　cases；　a　remarkable　power　of
contractility　quite　like　that　of　the　substance　of　animals。　And　about　24
or　25　years　ago；　namely；　about　the　year　1846；　to　the　best　of　my
recollection；　a　very　eminent　German　botanist；　Hugo　Von　Mohl；　conferred
upon　this　substance　which　is　found　in　the　interior　of　the　plant　cell；
and　which　is　identical　with　the　matter　found　in　the　inside　of　the　yeast
cell；　and　which　again　contains　an　animal　substance　similar　to　that　of
which　we　ourselves　are　made　uphe　conferred　upon　this　that　title　of
〃protoplasm；〃　which　has　brought　other　people　a　great　deal　of　trouble
since！　　I　beg　particularly　to　say　that；　because　I　find　many　people
suppose　that　I　was　the　inventor　of　that　term；　whereas　it　has　been　in
existence　for　at　least　twenty…five　years。　　And　then　other　observers；
taking　the　question　up；　came　to　this　astonishing　conclusion　（working
from　this　basis　of　the　yeast）；　that　the　differences　between　animals　and
plants　are　not　so　much　in　the　fundamental　substances　which　compose　them；
not　in　the　protoplasm；　but　in　the　manner　in　which　the　cells　of　which
their　bodies　are　built　up　have　become　modified。　　There　is　a　sense　in
which　it　is　trueand　the　analogy　was　pointed　out　very　many　years　ago
by　some　French　botanists　and　chemiststhere　is　a　sense　in　which　it　is
true　that　every　plant　is　substantially　an　enormous　aggregation　of
bodies　similar　to　yeast　cells；　each　having　to　a　certain　extent　its　own
independent　life。　　And　there　is　a　sense　in　which　it　is　also　perfectly
truealthough　it　would　be　impossible　for　me　to　give　the　statement　to
you　with　proper　qualifications　and　limitations　on　an　occasion　like
thisbut　there　is　also　a　sense　in　which　it　is　true　that　every　animal
body　is　made　up　of　an　aggregation　of　minute　particles　of　protoplasm；
comparable　each　of　them　to　the　individual　separate　yeast　plant。　　And
those　who　are　acquainted　with　the　history　of　the　wonderful　revolution
which　has　been　worked　in　our　whole　conception　of　these　matters　in　the
last　thirty　years；　will　bear　me　out　in　saying　that　the　first　germ　of
them；　to　a　very　great　extent；　was　made　to　grow　and　fructify　by　the　study
of　the　yeast　plant；　which　presents　us　with　living　matter　in　almost　its
simplest　condition。

Then　there　is　yet　one　last　and　most　important　bearing　of　this　yeast
question。　　There　is　one　direction　probably　in　which　the　effects　of　the
careful　study　of　the　nature　of　fermentation　will　yield　results　more
practically　valuable　to　mankind　than　any　other。　　Let　me　recall　to　your
minds　the　fact　which　I　stated　at　the　beginning　of　this　lecture。　Suppose
that　I　had　here　a　solution　of　pure　sugar　with　a　little　mineral　matter
in　it；　and　suppose　it　were　possible　for　me　to　take　upon　the　point　of　a
needle　one　single；　solitary　yeast　cell；　measuring　no　more　perhaps　than
the　three…thousandth　of　an　inch　in　diameternot　bigger　than　one　of
those　little　coloured　specks　of　matter　in　my　own　blood　at　this　moment；
the　weight　of　which　it　would　be　difficult　to　express　in　the　fraction　of
a　grainand　put　it　into　this　solution。　　From　that　single　one；　if　the
solution　were　kept　at　a　fair　temperature　in　a　warm　summer's　day；　there
would　be　generated；　in　the　course　of　a　week；　enough　torulae　to　form　a
scum　at　the　top　and　to　form　lees　at　the　bottom；　and　to　change　the
perfectly　tasteless　and　entirely　harmless　fluid；　syrup；　into　a　solution
impregnated　with　the　poisonous　gas　carbonic　acid；　impregnated　with　the
poisonous　substance　alcohol；　and　that；　in　virtue　of　the　changes　worked
upon　the　sugar　by　the　vital　activity　of　these　infinitesimally　small
plants。　　Now　you　see　that　this　is　a　case　of　infection。　　And　from　the
time　that　the　phenomenon　of　fermentation　were　first　carefully　studied；
it　has　constantly　been　suggested　to　the　minds　of　thoughtful　physicians
that　there　was　a　something　astoundingly　similar　between　this　phenomena
of　the　propagation　of　fermentation　by　infection　and　contagion；　and　the
phenomena　of　the　propagation　of　diseases　by　infection　and　contagion。
Out　of　this　suggestion　has　grown　that　remarkable　theory　of　many
diseases　which　has　been　called　the　〃germ　theory　of　disease；〃