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.

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.

Charles Darwin

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