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