in the ground. The living roots of plants, moreover, as Sachs and others have shown, quickly corrode and leave their impressions on polished slabs of marble, dolomite and phosphate of lime. They will attack even basalt and sandstone.[1] But we are not here concerned with agencies which are wholly independent of the action of worms.
The combination of any acid with a base is much facilitated by agitation, as fresh surfaces are thus continually brought into contact. This will be thoroughly effected with the particles of stone and earth in the intestines of worms, during the digestive process; and it should be remembered that the entire mass of the mould over every field, passes, in the course of a few years, through their alimentary canals. Moreover as the old burrows slowly collapse, and as fresh castings are continually brought to the surface, the whole superficial layer of mould slowly revolves or circulates; and the friction of the particles one with another will rub off the finest films of disintegrated matter as soon as
- ↑ See, for references on this subject, S. W. Johnson, "How Crops Feed," 1870, p. 326.
