第47章

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, thatduring 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. 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 only1.76 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.

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; 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 mouldis 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, nitric and sulphuric acids, diluted as in the Pharmacopoeia,produced less effect than did the acids in the intestines of the worms.