the formation of vegetable mould-第29节

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the　oxide。　　Acetic　acid　produced　hardly　any　effect　on　his　sand；　and　
even　hydrochloric；　nitric　and　sulphuric　acids；　diluted　as　in　the　
Pharmacopoeia；　produced　less　effect　than　did　the　acids　in　the　
intestines　of　the　worms。

Mr。　A。　A。　Julien　has　lately　collected　all　the　extant　information　
about　the　acids　generated　in　humus；　which；　according　to　some　
chemists；　amount　to　more　than　a　dozen　different　kinds。　　These　
acids；　as　well　as　their　acid　salts　（i。e。；　in　combination　with　
potash；　soda；　and　ammonia）；　act　energetically　on　carbonate　of　lime　
and　on　the　oxides　of　iron。　　It　is　also　known　that　some　of　these　
acids；　which　were　called　long　ago　by　Thenard　azohumic；　are　enabled　
to　dissolve　colloid　silica　in　proportion　to　the　nitrogen　which　they　
contain。　｛64｝　　In　the　formation　of　these　latter　acids　worms　
probably　afford　some　aid；　for　Dr。　H。　Johnson　informs　me　that　by　
Nessler's　test　he　found　0。018　per　cent。　of　ammonia　in　their　
castings。

It　may　be　here　added　that　I　have　recently　been　informed　by　Dr。　
Gilbert　〃that　several　square　yards　on　his　lawn　were　swept　clean；　
and　after　two　or　three　weeks　all　the　worm…castings　on　the　space　
were　collected　and　dried。　　These　were　found　to　contain　0。35　of　
nitrogen。　　This　is　from　two　to　three　times　as　much　as　we　find　in　
our　ordinary　arable　surface…soil；　more　than　in　our　ordinary　pasture　
surface…soil；　but　less　than　in　rich　kitchen…garden　mould。　　
Supposing　a　quantity　of　castings　equal　to　10　tons　in　the　dry　state　
were　annually　deposited　on　an　acre；　this　would　represent　a　manuring　
of　78　lbs。　of　nitrogen　per　acre　per　annum；　and　this　is　very　much　
more　than　the　amount　of　nitrogen　in　the　annual　yield　of　hay　per　
acre；　if　raised　without　any　nitrogenous　manure。　　Obviously；　so　far　
as　the　nitrogen　in　the　castings　is　derived　from　surface…growth　or　
from　surface…soil；　it　is　not　a　gain　to　the　latter；　but　so　far　as　it　
is　derived　from　below；　it　is　a　gain。〃

The　several　humus…acids；　which　appear；　as　we　have　just　seen；　to　be　
generated　within　the　bodies　of　worms　during　the　digestive　process；　
and　their　acid　salts；　play　a　highly　important　part；　according　to　
the　recent　observations　of　Mr。　Julien；　in　the　disintegration　of　
various　kinds　of　rocks。　　It　has　long　been　known　that　the　carbonic　
acid；　and　no　doubt　nitric　and　nitrous　acids；　which　are　present　in　
rain…water；　act　in　like　manner。　　There　is；　also；　a　great　excess　of　
carbonic　acid　in　all　soils；　especially　in　rich　soils；　and　this　is　
dissolved　by　the　water　in　the　ground。　　The　living　roots　of　plants；　
moreover；　as　Sachs　and　others　have　shown；　quickly　corrode　and　leave　
their　impressions　on　polished　slabs　of　marble；　dolomite　and　
phosphate　of　lime。　　They　will　attack　even　basalt　and　sandstone。　
｛65｝　　But　we　are　not　here　concerned　with　agencies　which　are　wholly　
independent　of　the　action　of　worms。

The　combination　of　any　acid　with　a　base　is　much　facilitated　by　
agitation；　as　fresh　surfaces　are　thus　continually　brought　into　
contact。　　This　will　be　thoroughly　effected　with　the　particles　of　
stone　and　earth　in　the　intestines　of　worms；　during　the　digestive　
process；　and　it　should　be　remembered　that　the　entire　mass　of　the　
mould　over　every　field；　passes；　in　the　course　of　a　few　years；　
through　their　alimentary　canals。　　Moreover　as　the　old　burrows　
slowly　collapse；　and　as　fresh　castings　are　continually　brought　to　
the　surface；　the　whole　superficial　layer　of　mould　slowly　revolves　
or　circulates；　and　the　friction　of　the　particles　one　with　another　
will　rub　off　the　finest　films　of　disintegrated　matter　as　soon　as　
they　are　formed。　　Through　these　several　means；　minute　fragments　of　
rocks　of　many　kinds　and　mere　particles　in　the　soil　will　be　
continually　exposed　to　chemical　decomposition；　and　thus　the　amount　
of　soil　will　tend　to　increase。

As　worms　line　their　burrows　with　their　castings；　and　as　the　burrows　
penetrate　to　a　depth　of　5　or　6；　or　even　more　feet；　some　small　
amount　of　the　humus…acids　will　be　carried　far　down；　and　will　there　
act　on　the　underlying　rocks　and　fragments　of　rock。　　Thus　the　
thickness　of　the　soil；　if　none　be　removed　from　the　surface；　will　
steadily　though　slowly　tend　to　increase；　but　the　accumulation　will　
after　a　time　delay　the　disintegration　of　the　underlying　rocks　and　
of　the　more　deeply　seated　particles。　　For　the　humus…acids　which　are　
generated　chiefly　in　the　upper　layer　of　vegetable　mould；　are　
extremely　unstable　compounds；　and　are　liable　to　decomposition　
before　they　reach　any　considerable　depth。　｛66｝　　A　thick　bed　of　
overlying　soil　will　also　check　the　downward　extension　of　great　
fluctuations　of　temperature；　and　in　cold　countries　will　check　the　
powerful　action　of　frost。　　The　free　access　of　air　will　likewise　be　
excluded。　　From　these　several　causes　disintegration　would　be　almost　
arrested；　if　the　overlying　mould　were　to　increase　much　in　
thickness；　owing　to　none　or　little　being　removed　from　the　surface。　
｛67｝　　In　my　own　immediate　neighbourhood　we　have　a　curious　proof　how　
effectually　a　few　feet　of　clay　checks　some　change　which　goes　on　in　
flints；　lying　freely　exposed；　for　the　large　ones　which　have　lain　
for　some　time　on　the　surface　of　ploughed　fields　cannot　be　used　for　
building；　they　will　not　cleave　properly；　and　are　said　by　the　
workmen　to　be　rotten。　｛68｝　　It　is　therefore　necessary　to　obtain　
flints　for　building　purposes　from　the　bed　of　red　clay　overlying　the　
chalk　（the　residue　of　its　dissolution　by　rain…water）　or　from　the　
chalk　itself。

Not　only　do　worms　aid　directly　in　the　chemical　disintegration　of　
rocks；　but　there　is　good　reason　to　believe　that　they　likewise　act　
in　a　direct　and　mechanical　manner　on　the　smaller　particles。　　All　
the　species　which　swallow　earth　are　furnished　with　gizzards；　and　
these　are　lined　with　so　thick　a　chitinous　membrane；　that　Perrier　
speaks　of　it；　｛69｝　as　〃une　veritable　armature。〃　　The　gizzard　is　
surrounded　by　powerful　transverse　muscles；　which；　according　to　
Claparede；　are　about　ten　times　as　thick　as　the　longitudinal　ones；　
and　Perrier　saw　them　contracting　energetically。　　Worms　belonging　to　
one　genus；　Digaster；　have　two　distinct　but　quite　similar　gizzards；　
and　in　another　genus；　Moniligaster；　the　second　gizzard　consists　of　
four　pouches；　one　succeeding　the　other；　so　that　it　may　almost　be　
said　to　have　five　gizzards。　｛70｝　　In　the　same　manner　as　
gallinaceous　and　struthious　birds　swallow　stones　to　aid　in　the　
trituration　of　their　food；　so　it　appears　to　be　with　terricolous　
worms。　　The　gizzards　of　thirty…eight　of　our　common　worms　were　
opened；　and　in　twenty…five　of　them　small　stones　or　grains　of　sand；　
sometimes　together　with　the　hard　calcareous　concretions　formed　
within　the　anterior　calciferous　glands；　were　found；　and　in　two　
others　concretions　alone。　　In　the　gizzards　of　the　remaining　worms　
there　were　no　stones；　but　some　of　these　were　not　real　exceptions；　
as　the　gizzards　were　opened　late　in　the　autumn；　when　the　worms　had　
ceased　to　feed　and　their　gizzards　were　quite　empty。　｛71｝

When　worms　make　their　burrows　through　earth　abounding　with　little　
stones；　no　doubt　many　will　be　unavoidably　swallowed；　but　it　must　
not　be　supposed　that　this　fact　accounts　for　the　frequency　with　
which　stones　and　sand　are　found　in　their　gizzards。　　For　beads　of　
glass　and　fragments　of　brick　and　of　hard　tiles　were　scattered　over　
the　surface　of　the　earth；　in　pots　in　which　worms　were　kept　and　had　
already　made　their　burrows；　and　very　many　of　these　beads　and　
fragments　were　picked　up　and　swallowed　by　the　worms；　for　they　were　
found　in　their　castings；　intestines；　and　gizzards。　　They　even　
swallowed　the　coarse　red　dust；　formed　by　the　pounding　of　the　tiles。　　
Nor　can　it　be　supposed　that　they　mistook　the　beads　and　fragments　
for　food；　for　we　have　seen　that　their　taste　is　delicate　enough　to　
distinguish　between　different　kinds　of　leaves。　　It　is　therefore　
manifest　that　they　swallow　hard　objects；　such　as　bits　of　stone；　
beads　of　glass　and　angular　fragments　of　bricks　or　tiles　for　some　
special　purpose；　and　it　can　hardly　be　doubted　that　this　is　to　aid　
their　gizzards　in　crushing　and　grinding　the　earth；　which　they　so　
largely　consume。　　That　such　hard　objects　are　not　necessary　for　
crushing　leaves；　may　be　inferred　from　the　fact　that　certain　
species；　which　live　in　mud　or　water　and　feed　on　dead　or　living　
vegetable　matter；　but　which　do　not　swallow　earth；　are　not　provided　
with　gizzards；　｛72｝　and　therefore　cannot　have　the　power　of　
utilising　stones。

During　the　grinding　process；　the　particles　of　earth　must　be　rubbed　
against　one　another；　and　between　the　stones　and　the　tough　lining　
membrane　of　the　gizzard。　　The　softer　particles　will　thus　suffer　
some　attrition；　and　will　perhaps　even　be　crushed。　　This　conclusion　
is　supported　by　the　appearance　of　freshly　ejected　castings；　for　
these　often　reminded　me　of　the　appearance　of　paint　which　has　just　
been　ground　by　a　workman　between　two　flat　stones。　　Morren　remarks　
that　the　intestinal　canal　is　〃impleta　tenuissima　terra；　veluti　in　
pulverem　redacta。〃　｛73｝　　Perrier　also　speaks　of　〃l'etat　de　pate　
excessivement　fine　a　laquelle　est　reduite　la　terre　qu'ils　
rejettent；〃　&c。　｛74｝

As　the　amount　of　trituration　which　the　particles　of　earth　undergo　
in　the　gizzards　of　worms　pos