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 Thénard 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 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.”