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 humus. The latter are well known to have the power of de-oxidising or dissolving peroxide 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
