按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
sand also wears down the hardest rocks。 I have shown ｛60｝ that
during four months of the year a large quantity of dust is blown
from the north…western shores of Africa； and falls on the Atlantic
over a space of 1600 miles in latitude； and for a distance of from
300 to 600 miles from the coast。 But dust has been seen to fall at
a distance of 1030 miles from the shores of Africa。 During a stay
of three weeks at St。 Jago in the Cape Verde Archipelago； the
atmosphere was almost always hazy； and extremely fine dust coming
from Africa was continually falling。 In some of this dust which
fell in the open ocean at a distance of between 330 and 380 miles
from the African coast； there were many particles of stone； about
1/1000 of an inch square。 Nearer to the coast the water has been
seen to be so much discoloured by the falling dust； that a sailing
vessel left a track behind her。 In countries； like the Cape Verde
Archipelago； where it seldom rains and there are no frosts； the
solid rock nevertheless disintegrates； and in conformity with the
views lately advanced by a distinguished Belgian geologist； De
Koninck； such disintegration may be attributed in chief part to the
action of the carbonic and nitric acids； together with the nitrates
and nitrites of ammonia； dissolved in the dew。
In all humid； even moderately humid； countries； worms aid in the
work of denudation in several ways。 The vegetable mould which
covers； as with a mantle； the surface of the land； has all passed
many times through their bodies。 Mould differs in appearance from
the subsoil only in its dark colour； and in the absence of
fragments or particles of stone （when such are present in the
subsoil）； larger than those which can pass through the alimentary
canal of a worm。 This sifting of the soil is aided； as has already
been remarked； by burrowing animals of many kinds； especially by
ants。 In countries where the summer is long and dry； the mould in
protected places must be largely increased by dust blown from other
and more exposed places。 For instance； the quantity of dust
sometimes blown over the plains of La Plata； where there are no
solid rocks； is so great； that during the 〃gran seco；〃 1827 to
1830； the appearance of the land； which is here unenclosed； was so
completely changed that the inhabitants could not recognise the
limits of their own estates； and endless lawsuits arose。 Immense
quantities of dust are likewise blown about in Egypt and in the
south of France。 In China； as Richthofen maintains； beds appearing
like fine sediment； several hundred feet in thickness and extending
over an enormous area； owe their origin to dust blown from the high
lands of central Asia。 ｛61｝ In humid countries like Great Britain；
as long as the land remains in its natural state clothed with
vegetation； the mould in any one place can hardly be much increased
by dust； but in its present condition； the fields near high roads；
where there is much traffic； must receive a considerable amount of
dust； and when fields are harrowed during dry and windy weather；
clouds of dust may be seen to be blown away。 But in all these
cases the surface…soil is merely transported from one place to
another。 The dust which falls so thickly within our houses
consists largely of organic matter； and if spread over the land
would in time decay and disappear almost entirely。 It appears；
however； from recent observations on the snow…fields of the Arctic
regions； that some little meteoric dust of extra mundane origin is
continually falling。
The dark colour of ordinary mould is obviously due to the presence
of decaying organic matter； which； however； is present in but small
quantities。 The loss of weight which mould suffers when heated to
redness seems to be in large part due to water in combination being
dispelled。 In one sample of fertile mould the amount of organic
matter was ascertained to be only 1。76 per cent。； in some
artificially prepared soil it was as much as 5。5 per cent。； and in
the famous black soil of Russia from 5 to even 12 per cent。 ｛62｝
In leaf…mould formed exclusively by the decay of leaves the amount
is much greater； and in peat the carbon alone sometimes amounts to
64 per cent。； but with these latter cases we are not here
concerned。 The carbon in the soil tends gradually to oxidise and
to disappear； except where water accumulates and the climate is
cool； ｛63｝ so that in the oldest pasture…land there is no great
excess of organic matter； notwithstanding the continued decay of
the roots and the underground stems of plants； and the occasional
addition of manure。 The disappearance of the organic matter from
mould is probably much aided by its being brought again and again
to the surface in the castings of worms。
Worms； on the other hand； add largely to the organic matter in the
soil by the astonishing number of half…decayed leaves which they
draw into their burrows to a depth of 2 or 3 inches。 They do this
chiefly for obtaining food； but partly for closing the mouths of
their burrows and for lining the upper part。 The leaves which they
consume are moistened； torn into small shreds； partially digested；
and intimately commingled with earth； and it is this process which
gives to vegetable mould its uniform dark tint。 It is known that
various kinds of acids are generated by the decay of vegetable
matter； and from the contents of the intestines of worms and from
their castings being acid； it seems probable that the process of
digestion induces an analogous chemical change in the swallowed；
triturated； and half…decayed leaves。 The large quantity of
carbonate of lime secreted by the calciferous glands apparently
serves to neutralise the acids thus generated； for the digestive
fluid of worms will not act unless it be alkaline。 As the contents
of the upper part of their intestines are acid； the acidity can
hardly be due to the presence of uric acid。 We may therefore
conclude that the acids in the alimentary canal of worms are formed
during the digestive process； and that probably they are nearly of
the same nature as those in ordinary mould or humus。 The latter
are well known to have the power of de…oxidising or dissolving per…
oxide of iron； as may be seen wherever peat overlies red sand； or
where a rotten root penetrates such sand。 Now I kept some worms in
a pot filled with very fine reddish sand； consisting of minute
particles of silex coated with the red oxide of iron； and the
burrows； which the worms made through this sand； were lined or
coated in the usual manner with their castings； formed of the sand
mingled with their intestinal secretions and the refuse of the
digested leaves； and this sand had almost wholly lost its red
colour。 When small portions of it were placed under the
microscope； most of the grains were seen to be transparent and
colourless； owing to the dissolution of the oxide； whilst almost
all the grains taken from other parts of the pot were coated with
the oxide。 Acetic acid produced hardly any effect on his sand； and
even hydrochloric； nitr