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. {220} 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
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