242 DISINTEGRATION CHAP. Y. 



earth ; and it is this process which gives to 

 vegetable mould its uniform dark tint. It is 

 known that various kinds of acids are gene- 

 rated by the decay of vegetable matter ; and 

 from the contents of the intestines of worms and 

 from their castings being acid, it seems pro- 

 bable 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 neutra- 

 lise 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 in- 

 testines are acid, the acidity can hardly be due 

 to the presence of uric acid. We may there- 

 fore conclude that the acids in the alimentary 

 canal of worms are formed during the diges- 

 tive 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 



