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. {217a} 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; {217b} 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
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