210 DISINTEGRATION Chap. V. 



earth ; and it is this process which gives to 

 vegetable mould its uniform dark tint. It is 

 known that various kinds of acids are gen- 

 erated 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 

 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 



