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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 a